ChemTEB Classification Datasets
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Chemical Classification Datasets for Evaluating Text Embedding Models
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* Georges Friedel (1904) "Étude sur les groupements cristallins", Extrait du Bulletin de la Société de lIndustrie minérale', Quatrième série, Tomes III e IV. Saint-Étienne, Société de l’Imprimerie Théolier J. Thomas et C., 485 pp.
* Georges Friedel (1920) "Contribution à létude géométrique des macles", Bulletin de la Société française de Minéralogie' 43: 246-295.
* Georges Friedel (1926) Leçons de Cristallographie, Berger-Levrault, Nancy, Paris, Strasbourg XIX+602 pp.
* Georges Friedel (1933) "Sur un nouveau type de macles", Bulletin de la Société française de Minéralogie 56: 262-274.
* J.D.H. Donnay (1940) "Width of Albite-Twinning Lamellae", Am. Mineral., 25: 578-586. | 1 | Crystallography |
Thermometric titrations of silver nitrate with halides and cyanide are all possible. The reaction of silver nitrate with chloride is strongly exothermic. For instance, the reaction enthalpy of Ag with Cl is a high −61.2 kJ/mol. This permits convenient determination of chloride with commonly available standard 0.1 mol/L AgNO. Endpoints are very sharp, and with care, chloride concentrations down to 15 mg/L can be analyzed. Bromide and chloride may be determined in admixture. | 0 | Chromatography + Titration + pH indicators |
A universal indicator is a pH indicator made of a solution of several compounds that exhibit various smooth colour changes over a wide range pH values to indicate the acidity or alkalinity of solutions. A universal indicator can be in paper form or present in a form of a solution. | 0 | Chromatography + Titration + pH indicators |
* Conventional (C)TEM-CBED: In CTEM-CBED different shape condenser apertures are used to obtain the intensity distribution over the entire Brillouin zone.
* Large Angle (LA)CBED: (LA)CBED is performed with a large incident angle, ranging from 1˚ to 10˚. LACBED makes it possible to obtain non-overlapping disks with a larger diameter than the one determined by the Bragg angle. With LACBED I one can obtain one selected CBED disk at a time on a detector. In LACBED II, with a slight change in the focusing conditions of the intermediate lens, bright field patterns and dark field patterns can be obtained simultaneously, without overlapping each other on the fluorescent screen. A disadvantage of LACBED is that it requires a large, flat specimen.
* 4D-STEM: In 4D-STEM a convergent probing beam is raster-scanned on a specimen in a 2D array and in each position of the array, a 2D diffraction pattern is obtained, thus generating a 4D data set. After acquisition, by using different phase techniques such as ptychography, one can recover the transmittion function and the induced phase shift. In some applications, 4D-STEM is called STEM-CBED.
* Beam Rocking (BR)-CBED: With this technique, by rocking the incident beam with a rocking coil placed above the specimen, a virtual convergent beam is produced. Given that the diameter of the beam on the specimen is a few micrometers, this method has made CBED possible for materials that are susceptible to strong convergent beams. Furthermore, the large size of the illuminated specimen area and the low density current of the beam make specimen contamination insignificant.
* BR-LACBED: In this technique, in addition to the rocking coil above the specimen, there is a rocking coil placed under the projector lens, which is used to bring the preferred beam to the STEM detector. Every time the incident beam is rocked, the second coil is simultaneously driven so that the beam always falls on the STEM detector.
* Signal processing and BR-CBED: In order to enhance contrast in BR-CBED, a band-pass filter can be used that filters a certain frequency band in the CBED pattern. The combination of these two techniques makes the symmetries appearing in the patterns more clear.
* CB-LEED (Low Energy Electron Diffraction): Rocking curves are analyzed at a single energy using a convergent probe. Advantages of this method are: mapping of LEED diffraction spots into CBLEED disks, the diffraction patterns originate from a localized region of the specimen which enables the extraction of localized structural information, mapping out of the surfaces, sensitivity enhancement of small atomic displacements etc.
*Ptychography is a technique for recovering the phase of the exit electron wave. The reconstruction is done by applying an iterative phase retrieval algorithm which returns a real-space image with both phase and amplitude information. By using electron ptychography, in 2018, images of MoS with an atomic resolution of 0.39 Å were reported by Jiang et al. which set the new world record for the highest resolution microscope.
* Microdiffraction, nanodiffraction: In the literature, there are several terms used to refer to electron diffraction patterns that are acquired with a convergent beam. Such terms are CBED, microdiffraction, nanodiffraction etc. When the CBED technique is used for the acquisition of conventional diffraction information like lattice structure and interplanar spacing from very small areas, then the term microdiffraction is used. On the other hand, the term nanodiffraction is used when very small probes (< 1 nm or less in diameter) are used. | 1 | Crystallography |
The dyes are immobilized on the column matrix effectively, since usually the dyes link to a monochlorotriazine or dichlorotriazine ring (triazine dye). This type of dyes works especially well on a support matrix with hydroxyl group. The commonly used supporting matrix would be cross-linked agarose (sepharose), sephadex, polyacrylamide, and silica.
An example for triazine linkage immobilization is Blue Sepharose, resulting from Cibacron blue FG3-A with monochlorotriazine covalently coupled with OH group of sepharose. This reaction form an ether linkage and also hydrogen chloride.
CHClNOS + CHO → CHNOS + HCl
Cibacron Blue FG3-A + Sepharose → Blue Sepharose + HCl | 0 | Chromatography + Titration + pH indicators |
A crystal cluster is a group of crystals which are formed in an open space environment and exhibit euhedral crystal form determined by their internal crystal structure. A cluster of small crystals coating the walls of a cavity are called druse. | 1 | Crystallography |
* 1925 – Laboratory of crystallography at the Mineralogical Museum (Leningrad).
* 1932 – Crystallographic section of the Lomonosov Institute of Geochemistry, Mineralogy and Petrography of the USSR Academy of Sciences.
* 1937 – Crystallographic Laboratory becomes part of the Geological Group of the USSR Academy of Sciences.
* 1941 – During World War II the majority of academic institutes were evacuated from Moscow to the East. The Crystallographic Laboratory continued its work in 1941-43 in the Sverdlovsk Oblast (in the Urals) where a series of important scientific and applied crystallographic problems were solved.
* 1943 – The Laboratory returns to Moscow and is transferred to the Department of Physical and Mathematical Sciences and renamed the Institute of Crystallography.
* 1944 – Organization of the Institute of Crystallography. Alexei Shubnikov was appointed Director of the Institute.
* 1956 – Founding of the journal Kristallografiya in which most of the institutes research is subsequently published. This journal is available in English translation as Soviet Physics Crystallography (ISSN 0038-5638) 1956-1992 (vols. 1-37) continued as Crystallography Reports' (ISSN 1063-7745) 1993- (vol. 38-)
* 1957 – Recognition outside the USSR of the establishment of the new field of antisymmetry and colour symmetry by A.V. Shubnikov and N.V. Belov
* 1962 – Boris Konstantinovich Weinstein is appointed Director of the Institute.
* 1969 – Award of the Order of the Red Banner of Labour.
* 1998 – Professor Mikhail Kovalchuk elected Director of the Institute.
* 2016 – The Institute was subsumed within the new «Crystallography and Photonics» Federal Research Center of the Russian Academy of Sciences (KiF RAS) which is now known as the «Crystallography and Photonics» FLNIK. | 1 | Crystallography |
Countercurrent chromatography and centrifugal partition chromatography are two different instrumental realization of the same liquid–liquid chromatographic theory. Countercurrent chromatography usually uses a planetary gear motion without rotary seals, while centrifugal partition chromatography uses circular rotation with rotary seals for liquid connection. CCC has interchanging mixing and settling zones in the coil tube, so atomization, extraction and settling are time and zone separated. Inside centrifugal partition chromatography, all three steps happen continuously in one time, inside the cells.
Advantages of centrifugal partition chromatography:
* Higher flow rate for same volume size Laboratory scale example: 250 mL centrifugal partition chromatography has optimal flow rate of 5–15 mL/min, 250 mL countercurrent chromatography has optimal flow rate of 1–3 mL/min. Process scale example: 25 L countercurrent chromatography has optimal flow rate of 100–300 ml/min, 25 L centrifugal partition chromatography has optimal flow rate of 1000–3000 ml/min.
* Higher productivity (due to higher flow rate and faster separation time)
* Scalable up to tonnes per month
* Better stationary phase retention for most phases
Disadvantages of centrifugal partition chromatography:
* Higher pressure than CCC (typical operation pressures of 40–160 bar vs 5–25 bar)
* Rotary seal wear over time | 0 | Chromatography + Titration + pH indicators |
Abnormal or discontinuous grain growth, also referred to as exaggerated or secondary recrystallisation grain growth, is a grain growth phenomenon in which certain energetically favorable grains (crystallites) grow rapidly in a matrix of finer grains, resulting in a bimodal grain-size distribution.
In ceramic materials, this phenomenon can result in the formation of elongated prismatic, acicular (needle-like) grains in a densified matrix. This microstructure has the potential to improve fracture toughness by impeding the propagation of cracks. | 1 | Crystallography |
The Goldschmidt tolerance factor () is a dimensionless number that is calculated from the ratio of the ionic radii:
In an ideal cubic perovskite structure, the lattice parameter (i.e., length) of the unit cell (a) can be calculated using the following equation: | 1 | Crystallography |
In coordination chemistry and crystallography, the geometry index or structural parameter () is a number ranging from 0 to 1 that indicates what the geometry of the coordination center is. The first such parameter for 5-coordinate compounds was developed in 1984. Later, parameters for 4-coordinate compounds were developed. | 1 | Crystallography |
Iodometry in its many variations is extremely useful in volumetric analysis. Examples include the determination of copper(II), chlorate, hydrogen peroxide, and dissolved oxygen:
Available chlorine refers to chlorine liberated by the action of dilute acids on hypochlorite. Iodometry is commonly employed to determine the active amount of hypochlorite in bleach responsible for the bleaching action. In this method, excess but known amount of iodide is added to known volume of sample, in which only the active (electrophilic) can oxidize iodide to iodine. The iodine content and thus the active chlorine content can be determined with iodometry.
The determination of arsenic(V) compounds is the reverse of the standardization of iodine solution with sodium arsenite, where a known and excess amount of iodide is added to the sample:
For analysis of antimony(V) compounds, some tartaric acid is added to solubilize the antimony(III) product. | 0 | Chromatography + Titration + pH indicators |
The "hard ionization" process of electron ionization can be softened by the cooling of the molecules before their ionization, resulting in mass spectra that are richer in information. In this method named cold electron ionization (cold-EI) the molecules exit the GC column, mixed with added helium make up gas and expand into vacuum through a specially designed supersonic nozzle, forming a supersonic molecular beam (SMB). Collisions with the make up gas at the expanding supersonic jet reduce the internal vibrational (and rotational) energy of the analyte molecules, hence reducing the degree of fragmentation caused by the electrons during the ionization process. Cold-EI mass spectra are characterized by an abundant molecular ion while the usual fragmentation pattern is retained, thus making cold-EI mass spectra compatible with library search identification techniques. The enhanced molecular ions increase the identification probabilities of both known and unknown compounds, amplify isomer mass spectral effects and enable the use of isotope abundance analysis for the elucidation of elemental formulas. | 0 | Chromatography + Titration + pH indicators |
The volumetric titration is based on the same principles as the coulometric titration, except that the anode solution above now is used as the titrant solution. The titrant consists of an alcohol (ROH), base (B), and a known concentration of . Pyridine has been used as the base in this case.
One mole of is consumed for each mole of . The titration reaction proceeds as above, and the end point may be detected by a bipotentiometric method as described above. | 0 | Chromatography + Titration + pH indicators |
Racemic crystallography is a technique used in structural biology where crystals of a protein molecule are developed from an equimolar mixture of an L-protein molecule of natural chirality and its D-protein mirror image. L-protein molecules consist of left-handed L-amino acids and the achiral amino acid glycine, whereas the mirror image D-protein molecules consist of right-handed D-amino acids and glycine. Typically, both the L-protein and the D-protein are prepared by total chemical synthesis. | 1 | Crystallography |
Robert Travis Kennedy is an American chemist specializing in bioanalytical chemistry including liquid chromatography, capillary electrophoresis, and microfluidics. He is currently the Hobart H. Willard Distinguished University Professor of Chemistry and the chair of the department of chemistry at the University of Michigan. He holds joint appointments with the Department of Pharmacology and Department Macromolecular Science and Engineering. Kennedy is an Associate Editor of Analytical Chemistry and ACS Measurement Science AU. | 0 | Chromatography + Titration + pH indicators |
Since it is accepted that ion suppression has the potential to affect the other analytical parameters of any assay, a prudent approach to any LC-MS method development should include an evaluation of ion-suppression. There are two accepted protocols by which this may be achieved, described as follows. | 0 | Chromatography + Titration + pH indicators |
The extrusion of stationary phase from the column at the end of a separation experiment by stopping rotation and pumping solvent or gas through the column was used by CCC practitioners before the term EECCC was suggested. In elution-extrusion mode (EECCC), The mobile phase is extruded after a certain point by switching the phase being pumped into the system whilst maintaining rotation. For example, if the separation has been initiated with the aqueous phase as the mobile phase at a certain point the organic phase is pumped through the column which effectively pushes out both phases that are present in the column at the time of switching. The complete sample is eluted in the order of polarity (either normal or reversed) without loss of resolution by diffusion. It requires only one column volume of solvent phase and leaves the column full of fresh stationary phase for the subsequent separation. | 0 | Chromatography + Titration + pH indicators |
In this approach, the sample is spiked with a species (internal standard) which is used to normalise the response of analyte, compensating for variables at any stage of the sample preparation and analysis, including ion suppression.
It is important that the internal standard displays very similar (ideally identical) properties, with respect to detector response (i.e. ionisation), as the analyte of interest. To simplify the selection of internal standard, most laboratories use an analogous stable isotope in an isotope dilution type analysis. The stable isotope is almost guaranteed to be chemically and physically as close as possible to the analyte of interest, hence producing an almost identical detector response in addition to behaving identically during sample preparation and chromatographic resolution. To this end, the ion suppression experienced by both the analyte and the internal standard should be identical. It is important to note that an excessively high concentration of stable isotope internal standard may cause ion suppression itself, since it will co-elute with the analyte of interest. Hence, the internal standard should be added at an appropriate concentration. | 0 | Chromatography + Titration + pH indicators |
There are two simple regular lattices that achieve this highest average density. They are called face-centered cubic (FCC) (also called cubic close packed) and hexagonal close-packed (HCP), based on their symmetry. Both are based upon sheets of spheres arranged at the vertices of a triangular tiling; they differ in how the sheets are stacked upon one another. The FCC lattice is also known to mathematicians as that generated by the A root system. | 1 | Crystallography |
Several software graphic tools will let you create 2D patterns using wallpaper symmetry groups. Usually you can edit the original tile and its copies in the entire pattern are updated automatically.
* [http://www.madpattern.com/ MadPattern], a free set of Adobe Illustrator templates that support the 17 wallpaper groups
* [http://www.peda.com/tess/ Tess], a shareware tessellation program for multiple platforms, supports all wallpaper, frieze, and rosette groups, as well as Heesch tilings.
*[http://math.hws.edu/eck/js/symmetry/wallpaper.html Wallpaper Symmetry] is a free online JavaScript drawing tool supporting the 17 groups. The [http://math.hws.edu/eck/js/symmetry/symmetry-info.html main page] has an explanation of the wallpaper groups, as well as drawing tools and explanations for the other planar symmetry groups as well.
* [https://en.oiler.education/tales TALES GAME], a free software designed for educational purposes which includes the tessellation function.
* [http://www.scienceu.com/geometry/handson/kali/ Kali] , online graphical symmetry editor Java applet (not supported by default in browsers).
* [http://www.geometrygames.org/Kali/index.html Kali] , free downloadable Kali for Windows and Mac Classic.
* Inkscape, a free vector graphics editor, supports all 17 groups plus arbitrary scales, shifts, rotates, and color changes per row or per column, optionally randomized to a given degree. (See [http://tavmjong.free.fr/INKSCAPE/MANUAL/html/Tiles-Symmetries.html])
* [http://www.artlandia.com/products/SymmetryWorks/ SymmetryWorks] is a commercial plugin for Adobe Illustrator, supports all 17 groups.
* [https://eschersket.ch/ EscherSketch] is a free online JavaScript drawing tool supporting the 17 groups.
* [https://repper.app/ Repper] is a commercial online drawing tool supporting the 17 groups plus a number of non-periodic tilings | 1 | Crystallography |
In contrast with crystals, liquids have no long-range order (in particular, there is no regular lattice), so the structure factor does not exhibit sharp peaks. They do however show a certain degree of short-range order, depending on their density and on the strength of the interaction between particles. Liquids are isotropic, so that, after the averaging operation in Equation (), the structure factor only depends on the absolute magnitude of the scattering vector . For further evaluation, it is convenient to separate the diagonal terms in the double sum, whose phase is identically zero, and therefore each contribute a unit constant:
One can obtain an alternative expression for in terms of the radial distribution function : | 1 | Crystallography |
The eluent (mobile phase) should be the appropriate solvent to dissolve the polymer, should not interfere with the response of the polymer analyzed, and should wet the packing surface and make it inert to interactions with the polymers. The most common eluents for polymers that dissolve at room temperature GPC are tetrahydrofuran (THF), o-dichlorobenzene and trichlorobenzene at 130–150 °C for crystalline polyalkynes and hexafluoroisopropanol (HFIP) for crystalline condensation polymers such as polyamides and polyesters. | 0 | Chromatography + Titration + pH indicators |
A RHEED system requires an electron source (gun), photoluminescent detector screen and a sample with a clean surface, although modern RHEED systems have additional parts to optimize the technique. The electron gun generates a beam of electrons which strike the sample at a very small angle relative to the sample surface. Incident electrons diffract from atoms at the surface of the sample, and a small fraction of the diffracted electrons interfere constructively at specific angles and form regular patterns on the detector. The electrons interfere according to the position of atoms on the sample surface, so the diffraction pattern at the detector is a function of the sample surface. Figure 1 shows the most basic setup of a RHEED system. | 1 | Crystallography |
The ancient world lacked standardized forensic practices, which enabled criminals to escape punishment. Criminal investigations and trials relied heavily on forced confessions and witness testimony. However, ancient sources do contain several accounts of techniques that foreshadow concepts in forensic science developed centuries later.
The first written account of using medicine and entomology to solve criminal cases is attributed to the book of Xi Yuan Lu (translated as Washing Away of Wrongs), written in China in 1248 by Song Ci (, 1186–1249), a director of justice, jail and supervision, during the Song dynasty.
Song Ci introduced regulations concerning autopsy reports to court, how to protect the evidence in the examining process, and explained why forensic workers must demonstrate impartiality to the public. He devised methods for making antiseptic and for promoting the reappearance of hidden injuries to dead bodies and bones (using sunlight and vinegar under a red-oil umbrella); for calculating the time of death (allowing for weather and insect activity); described how to wash and examine the dead body to ascertain the reason for death. At that time the book had described methods for distinguishing between suicide and faked suicide. He wrote the book on forensics stating that all wounds or dead bodies should be examined, not avoided. The book became the first form of literature to help determine the cause of death.
In one of Song Cis accounts (Washing Away of Wrongs'), the case of a person murdered with a sickle was solved by an investigator who instructed each suspect to bring his sickle to one location. (He realized it was a sickle by testing various blades on an animal carcass and comparing the wounds.) Flies, attracted by the smell of blood, eventually gathered on a single sickle. In light of this, the owner of that sickle confessed to the murder. The book also described how to distinguish between a drowning (water in the lungs) and strangulation (broken neck cartilage), and described evidence from examining corpses to determine if a death was caused by murder, suicide or accident.
Methods from around the world involved saliva and examination of the mouth and tongue to determine innocence or guilt, as a precursor to the Polygraph test. In ancient India, some suspects were made to fill their mouths with dried rice and spit it back out. Similarly, in ancient China, those accused of a crime would have rice powder placed in their mouths. In ancient middle-eastern cultures, the accused were made to lick hot metal rods briefly. It is thought that these tests had some validity since a guilty person would produce less saliva and thus have a drier mouth; the accused would be considered guilty if rice was sticking to their mouths in abundance or if their tongues were severely burned due to lack of shielding from saliva. | 0 | Chromatography + Titration + pH indicators |
Nickel can be titrated thermometrically using di-sodium dimethylglyoximate as titrant. The chemistry is analogous to the classic gravimetric procedure, but the time taken for a determination can be reduced from many hours to a few minutes. Potential interferences need to be considered. | 0 | Chromatography + Titration + pH indicators |
Isometries of the Euclidean plane fall into four categories (see the article Euclidean plane isometry for more information).
*Translations, denoted by T, where v is a vector in R. This has the effect of shifting the plane applying displacement vector v.
*Rotations, denoted by R, where c is a point in the plane (the centre of rotation), and θ is the angle of rotation.
*Reflections, or mirror isometries, denoted by F, where L is a line in R. (F is for "flip"). This has the effect of reflecting the plane in the line L, called the reflection axis or the associated mirror.
*Glide reflections, denoted by G, where L is a line in R and d is a distance. This is a combination of a reflection in the line L and a translation along L by a distance d. | 1 | Crystallography |
Initial glance, forensic intelligence may appear as a nascent facet of forensic science facilitated by advancements in information technologies such as computers, databases, and data-flow management software. However, a more profound examination reveals that forensic intelligence represents a genuine and emerging inclination among forensic practitioners to actively participate in investigative and policing strategies. In doing so, it elucidates existing practices within scientific literature, advocating for a paradigm shift from the prevailing conception of forensic science as a conglomerate of disciplines merely aiding the criminal justice system. Instead, it urges a perspective that views forensic science as a discipline studying the informative potential of traces—remnants of criminal activity. Embracing this transformative shift poses a significant challenge for education, necessitating a shift in learners' mindset to accept concepts and methodologies in forensic intelligence.
Recent calls advocating for the integration of forensic scientists into the criminal justice system, as well as policing and intelligence missions, underscore the necessity for the establishment of educational and training initiatives in the field of forensic intelligence. This article contends that a discernible gap exists between the perceived and actual comprehension of forensic intelligence among law enforcement and forensic science managers, positing that this asymmetry can be rectified only through educational interventions
The primary challenge in forensic intelligence education and training is identified as the formulation of programs aimed at heightening awareness, particularly among managers, to mitigate the risk of making suboptimal decisions in information processing. The paper highlights two recent European courses as exemplars of educational endeavors, elucidating lessons learned and proposing future directions at an initial glance, forensic intelligence may appear as a nascent facet of forensic science facilitated by advancements in information technologies such as computers, databases, and data-flow management software.
However, a more profound examination reveals that forensic intelligence represents a genuine and emerging inclination among forensic practitioners to actively participate in investigative and policing strategies. In doing so, it elucidates existing practices within scientific literature, advocating for a paradigm shift from the prevailing conception of forensic science as a conglomerate of disciplines merely aiding the criminal justice system. Instead, it urges a perspective that views forensic science as a discipline studying the informative potential of traces—remnants of criminal activity. Embracing this transformative shift poses a significant challenge for education, necessitating a shift in learners' mindset to accept concepts and methodologies in forensic intelligence.
The overarching conclusion is that the heightened focus on forensic intelligence has the potential to rejuvenate a proactive approach to forensic science, enhance quantifiable efficiency, and foster greater involvement in investigative and managerial decision-making. A novel educational challenge is articulated for forensic science university programs worldwide: a shift in emphasis from a fragmented criminal trace analysis to a more comprehensive security problem-solving approach. | 0 | Chromatography + Titration + pH indicators |
When a crystalline material that contains atoms with uncompensated electron spins is cooled down, ordering of these spins generally occurs once the thermal energy is small enough not to overrule the interactions between neighboring spins. If the ordering does not exhibit the same symmetry as the original unit cell of the crystallographic lattice, a superstructure will result. In this case, the superspots are typically only visible in neutron diffraction patterns, because the neutron is scattered both by the nucleus and by the magnetic moments of the electron spins. | 1 | Crystallography |
The discreteness condition means that there is some positive real number ε, such that for every translation T in the group, the vector v has length at least ε (except of course in the case that v is the zero vector, but the independent translations condition prevents this, since any set that contains the zero vector is linearly dependent by definition and thus disallowed).
The purpose of this condition is to ensure that the group has a compact fundamental domain, or in other words, a "cell" of nonzero, finite area, which is repeated through the plane. Without this condition, one might have for example a group containing the translation T for every rational number x, which would not correspond to any reasonable wallpaper pattern.
One important and nontrivial consequence of the discreteness condition in combination with the independent translations condition is that the group can only contain rotations of order 2, 3, 4, or 6; that is, every rotation in the group must be a rotation by 180°, 120°, 90°, or 60°. This fact is known as the crystallographic restriction theorem, and can be generalised to higher-dimensional cases. | 1 | Crystallography |
In macromolecular crystallography, the term additive is used instead of adjutant. An additive can either interact directly with the protein, and become incorporated at a fixed position in the resulting crystal or have a role within the disordered solvent, that in protein crystals constitute roughly 50% of the lattice volume.
Polyethylene glycols of various molecular weights and high-ionic strength salts such as ammonium sulfate and sodium citrate that induce protein precipitation when used in high concentrations are classified as precipitants, while certain other salts such as zinc sulfate or calcium sulfate that may cause a protein to precipitate vigorously even when used in small amounts are considered adjutants. Crystallization adjutants are considered additives when they are effective at relatively low concentrations.
The distinction between buffers and adjutants is also fuzzy. Buffer molecules can become part of the lattice (for example HEPES in becomes incorporated in crystals of human neutrophil collagenase) but their main use is to maintain the rather precise pH requirements for crystallization that many proteins have. Commonly used buffers such as citrate have a high ionic strength and at the typical buffer concentrations they also act as precipitants. Various species such as Ca and Zn are a biological requirement for certain proteins to fold correctly and certain co-factors are needed to maintain a well defined conformation. Certain strategies, like replacing precipitants and buffers with others intended to have a similar effect, have been used to differentiate between the roles played in protein crystallization by the various components in the crystallization solution. | 1 | Crystallography |
Quinaldine red (pronounced , abbreviated QR) is a dark green–red or black solid that does not dissolve easily in water (it is partly miscible). In addition to being used as colored indicator, quinaldine red is also used as a fluorescence probe and an agent in bleaching. | 0 | Chromatography + Titration + pH indicators |
Ion exchange chromatography can be used to separate proteins because they contain charged functional groups. The ions of interest (in this case charged proteins) are exchanged for another ions (usually H) on a charged solid support. The solutes are most commonly in a liquid phase, which tends to be water. Take for example proteins in water, which would be a liquid phase that is passed through a column. The column is commonly known as the solid phase since it is filled with porous synthetic particles that are of a particular charge. These porous particles are also referred to as beads, may be aminated (containing amino groups) or have metal ions in order to have a charge. The column can be prepared using porous polymers, for macromolecules of a mass of over 100 000 Da, the optimum size of the porous particle is about 1 μm. This is because slow diffusion of the solutes within the pores does not restrict the separation quality. The beads containing positively charged groups, which attract the negatively charged proteins, are commonly referred to as anion exchange resins. The amino acids that have negatively charged side chains at pH 7 (pH of water) are glutamate and aspartate. The beads that are negatively charged are called cation exchange resins, as positively charged proteins will be attracted. The amino acids that have positively charged side chains at pH 7 are lysine, histidine and arginine.
The isoelectric point is the pH at which a compound - in this case a protein - has no net charge. A protein's isoelectric point or PI can be determined using the pKa of the side chains, if the amino (positive chain) is able to cancel out the carboxyl (negative) chain, the protein would be at its PI. Using buffers instead of water for proteins that do not have a charge at pH 7, is a good idea as it enables the manipulation of pH to alter ionic interactions between the proteins and the beads. Weakly acidic or basic side chains are able to have a charge if the pH is high or low enough respectively. Separation can be achieved based on the natural isoelectric point of the protein. Alternatively a peptide tag can be genetically added to the protein to give the protein an isoelectric point away from most natural proteins (e.g., 6 arginines for binding to a cation-exchange resin or 6 glutamates for binding to an anion-exchange resin such as DEAE-Sepharose).
Elution by increasing ionic strength of the mobile phase is more subtle. It works because ions from the mobile phase interact with the immobilized ions on the stationary phase, thus "shielding" the stationary phase from the protein, and letting the protein elute.
Elution from ion-exchange columns can be sensitive to changes of a single charge- chromatofocusing. Ion-exchange chromatography is also useful in the isolation of specific multimeric protein assemblies, allowing purification of specific complexes according to both the number and the position of charged peptide tags. | 0 | Chromatography + Titration + pH indicators |
In physics, the terms order and disorder designate the presence or absence of some symmetry or correlation in a many-particle system.
In condensed matter physics, systems typically are ordered at low temperatures; upon heating, they undergo one or several phase transitions into less ordered states.
Examples for such an order-disorder transition are:
* the melting of ice: solid–liquid transition, loss of crystalline order;
* the demagnetization of iron by heating above the Curie temperature: ferromagnetic–paramagnetic transition, loss of magnetic order.
The degree of freedom that is ordered or disordered can be translational (crystalline ordering), rotational (ferroelectric ordering), or a spin state (magnetic ordering).
The order can consist either in a full crystalline space group symmetry, or in a correlation. Depending on how the correlations decay with distance, one speaks of long range order or short range order.
If a disordered state is not in thermodynamic equilibrium, one speaks of quenched disorder. For instance, a glass is obtained by quenching (supercooling) a liquid. By extension, other quenched states are called spin glass, orientational glass. In some contexts, the opposite of quenched disorder is annealed disorder. | 1 | Crystallography |
Field-flow fractionation (FFF) can be considered as an alternative to GPC, especially when particles or high molar mass polymers cause clogging of the column, shear degradation is an issue or agglomeration takes place but cannot be made visible. FFF is separation in an open flow channel without having a static phase involved so no interactions occur. With one field-flow fractionation version, thermal field-flow fractionation, separation of polymers having the same size but different chemical compositions is possible. | 0 | Chromatography + Titration + pH indicators |
The intense color from which the compound gets its name results from irradiation and subsequent excitation and relaxation of the extended π electron system across the R-N=N-R linked phenols. Absorption of these electrons falls in the visible region of the electromagnetic spectrum. Azo violets intense indigo color (λ 432 nm) approximates Pantone R: 102 G: 15 B: 240. | 0 | Chromatography + Titration + pH indicators |
The process of crystal identification involves comparing them to images of crystals in different solvents found in published sources. Although the shape of the crystals depends on the solvent and, to a certain degree, the substance concentration, it is usually possible to recognize the fundamental crystalline forms. Care should be taken to differentiate between undissolved substances, which might be crystalline but lack a characteristic shape, and recrystallized substances. Microcrystal samples cannot be preserved for long, as they start to degrade within hours or days.
Distinguishing between gyrophoric acid and lecanoric acid using thin-layer chromatography can be challenging. However, if one of these substances is known to be present, a microcrystal test can help differentiate them. In the GAW solvent system, lecanoric acid forms long, curved crystal clusters, although the results can be inconsistent, especially in the presence of other substances. Gyrophoric acid, when present in the GE solvent system, may manifest as small, fine crystal clusters or rounded aggregations of tiny crystals. Lecanoric acid in the GE solvent system produces needle-like crystal clusters, but these are not as well-formed as in GAW. These tests can help distinguish Punctelia borreri (which contains gyrophoric acid) from Punctelia subrudecta (which contains lecanoric acid).
When two substances generate similar-looking crystals, their optical properties can be used to differentiate between them. Certain crystals alter the polarization plane of transmitted light, and when rotated between crossed polarizers, they alternate between bright and dark every 90°. The extinction angle is the angle between a specific crystal axis and the filter's polarization plane when the crystal appears dark (in extinction). For instance, this method can be employed to distinguish between perlatolic acid and imbricaric acid, which both form long, straight crystals in the GE solvent system but exhibit extinction angles of 0° and 45°, respectively, in relation to their long axis. | 1 | Crystallography |
pHydrion is the trademarked name for a popular line of chemical test products, marketed by Micro Essential Laboratory, Inc., the original manufacturer of Hydrion and pHydrion products. The trademarked pHydrion product line comprises chemical test papers, chemical indicators, chemical test kits, chemical indicator kits, pH indicator pencils, chemical buffers, buffer salts, buffer preservatives, dispensers, color charts, and testing products, for use in testing, detecting, identifying, measuring, and indicating levels of pH, of sanitizers, and of other substances. | 0 | Chromatography + Titration + pH indicators |
Forensic science, also known as criminalistics, is the application of science principles and methods to support legal decision-making in matters of criminal and civil law.
During criminal investigation in particular, it is governed by the legal standards of admissible evidence and criminal procedure. It is a broad field utilizing numerous practices such as the analysis of DNA, fingerprints, bloodstain patterns, firearms, ballistics, toxicology, and fire debris analysis.
Forensic scientists collect, preserve, and analyze evidence during the course of an investigation. While some forensic scientists travel to the scene of the crime to collect the evidence themselves, others occupy a laboratory role, performing analysis on objects brought to them by other individuals. Others are involved in analysis of financial, banking, or other numerical data for use in financial crime investigation, and can be employed as consultants from private firms, academia, or as government employees.
In addition to their laboratory role, forensic scientists testify as expert witnesses in both criminal and civil cases and can work for either the prosecution or the defense. While any field could technically be forensic, certain sections have developed over time to encompass the majority of forensically related cases. | 0 | Chromatography + Titration + pH indicators |
The set of all lines through the origin in three-dimensional space forms a space called the real projective plane. This plane is difficult to visualize, because it cannot be embedded in three-dimensional space.
However, one can visualize it as a disk, as follows. Any line through the origin intersects the southern hemisphere ≤ 0 in a point, which can then be stereographically projected to a point on a disk in the XY plane. Horizontal lines through the origin intersect the southern hemisphere in two antipodal points along the equator, which project to the boundary of the disk. Either of the two projected points can be considered part of the disk; it is understood that antipodal points on the equator represent a single line in 3 space and a single point on the boundary of the projected disk (see quotient topology). So any set of lines through the origin can be pictured as a set of points in the projected disk. But the boundary points behave differently from the boundary points of an ordinary 2-dimensional disk, in that any one of them is simultaneously close to interior points on opposite sides of the disk (just as two nearly horizontal lines through the origin can project to points on opposite sides of the disk).
Also, every plane through the origin intersects the unit sphere in a great circle, called the trace of the plane. This circle maps to a circle under stereographic projection. So the projection lets us visualize planes as circular arcs in the disk. Prior to the availability of computers, stereographic projections with great circles often involved drawing large-radius arcs that required use of a beam compass. Computers now make this task much easier.
Further associated with each plane is a unique line, called the planes pole', that passes through the origin and is perpendicular to the plane. This line can be plotted as a point on the disk just as any line through the origin can. So the stereographic projection also lets us visualize planes as points in the disk. For plots involving many planes, plotting their poles produces a less-cluttered picture than plotting their traces.
This construction is used to visualize directional data in crystallography and geology, as described below. | 1 | Crystallography |
Another common method of converting between fractional and Cartesian coordinates involves the use of a cell tensor which contains each of the basis vectors of the space expressed in Cartesian coordinates. | 1 | Crystallography |
Since refinement depends on finding the best fit between a calculated and experimental pattern, it is important to have a numerical figure of merit quantifying the quality of the fit. Below are the figures of merit generally used to characterize the quality of a refinement. They provide insight to how well the model fits the observed data.
Profile residual (reliability factor):
Weighted profile residual:
Bragg residual:
Expected profile residual:
Goodness of fit:
It is worth mentioning that all but one () figure of merit include a contribution from the background. There are some concerns about the reliability of these figures, as well there is no threshold or accepted value which dictates what represents a good fit. The most popular and conventional figure of merit used is the goodness of fit which should approach unity given a perfect fit, though this is rarely the case. In practice, the best way to assess quality is a visual analysis of the fit by plotting the difference between the observed and calculated data plotted on the same scale. | 1 | Crystallography |
Polymorphism in binary metal oxides has attracted much attention because these materials are of significant economic value. One set of famous examples have the composition SiO, which form many polymorphs. Important ones include: α-quartz, β-quartz, tridymite, cristobalite, moganite, coesite, and stishovite. | 1 | Crystallography |
Sir William Herschel was one of the first to advocate the use of fingerprinting in the identification of criminal suspects. While working for the Indian Civil Service, he began to use thumbprints on documents as a security measure to prevent the then-rampant repudiation of signatures in 1858.
In 1877 at Hooghly (near Kolkata), Herschel instituted the use of fingerprints on contracts and deeds, and he registered government pensioners fingerprints to prevent the collection of money by relatives after a pensioners death.
In 1880, Henry Faulds, a Scottish surgeon in a Tokyo hospital, published his first paper on the subject in the scientific journal Nature, discussing the usefulness of fingerprints for identification and proposing a method to record them with printing ink. He established their first classification and was also the first to identify fingerprints left on a vial. Returning to the UK in 1886, he offered the concept to the Metropolitan Police in London, but it was dismissed at that time.
Faulds wrote to Charles Darwin with a description of his method, but, too old and ill to work on it, Darwin gave the information to his cousin, Francis Galton, who was interested in anthropology. Having been thus inspired to study fingerprints for ten years, Galton published a detailed statistical model of fingerprint analysis and identification and encouraged its use in forensic science in his book Finger Prints. He had calculated that the chance of a "false positive" (two different individuals having the same fingerprints) was about 1 in 64 billion.
Juan Vucetich, an Argentine chief police officer, created the first method of recording the fingerprints of individuals on file. In 1892, after studying Galtons pattern types, Vucetich set up the worlds first fingerprint bureau. In that same year, Francisca Rojas of Necochea was found in a house with neck injuries whilst her two sons were found dead with their throats cut. Rojas accused a neighbour, but despite brutal interrogation, this neighbour would not confess to the crimes. Inspector Alvarez, a colleague of Vucetich, went to the scene and found a bloody thumb mark on a door. When it was compared with Rojas' prints, it was found to be identical with her right thumb. She then confessed to the murder of her sons.
A Fingerprint Bureau was established in Calcutta (Kolkata), India, in 1897, after the Council of the Governor General approved a committee report that fingerprints should be used for the classification of criminal records. Working in the Calcutta Anthropometric Bureau, before it became the Fingerprint Bureau, were Azizul Haque and Hem Chandra Bose. Haque and Bose were Indian fingerprint experts who have been credited with the primary development of a fingerprint classification system eventually named after their supervisor, Sir Edward Richard Henry. The Henry Classification System, co-devised by Haque and Bose, was accepted in England and Wales when the first United Kingdom Fingerprint Bureau was founded in Scotland Yard, the Metropolitan Police headquarters, London, in 1901. Sir Edward Richard Henry subsequently achieved improvements in dactyloscopy.
In the United States, Henry P. DeForrest used fingerprinting in the New York Civil Service in 1902, and by December 1905, New York City Police Department Deputy Commissioner Joseph A. Faurot, an expert in the Bertillon system and a fingerprint advocate at Police Headquarters, introduced the fingerprinting of criminals to the United States. | 0 | Chromatography + Titration + pH indicators |
Perovskite structures are adopted by many oxides that have the chemical formula ABO. The idealized form is a cubic structure (space group Pmm, no. 221) which is rarely encountered. The orthorhombic (e.g. space group Pnma, no. 62, or Amm2, no. 68) and tetragonal (e.g. space group I4/mcm, no. 140, or P4mm, no. 99) phases are the most common non-cubic variants. Although the perovskite structure is named after CaTiO, this mineral forms a non-idealized form. SrTiO and CaRbF are examples of cubic perovskites. Barium titanate is an example of a perovskite which can take on the rhombohedral (space group R3m, no. 160), orthorhombic, tetragonal and cubic forms depending on temperature.
In the idealized cubic unit cell of such a compound, the type A atom sits at cube corner position (0, 0, 0), the type B atom sits at the body-center position (1/2, 1/2, 1/2) and oxygen atoms sit at face centered positions (1/2, 1/2, 0), (1/2, 0, 1/2) and (0, 1/2, 1/2). The diagram to the right shows edges for an equivalent unit cell with A in the cube corner position, B at the body center, and O at face-centered positions.
Four general categories of cation-pairing are possible: ABX, or 1:2 perovskites; ABX, or 2:4 perovskites; ABX, or 3:3 perovskites; and ABX, or 1:5 perovskites.
The relative ion size requirements for stability of the cubic structure are quite stringent, so slight buckling and distortion can produce several lower-symmetry distorted versions, in which the coordination numbers of A cations, B cations or both are reduced. Tilting of the BO octahedra reduces the coordination of an undersized A cation from 12 to as low as 8. Conversely, off-centering of an undersized B cation within its octahedron allows it to attain a stable bonding pattern. The resulting electric dipole is responsible for the property of ferroelectricity and shown by perovskites such as BaTiO that distort in this fashion.
Complex perovskite structures contain two different B-site cations. This results in the possibility of ordered and disordered variants. | 1 | Crystallography |
Kagome () is a traditional Japanese woven bamboo pattern; its name is composed from the words kago, meaning "basket", and me, meaning "eye(s)", referring to the pattern of holes in a woven basket.
The kagome pattern is common in bamboo weaving in East Asia. In 2022, archaeologists found bamboo weaving remains at the Dongsunba ruins in Chongqing, China, 200 BC. After 2200 years, the kagome pattern is still clear.
It is a woven arrangement of laths composed of interlaced triangles such that each point where two laths cross has four neighboring points, forming the pattern of a trihexagonal tiling. The woven process gives the Kagome a chiral wallpaper group symmetry, p6 (632). | 1 | Crystallography |
Aurin (C.I. 43800), sometimes named rosolic acid or corallin is an organic compound, forming yellowish or deep-red crystals with greenish metallic luster. It is practically insoluble in water, freely soluble in alcohol. It is soluble in strong acids to form yellow solution, or in aqueous alkalis to form carmine red solutions. Due to this behaviour it can be used as pH indicator with pH transition range 5.0 - 6.8. It is used as an intermediate in manufacturing of dyes. | 0 | Chromatography + Titration + pH indicators |
A protocrystalline phase is a distinct phase occurring during crystal growth, which evolves into a microcrystalline form. The term is typically associated with silicon films in optical applications such as solar cells. | 1 | Crystallography |
Crystal structure is described in terms of the geometry of arrangement of particles in the unit cells. The unit cell is defined as the smallest repeating unit having the full symmetry of the crystal structure. The geometry of the unit cell is defined as a parallelepiped, providing six lattice parameters taken as the lengths of the cell edges (a, b, c) and the angles between them (α, β, γ). The positions of particles inside the unit cell are described by the fractional coordinates (x, y, z) along the cell edges, measured from a reference point. It is thus only necessary to report the coordinates of a smallest asymmetric subset of particles, called the crystallographic asymmetric unit. The asymmetric unit may be chosen so that it occupies the smallest physical space, which means that not all particles need to be physically located inside the boundaries given by the lattice parameters. All other particles of the unit cell are generated by the symmetry operations that characterize the symmetry of the unit cell. The collection of symmetry operations of the unit cell is expressed formally as the space group of the crystal structure. | 1 | Crystallography |
Because of her interest in nutrition and nutrients, Coward was one of the early adopters of chromatography following its introduction in 1906-1911 by M. S. Tswett.
Carotenoids, a class of structurally similar pigment molecules that include carotenes and xanthophylls, were of particular interest in nutritional research due to their demonstrated importance in animal studies. In his pioneering chromatographic research, Tswett showed the presence of four different xanthophylls in his studies of plant extracts, separated through the use of adsorption chromatography. Following L. S. Palmers descriptions of Tswetts experiment in 1922, Coward replicated the methodology, the results of which she published in 1923. During these studies Coward noted the presence of additional pigment (which would later be determined to be carotenes) in the eluent fractions, nearly developing a chromatographic method for the isolation of vitamin A from the carotenoids. This experiment made her the fifth scientist to adopt the use of chromatography, during a "dormant" period before the techniques popularization in the 1930s.
This early research applying adsorption chromatography would continue in her role at the Royal Pharmaceutical Society, in conjunction with other analytical methods. | 0 | Chromatography + Titration + pH indicators |
Yellow H-A or Reactive Yellow 3 has a formula of CHClNOS and a molecular weight of 593 g/mol, containing a monochlorotriazine ring. On agarose as supporting matrix, it was seen to purify cholesteryl ester transfer protein. | 0 | Chromatography + Titration + pH indicators |
There is compelling evidence that components within some UCMs are toxic to marine organisms. The clearance rate (also known as feeding feed) of mussels was reduced by 40% following exposure to a monoaromatic UCM derived from a Norwegian crude oil. The toxicity of monoaromatic UCM components was further evidenced by an elegant set of experiments using transplantations of clean and polluted mussels. Recent analysis by GC×GC-ToF-MS of UCMs extracted from the mussel tissues, has shown that they contain a vast array of both known and unknown compounds. The comparative analysis of UCMs extracted from mussels known to possess high, moderate and low Scope for Growth (SfG), a measure of the capacity for growth and reproduction, revealed that branched alkylbenzenes represented the largest structural class within the UCM of mussels with low SfG; also, branched isomers of alkyltetralins, alkylindanes and alkylindenes were prominent in the stressed mussels. Laboratory toxicity tests using both commercially available and specially synthesised compounds revealed that such branched alkylated structures were capable of producing the observed poor health of the mussels. The reversible effects observed in mussels following exposure to the UCM hydrocarbons identified to date are consistent with non-specific narcosis (also known as baseline) mode of action of toxicity. There is no evidence that toxic UCM components can biomagnify through the food chain. Crabs (Carcinus maenas) that were fed a diet of mussels contaminated with environmentally realistic concentrations of branched alkylbenzenes, suffered behavioural disruption but only a small concentration of the compounds were retained in the midgut of the crabs. Within marsh sediments still contaminated with high concentrations of UCM hydrocarbons from the Florida barge oil spill in 1969 (see above,) the behaviour and feeding of fiddler crabs (Uca pugnax) was reported to be affected. | 0 | Chromatography + Titration + pH indicators |
Factors influencing habit include: a combination of two or more crystal forms; trace impurities present during growth; crystal twinning and growth conditions (i.e., heat, pressure, space); and specific growth tendencies such as growth striations. Minerals belonging to the same crystal system do not necessarily exhibit the same habit. Some habits of a mineral are unique to its variety and locality: For example, while most sapphires form elongate barrel-shaped crystals, those found in Montana form stout tabular crystals. Ordinarily, the latter habit is seen only in ruby. Sapphire and ruby are both varieties of the same mineral: corundum.
Some minerals may replace other existing minerals while preserving the originals habit, i.e. pseudomorphous replacement. A classic example is tigers eye quartz, crocidolite asbestos replaced by silica. While quartz typically forms prismatic (elongate, prism-like) crystals, in tigers eye the original fibrous' habit of crocidolite is preserved. | 1 | Crystallography |
Sample thickness can be estimated using twoBeamGUI from a convergent beam electron diffraction pattern (CBED) in two beam approximation. The procedure is based on an automated extraction of the intensity profile across the diffracted disk in the following steps:
# diffraction disk radius is determined using multi-scale Hough transform,
# the transmitted and diffracted disks are localized and the reflection is indexed,
# the disks are horizontally aligned, cropped out and profiles are measured across the disks,
# the profile across the diffracted disk is matched with a series of profiles automatically simulated for given material, reflection and specified thickness range.
Once the procedure is completed, the measured profile and the most similar simulated profile are displayed with the diffracted disk on the background. This allows the user to verify correctness of the automated estimate and easily check the similarity of other intensity profiles within the specified thickness range. | 1 | Crystallography |
Countercurrent chromatography (CCC, also counter-current chromatography) is a form of liquid–liquid chromatography that uses a liquid stationary phase that is held in place by inertia of the molecules composing the stationary phase accelerating toward the center of a centrifuge due to centripetal force and is used to separate, identify, and quantify the chemical components of a mixture. In its broadest sense, countercurrent chromatography encompasses a collection of related liquid chromatography techniques that employ two immiscible liquid phases without a solid support. The two liquid phases come in contact with each other as at least one phase is pumped through a column, a hollow tube or a series of chambers connected with channels, which contains both phases. The resulting dynamic mixing and settling action allows the components to be separated by their respective solubilities in the two phases. A wide variety of two-phase solvent systems consisting of at least two immiscible liquids may be employed to provide the proper selectivity for the desired separation.
Some types of countercurrent chromatography, such as dual flow CCC, feature a true countercurrent process where the two immiscible phases flow past each other and exit at opposite ends of the column. More often, however, one liquid acts as the stationary phase and is retained in the column while the mobile phase is pumped through it. The liquid stationary phase is held in place by gravity or inertia of the molecules composing the stationary phase accelerating toward the center of a centrifuge due to centripetal force. An example of a gravity method is called droplet counter current chromatography (DCCC). There are two modes by which the stationary phase is retained by centripetal force: hydrostatic and hydrodynamic. In the hydrostatic method, the column is rotated about a central axis. Hydrostatic instruments are marketed under the name centrifugal partition chromatography (CPC). Hydrodynamic instruments are often marketed as high-speed or high-performance countercurrent chromatography (HSCCC and HPCCC respectively) instruments which rely on the Archimedes' screw force in a helical coil to retain the stationary phase in the column.
The components of a CCC system are similar to most liquid chromatography configurations, such as high-performance liquid chromatography (HPLC). One or more pumps deliver the phases to the column which is the CCC instrument itself. Samples are introduced into the column through a sample loop filled with an automated or manual syringe. The outflow is monitored with various detectors such as ultraviolet–visible spectroscopy or mass spectrometry. The operation of the pumps, CCC instrument, sample injection, and detection may be controlled manually or with a microprocessor. | 0 | Chromatography + Titration + pH indicators |
Stereographic projection is also applied to the visualization of polytopes. In a Schlegel diagram, an -dimensional polytope in is projected onto an -dimensional sphere, which is then stereographically projected onto . The reduction from to can make the polytope easier to visualize and understand. | 1 | Crystallography |
The titration process creates solutions with compositions ranging from pure acid to pure base. Identifying the pH associated with any stage in the titration process is relatively simple for monoprotic acids and bases. The presence of more than one acid or base group complicates these computations. Graphical methods, such as the equiligraph, have long been used to account for the interaction of coupled equilibria. | 0 | Chromatography + Titration + pH indicators |
In crystallography, the basis and lattice are treated separately. For a perfect crystal the lattice gives the reciprocal lattice, which determines the positions (angles) of diffracted beams, and the basis gives the structure factor which determines the amplitude and phase of the diffracted beams:
where the sum is over all atoms in the unit cell, are the positional coordinates of the -th atom, and is the scattering factor of the -th atom. The coordinates have the directions and dimensions of the lattice vectors . That is, (0,0,0) is at the lattice point, the origin of position in the unit cell; (1,0,0) is at the next lattice point along and (1/2, 1/2, 1/2) is at the body center of the unit cell. defines a reciprocal lattice point at which corresponds to the real-space plane defined by the Miller indices (see Bragg's law).
is the vector sum of waves from all atoms within the unit cell. An atom at any lattice point has the reference phase angle zero for all since then is always an integer. A wave scattered from an atom at (1/2, 0, 0) will be in phase if is even, out of phase if is odd.
Again an alternative view using convolution can be helpful. Since [crystal structure] = [lattice] [basis], [crystal structure] = [lattice] [basis]; that is, scattering [reciprocal lattice] [structure factor]. | 1 | Crystallography |
An effective sample preparation protocol, usually involving either liquid-liquid extraction (LLE) or solid phase extraction (SPE) and frequently derivatisation can remove ion suppressing species from the sample matrix prior to analysis. These common approaches may also remove other interferences, such as isobaric species.
Protein precipitation is another method that can be employed for small molecule analysis. Removal of all protein species from the sample matrix may be effective in some cases, although for many analytes, ion suppressing species are not of protein origin and so this technique is often used in conjunction with extraction and derivatisation. | 0 | Chromatography + Titration + pH indicators |
Dialysis is useful for many of the same desalting and buffer exchange applications performed with gel filtration chromatography, as both methods are based on similar molecular weight cut-off limits. Gel filtration has the advantage of speed (a few minutes vs. hours for dialysis) along with the ability to remove contaminants from relatively small-volume samples compared to dialysis which is an important feature when working with toxic or radioactive substances. Dialysis, on the other hand, is much less dependent on sample size as related to device format. For dialysis applications, achieving a high percentage sample recovery and molecule removal is generally straight forward with little optimization. For gel filtration applications it is important to select a column size and format that is suitable for your sample. | 0 | Chromatography + Titration + pH indicators |
Ion chromatography has advanced through the accumulation of knowledge over a course of many years. Starting from 1947, Spedding and Powell used displacement ion-exchange chromatography for the separation of the rare earths. Additionally, they showed the ion-exchange separation of 14N and 15N isotopes in ammonia. At the start of the 1950s, Kraus and Nelson demonstrated the use of many analytical methods for metal ions dependent on their separation of their chloride, fluoride, nitrate or sulfate complexes by anion chromatography. Automatic in-line detection was progressively introduced from 1960 to 1980 as well as novel chromatographic methods for metal ion separations. A groundbreaking method by Small, Stevens and Bauman at Dow Chemical Co. unfolded the creation of the modern ion chromatography. Anions and cations could now be separated efficiently by a system of suppressed conductivity detection. In 1979, a method for anion chromatography with non-suppressed conductivity detection was introduced by Gjerde et al. Following it in 1980, was a similar method for cation chromatography.
As a result, a period of extreme competition began within the IC market, with supporters for both suppressed and non-suppressed conductivity detection. This competition led to fast growth of new forms and the fast evolution of IC. A challenge that needs to be overcome in the future development of IC is the preparation of highly efficient monolithic ion-exchange columns and overcoming this challenge would be of great importance to the development of IC.
The boom of Ion exchange chromatography primarily began between 1935 and 1950 during World War II and it was through the "Manhattan project" that applications and IC were significantly extended. Ion chromatography was originally introduced by two English researchers, agricultural Sir Thompson and chemist J T Way. The works of Thompson and Way involved the action of water-soluble fertilizer salts, ammonium sulfate and potassium chloride. These salts could not easily be extracted from the ground due to the rain. They performed ion methods to treat clays with the salts, resulting in the extraction of ammonia in addition to the release of calcium. It was in the fifties and sixties that theoretical models were developed for IC for further understanding and it was not until the seventies that continuous detectors were utilized, paving the path for the development from low-pressure to high-performance chromatography. Not until 1975 was "ion chromatography" established as a name in reference to the techniques, and was thereafter used as a name for marketing purposes. Today IC is important for investigating aqueous systems, such as drinking water. It is a popular method for analyzing anionic elements or complexes that help solve environmentally relevant problems. Likewise, it also has great uses in the semiconductor industry.
Because of the abundant separating columns, elution systems, and detectors available, chromatography has developed into the main method for ion analysis.
When this technique was initially developed, it was primarily used for water treatment. Since 1935, ion exchange chromatography rapidly manifested into one of the most heavily leveraged techniques, with its principles often being applied to majority of fields of chemistry, including distillation, adsorption, and filtration. | 0 | Chromatography + Titration + pH indicators |
A positive test is represented by the development of red color in 15 minutes or more after addition of the reagents, indicating the presence of diacetyl, the oxidation product of acetoin. The test should be red, after standing for 1 hour because negative VP cultures may produce copper-like colour potentially resulting in a false positive interpretation, also because due to action of the reagents when mixed. | 0 | Chromatography + Titration + pH indicators |
The chemical components of orcein were elucidated only in the 1950s by Hans Musso. The structures are shown below. A paper originally published in 1961, embodying most of Musso's work on components of orcein and litmus, was translated into English and published in 2003 in a special issue of the journal Biotechnic & Histochemistry (Vol 78, No. 6) devoted to the dye. A single alternative structural formula for orcein, possibly incorrect, is given by the National Library of Medicine and Emolecules.
Orcein is a reddish-brown dye, orchil is a purple-blue dye. Orcein is also used as a stain in microscopy to visualize chromosomes, elastic fibers, Hepatitis B surface antigens, and copper-associated proteins.
Orcein is not approved as a food dye (banned in Europe since January 1977), with E number E121 before 1977 and E182 after. Its CAS number is . Its chemical formula is CHNO. It forms dark brown crystals. It is a mixture of phenoxazone derivates - hydroxyorceins, aminoorceins, and aminoorceinimines. | 0 | Chromatography + Titration + pH indicators |
In crystallography, a Strukturbericht designation or Strukturbericht type is a system of detailed crystal structure classification by analogy to another known structure. The designations were intended to be comprehensive but are mainly used as supplement to space group crystal structures designations, especially historically. Each Strukturbericht designation is described by a single space group, but the designation includes additional information about the positions of the individual atoms, rather than just the symmetry of the crystal structure. While Strukturbericht symbols exist for many of the earliest observed and most common crystal structures, the system is not comprehensive, and is no longer being updated. Modern databases such as Inorganic Crystal Structure Database index thousands of structure types directly by the prototype compound (i.e. "the NaCl structure" instead of "the B1 structure"). These are essentially equivalent to the old Stukturbericht designations. | 1 | Crystallography |
Azo violet (Magneson I; p-nitrobenzeneazoresorcinol) is an azo compound with the chemical formula CHNO. It is used commercially as a violet dye and experimentally as a pH indicator, appearing yellow below pH 11, and violet above pH 13. It also turns deep blue in the presence of magnesium salt in a slightly alkaline, or basic, environment. Azo violet may also be used to test for the presence of ammonium ions. The color of ammonium chloride or ammonium hydroxide solution will vary depending upon the concentration of azo violet used. Magneson I is used to test Be also; it produces an orange-red lake with Be(II) in alkaline medium. | 0 | Chromatography + Titration + pH indicators |
Phase transitions (phase changes) that help describe polymorphism include polymorphic transitions as well as melting and vaporization transitions. According to IUPAC, a polymorphic transition is "A reversible transition of a solid crystalline phase at a certain temperature and pressure (the inversion point) to another phase of the same chemical composition with a different crystal structure." Additionally, Walter McCrone described the phases in polymorphic matter as "different in crystal structure but identical in the liquid or vapor states." McCrone also defines a polymorph as “a crystalline phase of a given compound resulting from the possibility of at least two different arrangements of the molecules of that compound in the solid state.” These defining facts imply that polymorphism involves changes in physical properties but cannot include chemical change. Some early definitions do not make this distinction.
Eliminating chemical change from those changes permissible during a polymorphic transition delineates polymorphism. For example, isomerization can often lead to polymorphic transitions. However, tautomerism (dynamic isomerization) leads to chemical change, not polymorphism. As well, allotropy of elements and polymorphism have been linked historically. However, allotropes of an element are not always polymorphs. A common example is the allotropes of carbon, which include graphite, diamond, and londsdaleite. While all three forms are allotropes, graphite is not a polymorph of diamond and londsdaleite. The reason is that graphite is chemically distinct, having sp hybridized bonding, while diamond, and londsdaleite are chemically identical, both having sp hybridized bonding. Diamond and londsdaleite differ in their crystal structures but do not differ chemically. Isomerization and allotropy are only two of the phenomena linked to polymorphism. For additional information about identifying polymorphism and distinguishing it from other phenomena, see the review by Brog et al.
Polymorphism is of practical relevance to pharmaceuticals, agrochemicals, pigments, dyestuffs, foods, and explosives. | 1 | Crystallography |
The first representations of perfect quasicrystalline patterns can be found in several early Islamic works of art and architecture such as the Gunbad-i-Kabud tomb tower, the Darb-e Imam shrine and the Al-Attarine Madrasa. On July 16, 1945, in Alamogordo, New Mexico, the Trinity nuclear bomb test produced icosahedral quasicrystals. They went unnoticed at the time of the test but were later identified in samples of red Trinitite, a glass-like substance formed from fused sand and copper transmission lines. Identified in 2021, they are the oldest known anthropogenic quasicrystals.
In 1961, Hao Wang asked whether determining if a set of tiles admits a tiling of the plane is an algorithmically unsolvable problem or not. He conjectured that it is solvable, relying on the hypothesis that every set of tiles that can tile the plane can do it periodically (hence, it would suffice to try to tile bigger and bigger patterns until obtaining one that tiles periodically). Nevertheless, two years later, his student Robert Berger constructed a set of some 20,000 square tiles (now called "Wang tiles") that can tile the plane but not in a periodic fashion. As further aperiodic sets of tiles were discovered, sets with fewer and fewer shapes were found. In 1974 Roger Penrose discovered a set of just two tiles, now referred to as Penrose tiles, that produced only non-periodic tilings of the plane. These tilings displayed instances of fivefold symmetry. One year later Alan Mackay showed theoretically that the diffraction pattern from the Penrose tiling had a two-dimensional Fourier transform consisting of sharp delta peaks arranged in a fivefold symmetric pattern. Around the same time, Robert Ammann created a set of aperiodic tiles that produced eightfold symmetry.
In 1972, de Wolf and van Aalst reported that the diffraction pattern produced by a crystal of sodium carbonate cannot be labeled with three indices but needed one more, which implied that the underlying structure had four dimensions in reciprocal space. Other puzzling cases have been reported, but until the concept of quasicrystal came to be established, they were explained away or denied.
Shechtman first observed ten-fold electron diffraction patterns in 1982, while conducting a routine study of an aluminium–manganese alloy, AlMn, at the US National Bureau of Standards (later NIST). Shechtman related his observation to Ilan Blech, who responded that such diffractions had been seen before. Around that time, Shechtman also related his finding to John W. Cahn of the NIST, who did not offer any explanation and challenged him to solve the observation. Shechtman quoted Cahn as saying: "Danny, this material is telling us something, and I challenge you to find out what it is".
The observation of the ten-fold diffraction pattern lay unexplained for two years until the spring of 1984, when Blech asked Shechtman to show him his results again. A quick study of Shechtman's results showed that the common explanation for a ten-fold symmetrical diffraction pattern, a type of crystal twinning, was ruled out by his experiments. Therefore, Blech looked for a new structure containing cells connected to each other by defined angles and distances but without translational periodicity. He decided to use a computer simulation to calculate the diffraction intensity from a cluster of such a material, which he termed as "multiple polyhedral", and found a ten-fold structure similar to what was observed. The multiple polyhedral structure was termed later by many researchers as icosahedral glass.
Shechtman accepted Blechs discovery of a new type of material and chose to publish his observation in a paper entitled "The Microstructure of Rapidly Solidified AlMn", which was written around June 1984 and published in a 1985 edition of Metallurgical Transactions A. Meanwhile, on seeing the draft of the paper, John Cahn suggested that Shechtmans experimental results merit a fast publication in a more appropriate scientific journal. Shechtman agreed and, in hindsight, called this fast publication "a winning move”. This paper, published in the Physical Review Letters, repeated Shechtman's observation and used the same illustrations as the original paper.
Originally, the new form of matter was dubbed "Shechtmanite". The term "quasicrystal" was first used in print by Steinhardt and Levine shortly after Shechtman's paper was published.
Also in 1985, Ishimasa et al. reported twelvefold symmetry in Ni-Cr particles. Soon, eightfold diffraction patterns were recorded in V-Ni-Si and Cr-Ni-Si alloys. Over the years, hundreds of quasicrystals with various compositions and different symmetries have been discovered. The first quasicrystalline materials were thermodynamically unstable—when heated, they formed regular crystals. However, in 1987, the first of many stable quasicrystals were discovered, making it possible to produce large samples for study and applications.
In 1992, the International Union of Crystallography altered its definition of a crystal, reducing it to the ability to produce a clear-cut diffraction pattern and acknowledging the possibility of the ordering to be either periodic or aperiodic.
In 2001, Paul Steinhardt of Princeton University hypothesized that quasicrystals could exist in nature and developed a method of recognition, inviting all the mineralogical collections of the world to identify any badly cataloged crystals. In 2007 Steinhardt received a reply by Luca Bindi, who found a quasicrystalline specimen from Khatyrka in the University of Florence Mineralogical Collection. The crystal samples were sent to Princeton University for other tests, and in late 2009, Steinhardt confirmed its quasicrystalline character. This quasicrystal, with a composition of AlCuFe, was named icosahedrite and it was approved by the International Mineralogical Association in 2010. Analysis indicates it may be meteoritic in origin, possibly delivered from a carbonaceous chondrite asteroid. In 2011, Bindi, Steinhardt, and a team of specialists found more icosahedrite samples from Khatyrka. A further study of Khatyrka meteorites revealed micron-sized grains of another natural quasicrystal, which has a ten-fold symmetry and a chemical formula of AlNiFe. This quasicrystal is stable in a narrow temperature range, from 1120 to 1200 K at ambient pressure, which suggests that natural quasicrystals are formed by rapid quenching of a meteorite heated during an impact-induced shock.
Shechtman was awarded the Nobel Prize in Chemistry in 2011 for his work on quasicrystals. "His discovery of quasicrystals revealed a new principle for packing of atoms and molecules," stated the Nobel Committee and pointed that "this led to a paradigm shift within chemistry." In 2014, Post of Israel issued a stamp dedicated to quasicrystals and the 2011 Nobel Prize.
While the first quasicrystals discovered were made out of intermetallic components, later on quasicrystals were also discovered in soft-matter and molecular systems. Soft quasicrystal structures have been found in supramolecular dendrimer liquids and ABC Star Polymers in 2004 and 2007. In 2009, it was found that thin-film quasicrystals can be formed by self-assembly of uniformly shaped, nano-sized molecular units at an air-liquid interface. It was demonstrated that these units can be both inorganic and organic. Additionally in the 2010s, two-dimensional molecular quasicrystals were discovered, driven by intermolecular interactions and interface-interactions.
In 2018, chemists from Brown University announced the successful creation of a self-constructing lattice structure based on a strangely shaped quantum dot. While single-component quasicrystal lattices have been previously predicted mathematically and in computer simulations, they had not been demonstrated prior to this. | 1 | Crystallography |
:Automated diffraction tomography (ADT) uses software to collect diffraction patterns over a series of slight tilt increments. In this way, a three-dimensional (tomographic) data set of reciprocal lattice intensities can be generated and used for structure determination. By coupling this technique with PED, the range and quality of the data set can be improved. The combination of ADT-PED has been employed effectively to investigate complex framework structures and beam-sensitive organic crystals | 1 | Crystallography |
Supercells are also commonly used in computational models of crystal defects to allow the use of periodic boundary conditions. | 1 | Crystallography |
For membrane proteins, the situation is more complicated because the system that is being crystallized is not the membrane protein itself but the micellar system in which the membrane protein is embedded.
The size of the protein-detergent mixed micelles are affected by both additives and detergents which will strongly influence the crystals obtained. In addition to varying the concentration of primary detergents, additives (lipids and alcohols) and secondary detergents can be used to modulate the size and shape of the detergent micelles. By reducing the size of the mixed micelles lattice forming protein-protein contacts are encouraged. Lipid cubic phases, spontaneous self-assembling liquid crystals or lipid mesophases have been used successfully in the crystallization of integral membrane proteins.
Temperature, salts, detergents, various additives are used in this system to tailor the cubic phase to suit the target protein. Typical detergents used are n-dodecyl-β-d-maltopyranoside, n-decyl-β-d-glucopyranoside, lauryldimethylamine oxide LDAO, n-hexyl-β-d-glucopyranoside, n-nonyl-β-d-glucopyranoside and n-octyl-β-d-glucopyranoside; the various lipids are dioleoyl phosphatidylcholine, dioleoyl phosphatidylethanolamine and monoolein. | 1 | Crystallography |
To perform microcrystallization, a small piece of lichen is extracted using acetone or other solvents, filtered, and evaporated to yield a residue. The residue is transferred to a microscope slide, and a drop of microcrystallization reagent is added before capping with a cover glass. Commonly used reagents include GAW (HO/glycerol/ethanol 1:1:1, v/v/v) and GE (acetic acid/glycerol 1:3). Slides using GE or GAW are gently heated and then allowed to cool, promoting the crystallization process. Once formed, crystals are best observed under polarized light with a 200–1,000-fold magnification.
This method requires basic laboratory equipment, including a microscope equipped for polarized light, test tubes, pipettes, a micro spirit-lamp or micro Bunsen burner, spatula or scalpel, and microscope slides and cover glasses. Lichen substances can be identified based on the distinctive shape and color of their crystals. | 1 | Crystallography |
In mathematics and solid state physics, the first Brillouin zone (named after Léon Brillouin) is a uniquely defined primitive cell in reciprocal space. In the same way the Bravais lattice is divided up into Wigner–Seitz cells in the real lattice, the reciprocal lattice is broken up into Brillouin zones. The boundaries of this cell are given by planes related to points on the reciprocal lattice. The importance of the Brillouin zone stems from the description of waves in a periodic medium given by Bloch's theorem, in which it is found that the solutions can be completely characterized by their behavior in a single Brillouin zone.
The first Brillouin zone is the locus of points in reciprocal space that are closer to the origin of the reciprocal lattice than they are to any other reciprocal lattice points (see the derivation of the Wigner–Seitz cell). Another definition is as the set of points in k-space that can be reached from the origin without crossing any Bragg plane. Equivalently, this is the Voronoi cell around the origin of the reciprocal lattice.
There are also second, third, etc., Brillouin zones, corresponding to a sequence of disjoint regions (all with the same volume) at increasing distances from the origin, but these are used less frequently. As a result, the first Brillouin zone is often called simply the Brillouin zone. In general, the n-th Brillouin zone consists of the set of points that can be reached from the origin by crossing exactly n − 1 distinct Bragg planes. A related concept is that of the irreducible Brillouin zone, which is the first Brillouin zone reduced by all of the symmetries in the point group of the lattice (point group of the crystal).
The concept of a Brillouin zone was developed by Léon Brillouin (1889–1969), a French physicist.
Within the Brillouin zone, a constant-energy surface represents the loci of all the -points (that is, all the electron momentum values) that have the same energy. Fermi surface is a special constant-energy surface that separates the unfilled orbitals from the filled ones at zero kelvin. | 1 | Crystallography |
The reciprocal lattice to an FCC lattice is the body-centered cubic (BCC) lattice, with a cube side of .
Consider an FCC compound unit cell. Locate a primitive unit cell of the FCC; i.e., a unit cell with one lattice point. Now take one of the vertices of the primitive unit cell as the origin. Give the basis vectors of the real lattice. Then from the known formulae, you can calculate the basis vectors of the reciprocal lattice. These reciprocal lattice vectors of the FCC represent the basis vectors of a BCC real lattice. The basis vectors of a real BCC lattice and the reciprocal lattice of an FCC resemble each other in direction but not in magnitude. | 1 | Crystallography |
A chloridometer is a measuring instrument used to determine the concentration of chloride ions (Cl) in a solution. It uses a process known as coulometric titration or amperostatic coulometry, the accepted electrochemistry reference method to determine the concentration of chloride in biological fluids, including blood serum, blood plasma, urine, sweat, and cerebrospinal fluid. The coulometry process generates silver ions, which react with the chloride to form silver chloride (AgCl).
The first chloridometer was designed by a team led by Ernest Cotlove in 1958.
Other methods to determine chloride concentration include photometric titration and isotope dilution mass spectrometry. | 0 | Chromatography + Titration + pH indicators |
Isometries requiring an odd number of mirrors — reflection and glide reflection — always reverse left and right. The even isometries — identity, rotation, and translation — never do; they correspond to rigid motions, and form a normal subgroup of the full Euclidean group of isometries. Neither the full group nor the even subgroup are abelian; for example, reversing the order of composition of two parallel mirrors reverses the direction of the translation they produce.
Since the even subgroup is normal, it is the kernel of a homomorphism to a quotient group, where the quotient is isomorphic to a group consisting of a reflection and the identity. However the full group is not a direct product, but only a semidirect product, of the even subgroup and the quotient group. | 1 | Crystallography |
Dozens of congenital metabolic diseases also known as inborn errors of metabolism (IEM) are now detectable by newborn screening tests, especially the testing using gas chromatography–mass spectrometry. GC–MS can determine compounds in urine even in minor concentration. These compounds are normally not present but appear in individuals suffering with metabolic disorders. This is increasingly becoming a common way to diagnose IEM for earlier diagnosis and institution of treatment eventually leading to a better outcome. It is now possible to test a newborn for over 100 genetic metabolic disorders by a urine test at birth based on GC–MS.
In combination with isotopic labeling of metabolic compounds, the GC–MS is used for determining metabolic activity. Most applications are based on the use of C as the labeling and the measurement of C-C ratios with an isotope ratio mass spectrometer (IRMS); an MS with a detector designed to measure a few select ions and return values as ratios. | 0 | Chromatography + Titration + pH indicators |
A comprehensive article on the modern trends and best practices of mobile phase selection in reversed-phase chromatography was published by Boyes and Dong. A mobile phase in reversed-phase chromatograpy consists of mixtures of water or aqueous buffers, to which organic solvents are added, to elute analytes from a reversed-phase column in a selective manner. The added organic solvents must be miscible with water, and the two most common organic solvents used are acetonitrile and methanol. Other solvents can also be used such as ethanol or 2-propanol (isopropyl alcohol) and tetrahydrofuran (THF). The organic solvent is called also a modifier, since it is added to the aqueous solution in the mobile phase in order to modify the polarity of the mobile phase. Water is the most polar solvent in the reversed phase mobile phase; therefore, lowering the polarity of the mobile phase by adding modifiers enhances its elution strength. The two most widely used organic modifiers are acetonitrile and methanol, although acetonitrile is the more popular choice. Isopropanol (2-propanol) can also be used, because of its strong eluting properties, but its use is limited by its high viscosity, which results in higher backpressures. Both acetonitrile and methanol are less viscous than isopropanol, although a mixture of 50:50 percent of methanol:water is also very viscous and causes high backpressures.
All three solvents are essentially UV transparent. This is a crucial property for common reversed phase chromatography since sample components are typically detected by UV detectors. Acetonitrile is more transparent than the others in low UV wavelengths range, therefore it is used almost exclusively when separating molecules with weak or no chromophores (UV-VIS absorbing groups), such as peptides. Most peptides only absorb at low wavelengths in the ultra-violet spectrum (typically less than 225 nm) and acetonitrile provides much lower background absorbance at low wavelengths than the other common solvents.
The pH of the mobile phase can have an important role on the retention of an analyte and can change the selectivity of certain analytes. For samples containing solutes with ionized functional groups, such as amines, carboxyls, phosphates, phosphonates, sulfates, and sulfonates, the ionization of these groups can be controlled using mobile phase buffers.
For example, carboxylic groups in solutes become increasingly negatively charged as the pH of the mobile phase rises above their pKa, hence the whole molecule becomes more polar and less retained on the a-polar stationary phase. In this case, raising the pH of the phase mobile above 4–5 = pH (which is the typical pKa range for carboxylic groups) increases their ionization, hence decreases their retention. Conversely, using a mobile phase at a pH lower than 4 will increase their retention, because it will decrease their ionization degree, rendering them less polar.
The same considerations apply to substances containing basic functional groups, such as amines, whose pKa ranges are around 8 and above, are retained more, as the pH of the mobile phase increases, approaching 8 and above, because they are less ionized, hence less polar. However, in the case of high pH mobile phases, most of the traditional silica gel based Reversed Phase columns are generally limited for use with mobile phases at pH 8 and above, therefore, control over the retention of amines in this range is limited.
The choice of buffer type is an important factor in RP-LC method development, as it can affect the retention, selectivity, and resolution of the analytes of interest. When selecting a buffer for RP-HPLC, there are a number of factors to consider, including:
* The desired pH of the mobile phase: Buffers are most effective around their pKa value, so it is important to choose a buffer with a pKa that is close to the desired mobile phase pH needed.
* The solubility of the buffer in the organic solvent: The buffer must be compatible with the organic solvent that is being used in the mobile phase, mostly with the common organic solvents mentioned above, acetonitrile, methanol, and isopropanol.
* The UV cut-off of the buffer: In case of UV detection, the buffer should have a UV absorption that is below the detection wavelength of the analytes of interest. This will prevent the buffer from interfering with the detection of that analytes.
* The compatibility of the buffer with the detector: If mass spectrometry (MS) is being used for detection, the buffer must be compatible with the mass spectrometry (MS) instrument. Some buffers, such as those containing phosphate salts, cannot be used with the MS detectors, as they are not volatile as needed, and they interfere with the MS detection by suppressing the analytes ionization, making them undetected by MS.
Some of the most common buffers used in RP-HPLC include:
* Phosphate buffers: Phosphate buffer is versatile and can be used to achieve a wide range of pH values, thanks to 3 pKa values. They also have very low UV background for UV detection. However, they are not appropriate for MS detection.
* Acetate buffers: Acetate buffers are also versatile and can be used to achieve range of pH values typically used in RP-LC. In terms of UV detection at sub 220 nm wavelength, it is not so favorable. The ammonium acetate buffer is compatible with MS.
* Formate buffers: Formate buffers is similar to the acetate buffer in terms of range of pHs used and limited UV detection under 225 nm. Its ammonium acetate is also compatible with MS.
* Ammonium buffers: Ammonium buffers are volatile and are often used in LC-MS methods. They also are limited for low UV detection.
Charged analytes can be separated on a reversed-phase column by the use of ion-pairing (also called ion-interaction). This technique is known as reversed-phase ion-pairing chromatography.
Elution can be performed isocratically (the water-solvent composition does not change during the separation process) or by using a solution gradient (the water-solvent composition changes during the separation process, usually by decreasing the polarity). | 0 | Chromatography + Titration + pH indicators |
Similar to bromocresol green, the structure of bromocresol purple changes with pH. Changing the level of acidity causes a shift in the equilibrium between two different structures that have different colors. In near-neutral or alkaline solution, the chemical has a sulfonate structure that gives the solution a purple color. As the pH decreases, it converts to a sultone (cyclic sulfonic ester) that colors the solution yellow. In some microbiology tests, this change is used as an indicator of bacterial growth. | 0 | Chromatography + Titration + pH indicators |
In October 2016, Christopher Monroe at the University of Maryland claimed to have created the worlds first discrete time crystal. Using the ideas proposed by Yao et al., his team trapped a chain of Yb ions in a Paul trap, confined by radio-frequency electromagnetic fields. One of the two spin states was selected by a pair of laser beams. The lasers were pulsed, with the shape of the pulse controlled by an acousto-optic modulator, using the Tukey window to avoid too much energy at the wrong optical frequency. The hyperfine electron states in that setup, S' and , have very close energy levels, separated by 12.642831 GHz. Ten Doppler-cooled ions were placed in a line 0.025 mm long and coupled together.
The researchers observed a subharmonic oscillation of the drive. The experiment showed "rigidity" of the time crystal, where the oscillation frequency remained unchanged even when the time crystal was perturbed, and that it gained a frequency of its own and vibrated according to it (rather than only the frequency of the drive). However, once the perturbation or frequency of vibration grew too strong, the time crystal "melted" and lost this subharmonic oscillation, and it returned to the same state as before where it moved only with the induced frequency.
Also in 2016, Mikhail Lukin at Harvard also reported the creation of a driven time crystal. His group used a diamond crystal doped with a high concentration of nitrogen-vacancy centers, which have strong dipole–dipole coupling and relatively long-lived spin coherence. This strongly interacting dipolar spin system was driven with microwave fields, and the ensemble spin state was determined with an optical (laser) field. It was observed that the spin polarization evolved at half the frequency of the microwave drive. The oscillations persisted for over 100 cycles. This subharmonic response to the drive frequency is seen as a signature of time-crystalline order.
In May 2018, a group in Aalto University reported that they had observed the formation of a time quasicrystal and its phase transition to a continuous time crystal in a Helium-3 superfluid cooled to within one ten thousandth of a kelvin from absolute zero (0.0001 K). On August 17, 2020 Nature Materials published a letter from the same group saying that for the first time they were able to observe interactions and the flow of constituent particles between two time crystals.
In February 2021 a team at Max Planck Institute for Intelligent Systems described the creation of time crystal consisting of magnons and probed them under scanning transmission X-ray microscopy to capture the recurring periodic magnetization structure in the first known video record of such type.
In July 2021, a team led by Andreas Hemmerich at the Institute of Laser Physics at the University of Hamburg presented the first realization of a time crystal in an open system, a so-called dissipative time crystal using ultracold atoms coupled to an optical cavity. The main achievement of this work is a positive application of dissipation – actually helping to stabilise the system's dynamics.
In November 2021, a collaboration between Google and physicists from multiple universities reported the observation of a discrete time crystal on Googles Sycamore processor, a quantum computing device. A chip of 20 qubits was used to obtain a many-body localization configuration of up and down spins and then stimulated with a laser to achieve a periodically driven "Floquet" system where all up spins are flipped for down and vice-versa in periodic cycles which are multiples of the lasers frequency. While the laser is necessary to maintain the necessary environmental conditions, no energy is absorbed from the laser, so the system remains in a protected eigenstate order.
Previously in June and November 2021 other teams had obtained virtual time crystals based on floquet systems under similar principles to those of the Google experiment, but on quantum simulators rather than quantum processors: first a group at the University of Maryland obtained time crystals on trapped-ions qubits using high frequency driving rather than many-body localization and then a collaboration between TU Delft and TNO in the Netherlands called Qutech created time crystals from nuclear spins in carbon-13 nitrogen-vacancy (NV) centers on a diamond, attaining longer times but fewer qubits.
In February 2022, a scientist at UC Riverside reported a dissipative time crystal akin to the system of July 2021 but all-optical, which allowed the scientist to operate it at room temperature. In this experiment injection locking was used to direct lasers at a specific frequency inside a microresonator creating a lattice trap for solitons at subharmonic frequencies.
In March 2022, a new experiment studying time crystals on a quantum processor was performed by two physicists at the university of Melbourne, this time using IBM's Manhattan and Brooklyn quantum processors observing a total of 57 qubits.
In June 2022, the observation of a continuous time crystal was reported by a team at the Institute of Laser Physics at the University of Hamburg, supervised by Hans Keßler and Andreas Hemmerich. In periodically driven systems, time-translation symmetry is broken into a discrete time-translation symmetry due to the drive. Discrete time crystals break this discrete time-translation symmetry by oscillating at a multiple of the drive frequency. In the new experiment, the drive (pump laser) was operated continuously, thus respecting the continuous time-translation symmetry. Instead of a subharmonic response, the system showed an oscillation with an intrinsic frequency and a time phase taking random values between 0 and 2π, as expected for spontaneous breaking of continuous time-translation symmetry. Moreover, the observed limit cycle oscillations were shown to be robust against perturbations of technical or fundamental character, such as quantum noise and, due to the openness of the system, fluctuations associated with dissipation. The system consisted of a Bose–Einstein condensate in an optical cavity, which was pumped with an optical standing wave oriented perpendicularly with regard to the cavity axis and was in a superradiant phase localizing at two bistable ground states between which it oscillated. | 1 | Crystallography |
The designations were established by the journal Zeitschrift für Kristallographie – Crystalline Materials, which published its first round of supplemental reviews under the name Strukturbericht from 1913-1928. These reports were collected into a book published in 1931 by Paul Peter Ewald and Carl Hermann which became Volume 1 of Strukturbericht. While the series was continued after the war under the name Structure reports, which was published through 1990, the series stopped generating new symbols. Instead, some new additional designations were given in books by Smithels, and Pearson.
For the first volume, the designation consisted of a capital letter (A,B,C,D,E,F,G,H,L,M,O) specifying a broad category of compounds, and then a number to specify a particular crystal structure. In the second volume, subscript numbers were added, some early symbols were modified (e.g. what was initially D1 became D0, noted in the tables below as "D1 → D0"), and the categories were modified (types I,K,S were added). In the third volume, the class I was renamed J. Later designations began to use a lower case letter in subscripts as well. | 1 | Crystallography |
Crystal violet can be used as an alternative to Coomassie brilliant blue (CBB) in staining of proteins separated by SDS-PAGE, reportedly showing a 5x improved sensitivity vs CBB. | 0 | Chromatography + Titration + pH indicators |
Sunset yellow is used in foods, condoms, cosmetics, and drugs. Sunset yellow FCF is used as an orange or yellow-orange dye. For example, it is used in candy, desserts, snacks, sauces, and preserved fruits.
Sunset yellow is often used in conjunction with E123, amaranth, to produce a brown colouring in both chocolates and caramel. | 0 | Chromatography + Titration + pH indicators |
The point groups are named according to their component symmetries. There are several standard notations used by crystallographers, mineralogists, and physicists.
For the correspondence of the two systems below, see crystal system. | 1 | Crystallography |
The crystallographic point group or crystal class is the mathematical group comprising the symmetry operations that leave at least one point unmoved and that leave the appearance of the crystal structure unchanged. These symmetry operations include
*Reflection, which reflects the structure across a reflection plane
*Rotation, which rotates the structure a specified portion of a circle about a rotation axis
*Inversion, which changes the sign of the coordinate of each point with respect to a center of symmetry or inversion point
*Improper rotation, which consists of a rotation about an axis followed by an inversion.
Rotation axes (proper and improper), reflection planes, and centers of symmetry are collectively called symmetry elements. There are 32 possible crystal classes. Each one can be classified into one of the seven crystal systems. | 1 | Crystallography |
The coupling of MS with LC systems is attractive because liquid chromatography can separate delicate and complex natural mixtures, which chemical composition needs to be well established (e.g., biological fluids, environmental samples, and drugs). Further, LC–MS has applications in volatile explosive residue analysis. Nowadays, LC–MS has become one of the most widely used chemical analysis techniques because more than 85% of natural chemical compounds are polar and thermally labile and GC-MS cannot process these samples. As an example, HPLC–MS is regarded as the leading analytical technique for proteomics and pharmaceutical laboratories. Other important applications of LC–MS include the analysis of food, pesticides, and plant phenols. | 0 | Chromatography + Titration + pH indicators |
Homoepitaxy is a kind of epitaxy performed with only one material, in which a crystalline film is grown on a substrate or film of the same material. This technology is often used to grow a more pure film than the substrate and to fabricate layers with different doping levels. In academic literature, homoepitaxy is often abbreviated to "homoepi".
Homotopotaxy is a process similar to homoepitaxy except that the thin-film growth is not limited to two-dimensional growth. Here the substrate is the thin-film material.
Heteroepitaxy is a kind of epitaxy performed with materials that are different from each other. In heteroepitaxy, a crystalline film grows on a crystalline substrate or film of a different material. This technology is often used to grow crystalline films of materials for which crystals cannot otherwise be obtained and to fabricate integrated crystalline layers of different materials. Examples include silicon on sapphire, gallium nitride (GaN) on sapphire, aluminium gallium indium phosphide (AlGaInP) on gallium arsenide (GaAs) or diamond or iridium, and graphene on hexagonal boron nitride (hBN).
Heteroepitaxy occurs when a film of different composition and/or crystalline films grown on a substrate. In this case, the amount of strain in the film is determined by the lattice mismatch Ԑ:
Where and are the lattice constants of the film and the substrate. The film and substrate could have similar lattice spacings but also different thermal expansion coefficients. If a film is grown at a high temperature, it can experience large strains upon cooling to room temperature. In reality, is necessary for obtaining epitaxy. If is larger than that, the film experiences a volumetric strain that builds with each layer until a critical thickness. With increased thickness, the elastic strain in the film is relieved by the formation of dislocations, which can become scattering centers that damage the quality of the structure. Heteroepitaxy is commonly used to create so-called bandgap systems thanks to the additional energy caused by de deformation. A very popular system with great potential for microelectronic applications is that of Si–Ge.
Heterotopotaxy is a process similar to heteroepitaxy except that thin-film growth is not limited to two-dimensional growth; the substrate is similar only in structure to the thin-film material.
Pendeo-epitaxy is a process in which the heteroepitaxial film is growing vertically and laterally simultaneously.
In 2D crystal heterostructure, graphene nanoribbons embedded in hexagonal boron nitride give an example of pendeo-epitaxy.
Grain-to-grain epitaxy involves epitaxial growth between the grains of a multicrystalline epitaxial and seed layer. This can usually occur when the seed layer only has an out-of-plane texture but no in-plane texture. In such a case, the seed layer consists of grains with different in-plane textures. The epitaxial overlayer then creates specific textures along each grain of the seed layer, due to lattice matching. This kind of epitaxial growth doesn't involve single-crystal films.
Epitaxy is used in silicon-based manufacturing processes for bipolar junction transistors (BJTs) and modern complementary metal–oxide–semiconductors (CMOS), but it is particularly important for compound semiconductors such as gallium arsenide. Manufacturing issues include control of the amount and uniformity of the depositions resistivity and thickness, the cleanliness and purity of the surface and the chamber atmosphere, the prevention of the typically much more highly doped substrate wafers diffusion of dopant to the new layers, imperfections of the growth process, and protecting the surfaces during manufacture and handling. | 1 | Crystallography |
Cerimetry or cerimetric titration, also known as cerate oximetry, is a method of volumetric chemical analysis developed by Ion Atanasiu. It is a redox titration in which an iron(II)–1,10-phenanthroline complex (ferroin) color change indicates the end point. Ferroin can be reversibly discolored in its oxidized form upon titration with a Ce solution. The use of cerium(IV) salts as reagents for volumetric analysis was first proposed in the middle of 19th century, but systematic studies did not start until about 70 years later. Standard solutions can be prepared from different Ce salts, but often cerium sulfate is chosen.
Since cerimetry is linked to the Fe/Fe redox pair, it can be used for analyses of nonstoichiometric levels that either oxidize Fe or reduce Fe. For the case of oxidation, a precise excess of high-purity crystalline Mohr's salt is added upon the oxide digestion in aqueous hydrogen chloride (HCl), while for the case of reduction, an excess of 1 M iron trichloride (FeCl) is added. In both cases, Fe ions will be titrated subsequently. Because the Ce solution is prone to hydrolysis, the titration is done in a strongly HCl-acidic solution into which some phosphoric acid (HPO) is added to obtain a less colored phosphato complex of Fe.
According to tabulated values of standard potentials at pH 0 for the first-row transition metals, any nonstoichiometry below the following oxidation states will reduce 1 M FeCl solution whereas any nonstoichiometry above them will oxidize the Mohrs salt: Ti, V, Cr, Mn, Co, and Ni. In addition, any nonstoichiometry in the Fe(III)–Fe(II) range is titrated directly with no additives, any nonstoichiometry below Fe will reduce 1 M FeCl whereas any nonstoichiometry above Fe will oxidize Mohrs salt. In the second- and third-row transition metals, only the early elements would be suitable for the titration, and the limiting oxidation states are Zr, Nb, Mo, Hf, Ta, and W. Standard potentials involving rhenium ions are too close to E for Fe/Fe as well as to each other. Nonstoichiometry of oxides containing several elements in oxidation states suitable for cerimetry is determined in one titration. | 0 | Chromatography + Titration + pH indicators |
Two-dimensional separations can be carried out in gas chromatography or liquid chromatography. Various different coupling strategies have been developed to "resample" from the first column into the second. Some important hardware for two-dimensional separations are Deans' switch and Modulator, which selectively transfer the first dimension eluent to second dimension column.
The chief advantage of two-dimensional techniques is that they offer a large increase in peak capacity, without requiring extremely efficient separations in either column. (For instance, if the first column offers a peak capacity (k)of 100 for a 10-minute separation, and the second column offers a peak capacity of 5 (k) in a 5-second separation, then the combined peak capacity may approach k × k=500, with the total separation time still ~ 10 minutes). 2D separations have been applied to the analysis of gasoline and other petroleum mixtures, and more recently to protein mixtures. | 0 | Chromatography + Titration + pH indicators |
In March 1951, the British Tabulating Machine Company (BTM) sent a team to Andrew Booths workshop. They then used his design to create the Hollerith Electronic Computer 1 (HEC 1) before the end of 1951. The computer was a direct copy of Andrew Booths circuits with extra Input/output interfaces. The HEC 2 was the HEC 1 with smarter metal casings and was built for the Business Efficiency Exhibition in 1953. A slightly modified version of the HEC 2 was then marketed as HEC2M and 8 were sold. The HEC2M was succeeded by the HEC4. Around 100 HEC4s were sold in the late 1950s.
The HEC used standard punched cards; the HEC 4 had a printer, too, and it featured several instructions (such as divide) and registers not found on the APEXC. | 1 | Crystallography |
Rotations, denoted by R, where c is a point in the plane (the centre of rotation), and θ is the angle of rotation. In terms of coordinates, rotations are most easily expressed by breaking them up into two operations. First, a rotation around the origin is given by
These matrices are the orthogonal matrices (i.e. each is a square matrix whose transpose is its inverse, i.e. ), with determinant 1 (the other possibility for orthogonal matrices is −1, which gives a mirror image, see below). They form the special orthogonal group SO(2).
A rotation around c can be accomplished by first translating c to the origin, then performing the rotation around the origin, and finally translating the origin back to c. That is,
or in other words,
Alternatively, a rotation around the origin is performed, followed by a translation:
A rotation can be seen as a composite of two non-parallel reflections. | 1 | Crystallography |
The subgroup structure suggests another way to compose an arbitrary isometry:
: Pick a fixed point, and a mirror through it.
# If the isometry is odd, use the mirror; otherwise do not.
# If necessary, rotate around the fixed point.
# If necessary, translate.
This works because translations are a normal subgroup of the full group of isometries, with quotient the orthogonal group; and rotations about a fixed point are a normal subgroup of the orthogonal group, with quotient a single reflection. | 1 | Crystallography |
right|thumb|Example and diagram for p3
left|thumb|Cell structure for p3
* Orbifold signature:
* Coxeter notation: [(3,3,3)] or [3]
* Lattice: hexagonal
* Point group: C
* The group p3 has three different rotation centres of order three (120°), but no reflections or glide reflections.
Imagine a tessellation of the plane with equilateral triangles of equal size, with the sides corresponding to the smallest translations. Then half of the triangles are in one orientation, and the other half upside down. This wallpaper group corresponds to the case that all triangles of the same orientation are equal, while both types have rotational symmetry of order three, but the two are not equal, not each others mirror image, and not both symmetric (if the two are equal it is p6, if they are each others mirror image it is p31m, if they are both symmetric it is p3m1; if two of the three apply then the third also, and it is p6m). For a given image, three of these tessellations are possible, each with rotation centres as vertices, i.e. for any tessellation two shifts are possible. In terms of the image: the vertices can be the red, the blue or the green triangles.
Equivalently, imagine a tessellation of the plane with regular hexagons, with sides equal to the smallest translation distance divided by . Then this wallpaper group corresponds to the case that all hexagons are equal (and in the same orientation) and have rotational symmetry of order three, while they have no mirror image symmetry (if they have rotational symmetry of order six it is p6, if they are symmetric with respect to the main diagonals it is p31m, if they are symmetric with respect to lines perpendicular to the sides it is p3m1; if two of the three apply then the third also, it is p6m). For a given image, three of these tessellations are possible, each with one third of the rotation centres as centres of the hexagons. In terms of the image: the centres of the hexagons can be the red, the blue or the green triangles.
;Examples of group p3 | 1 | Crystallography |
In crystallography, a fractional coordinate system is used in order to better reflect the symmetry of the underlying lattice of a crystal pattern (or any other periodic pattern in space). In a fractional coordinate system the basis vectors of the coordinate system are chosen to be lattice vectors and the basis is then termed a crystallographic basis (or lattice basis).
In a lattice basis, any lattice vector can be represented as,
There are an infinite number of lattice bases for a crystal pattern. However, these can be chosen in such a way that the simplest description of the pattern can be obtained. These bases are used in the International Tables of Crystallography Volume A and are termed conventional bases. A lattice basis is called primitive if the basis vectors are lattice vectors and all lattice vectors can be expressed as,
However, the conventional basis for a crystal pattern is not always chosen to be primitive. Instead, it is chosen so the number of orthogonal basis vectors is maximized. This results in some of the coefficients of the equations above being fractional. A lattice in which the conventional basis is primitive is called a primitive lattice, while a lattice with a non-primitive conventional basis is called a centered lattice.
The choice of an origin and a basis implies the choice of a unit cell which can further be used to describe a crystal pattern. The unit cell is defined as the parallelotope (i.e., generalization of a parallelogram (2D) or parallelepiped (3D) in higher dimensions) in which the coordinates of all points are such that, .
Furthermore, points outside of the unit cell can be transformed inside of the unit cell through standardization, the addition or subtraction of integers to the coordinates of points to ensure . In a fractional coordinate system, the lengths of the basis vectors and the angles between them are called the lattice parameters (lattice constants) of the lattice. In two- and three-dimensions, these correspond to the lengths and angles between the edges of the unit cell.
The fractional coordinates of a point in space in terms of the lattice basis vectors is defined as, | 1 | Crystallography |
*Glucose – 5 g/L
*Dipotassium phosphate – 5 g/L
*Proteose Peptone – 5 g/L
*Distilled water – 1000 mL
pH – 6.9 | 0 | Chromatography + Titration + pH indicators |
There are seven different kinds of lattice systems, and each kind of lattice system has four different kinds of centerings (primitive, base-centered, body-centered, face-centered). However, not all of the combinations are unique; some of the combinations are equivalent while other combinations are not possible due to symmetry reasons. This reduces the number of unique lattices to the 14 Bravais lattices.
The distribution of the 14 Bravais lattices into 7 lattice systems is given in the following table.
In geometry and crystallography, a Bravais lattice is a category of translative symmetry groups (also known as lattices) in three directions.
Such symmetry groups consist of translations by vectors of the form
:R = na + na + na,
where n, n, and n are integers and a, a, and a are three non-coplanar vectors, called primitive vectors.
These lattices are classified by the space group of the lattice itself, viewed as a collection of points; there are 14 Bravais lattices in three dimensions; each belongs to one lattice system only. They represent the maximum symmetry a structure with the given translational symmetry can have.
All crystalline materials (not including quasicrystals) must, by definition, fit into one of these arrangements.
For convenience a Bravais lattice is depicted by a unit cell which is a factor 1, 2, 3, or 4 larger than the primitive cell. Depending on the symmetry of a crystal or other pattern, the fundamental domain is again smaller, up to a factor 48.
The Bravais lattices were studied by Moritz Ludwig Frankenheim in 1842, who found that there were 15 Bravais lattices. This was corrected to 14 by A. Bravais in 1848. | 1 | Crystallography |
Initially these were described as ternary RE-B-Si compounds, but later carbon was included to improve the structure description that resulted in a quaternary RE-B-C-Si composition. REBCSi (RE=Y and Gd–Lu) have a unique crystal structure with two units – a cluster of B icosahedra and a Si ethane-like complex – and one bonding configuration (B)≡Si-C≡(B). A representative compound of this group is YBCSi (x=0.68). It has a trigonal crystal structure with space group Rm (No. 166) and lattice constants a = b = 1.00841(4) nm, c = 1.64714(5) nm, α = β = 90° and γ = 120°.
The crystal has layered structure. Figure 15 shows a network of boron icosahedra that spreads parallel to the (001) plane, connecting with four neighbors through B1–B1 bonds. The C3 and Si3 site atoms strengthen the network by bridging the boron icosahedra. Contrary to other boron-rich icosahedral compounds, the boron icosahedra from different layers are not directly bonded. The icosahedra within one layer are linked through Si ethane-like clusters with (B)≡Si-C≡(B) bonds, as shown in figures 16a and b.
There are eight atomic sites in the unit cell: one yttrium Y, four boron B1–B4, one carbon C3 and three silicon sites Si1–Si3. Atomic coordinates, site occupancy and isotropic displacement factors are listed in table Va; 68% of the Y sites are randomly occupied and remaining Y sites are vacant. All boron sites and Si1 and Si2 sites are fully occupied. The C3 and Si3 sites can be occupied by either carbon or silicon atoms (mixed occupancy) with a probability of about 50%. Their separation is only 0.413 Å, and thus either the C3 or Si3 sites, but not both, are occupied. These sites form Si-C pairs, but not Si-Si or C-C pairs. The distances between the C3 and Si3 sites and the surrounding sites for YBCSi are summarized in table Vb and the overall crystal structure is shown in figure 14.
Salvador et al. reported an isotypic terbium compound
TbCSi(B). Most parts of the crystal structure are the same as those described above; however, its bonding configuration is deduced as (B)≡C-C≡(B) instead of (B)≡Si-C≡(B). The authors intentionally added carbon to grow single crystals whereas the previous crystals were accidentally contaminated by carbon during their growth. Thus, higher carbon concentration was achieved. Existence of both bonding schemes of (B)≡Si-C≡(B) and (B)≡C-C≡(B) suggests the occupancy of the carbon sites of 50–100%. On the other hand, (B)≡Si-Si≡(B) bonding scheme is unlikely because of too short Si-Si distance, suggesting that the minimum carbon occupancy at the site is 50%. Some B atoms may replace C atoms at the C3 site, as previously assigned to the B site. However, the carbon occupation is more likely because the site is tetrahedrally coordinated whereas the B occupation of the site needs an extra electron to complete tetrahedral bonding. Thus, carbon is indispensable for this group of compounds. | 1 | Crystallography |
A "strong" ion exchanger will not lose the charge on its matrix once the column is equilibrated and so a wide range of pH buffers can be used. "Weak" ion exchangers have a range of pH values in which they will maintain their charge. If the pH of the buffer used for a weak ion exchange column goes out of the capacity range of the matrix, the column will lose its charge distribution and the molecule of interest may be lost. Despite the smaller pH range of weak ion exchangers, they are often used over strong ion exchangers due to their having greater specificity. In some experiments, the retention times of weak ion exchangers are just long enough to obtain desired data at a high specificity.
Resins (often termed beads) of ion exchange columns may include functional groups such as weak/strong acids and weak/strong bases. There are also special columns that have resins with amphoteric functional groups that can exchange both cations and anions. Some examples of functional groups of strong ion exchange resins are quaternary ammonium cation (Q), which is an anion exchanger, and sulfonic acid (S, -SOOH), which is a cation exchanger. These types of exchangers can maintain their charge density over a pH range of 0–14. Examples of functional groups of Weak ion exchange resins include diethylaminoethyl (DEAE, -CHN(CHH)), which is an anion exchanger, and carboxymethyl (CM, -CH-COOH), which is a cation exchanger. These two types of exchangers can maintain the charge density of their columns over a pH range of 5–9.
In ion chromatography, the interaction of the solute ions and the stationary phase based on their charges determines which ions will bind and to what degree. When the stationary phase features positive groups which attracts anions, it is called an anion exchanger; when there are negative groups on the stationary phase, cations are attracted and it is a cation exchanger. The attraction between ions and stationary phase also depends on the resin, organic particles used as ion exchangers.
Each resin features relative selectivity which varies based on the solute ions present who will compete to bind to the resin group on the stationary phase. The selectivity coefficient, the equivalent to the equilibrium constant, is determined via a ratio of the concentrations between the resin and each ion, however, the general trend is that ion exchangers prefer binding to the ion with a higher charge, smaller hydrated radius, and higher polarizability, or the ability for the electron cloud of an ion to be disrupted by other charges. Despite this selectivity, excess amounts of an ion with a lower selectivity introduced to the column would cause the lesser ion to bind more to the stationary phase as the selectivity coefficient allows fluctuations in the binding reaction that takes place during ion exchange chromatography.
Following table shows the commonly used ion exchangers | 0 | Chromatography + Titration + pH indicators |
In geophysics, a common assumption is that the rock formations of the crust are locally polar anisotropic (transversely isotropic); this is the simplest case of geophysical interest. Backus upscaling is often used to determine the effective transversely isotropic elastic constants of layered media for long wavelength seismic waves.
Assumptions that are made in the Backus approximation are:
* All materials are linearly elastic
* No sources of intrinsic energy dissipation (e.g. friction)
* Valid in the infinite wavelength limit, hence good results only if layer thickness is much smaller than wavelength
* The statistics of distribution of layer elastic properties are stationary, i.e., there is no correlated trend in these properties.
For shorter wavelengths, the behavior of seismic waves is described using the superposition of plane waves. Transversely isotropic media support three types of elastic plane waves:
* a quasi-P wave (polarization direction almost equal to propagation direction)
* a quasi-S wave
* a S-wave (polarized orthogonal to the quasi-S wave, to the symmetry axis, and to the direction of propagation).
Solutions to wave propagation problems in such media may be constructed from these plane waves, using Fourier synthesis. | 1 | Crystallography |
The growth (starting materials, flux, and crucible) are heated to form a complete liquid solution. The growth is cooled to a temperature where the solution is fully saturated. Further cooling causes crystals to precipitate from the solution, lowering the concentration of starting materials in solution, and lowering the temperature where the solution is fully saturated. The process is repeated, decreasing temperature and precipitating more crystals. The process is then stopped at a desired temperature, and the growth is removed from the furnace. Practically, the flux method is done by placing the growth into a programmable furnace:
# Ramp - The furnace is heated from an initial temperature to a maximum temperature, where the growth forms a complete liquid solution.
# Dwell - The furnace is maintained at the maximum temperature to homogenize the solution.
# Cool - The furnace is cooled to a desired temperature over a specified rate or time.
# Removal - The growth is removed from the furnace. The growth can be quenched, centrifuged, or simply removed if already at room temperature.
Additional steps may be added to this basic temperature profile, such as additional dwells or different cooling rates over different points of the cool. Crystallization can occur through spontaneous nucleation, encouragement with a seed, or through mechanical stress. | 1 | Crystallography |
Overshot titrations are a common phenomenon, and refer to a situation where the volume of titrant added during a chemical titration exceeds the amount required to reach the equivalence point. This excess titrant leads to an outcome where the solution becomes slightly more alkaline or over-acidified.
Overshooting the equivalence point can occur due to various factors, such as errors in burette readings, imperfect reaction stoichiometry, or issues with endpoint detection. The consequences of overshot titrations can affect the accuracy of the analytical results, particularly in quantitative analysis.
Researchers and analysts often employ corrective measures, such as back-titration and using more precise titration techniques, to mitigate the impact of overshooting and obtain reliable and precise measurements. Understanding the causes, consequences, and solutions related to overshot titrations is crucial in achieving accurate and reproducible results in the field of chemistry. | 0 | Chromatography + Titration + pH indicators |
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Wikipedia Crystallography vs Analytical Chemistry Binary Classification
This dataset is derived from the English Wikipedia articles and is designed for binary text classification tasks in the fields of crystallography and analytical chemistry. The dataset is divided into two classes based on the thematic content of the articles:
Crystallography: This class includes articles that focus on crystallography, the study of crystal structures and their properties. Topics may cover methods for determining crystal structures, the principles of X-ray diffraction, and the applications of crystallography in fields such as materials science, chemistry, and biology.
Analytical Chemistry: This class comprises articles related to various techniques in analytical chemistry, including:
- Chromatography: Articles discussing techniques for separating mixtures into their individual components, such as gas chromatography (GC) and liquid chromatography (LC).
- Titration: Articles covering methods for determining the concentration of a substance in a solution by adding a reagent of known concentration until a reaction is complete.
- pH Indicators: Articles about substances that change color based on the pH level of the solution they are in, used to measure acidity or alkalinity.
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