text
stringlengths 107
11k
| label
int64 0
1
| label_text
stringclasses 2
values |
|---|---|---|
Two-dimensional chromatography is a type of chromatographic technique in which the injected sample is separated by passing through two different separation stages. Two different chromatographic columns are connected in sequence, and the effluent from the first system is transferred onto the second column. Typically the second column has a different separation mechanism, so that bands that are poorly resolved from the first column may be completely separated in the second column. (For instance, a C18 reversed-phase chromatography column may be followed by a phenyl column.) Alternately, the two columns might run at different temperatures. During the second stage of separation the rate at which the separation occurs must be faster than the first stage, since there is still only a single detector. The plane surface is amenable to sequential development in two directions using two different solvents. | 0 | Chromatography + Titration + pH indicators |
A wallpaper is a mathematical object covering a whole Euclidean plane by repeating a motif indefinitely, in manner that certain isometries keep the drawing unchanged. For each wallpaper there corresponds a group of congruent transformations, with function composition as the group operation. Thus, a wallpaper group (or plane symmetry group or plane crystallographic group) is a mathematical classification of a two‑dimensional repetitive pattern, based on the symmetries in the pattern. Such patterns occur frequently in architecture and decorative art, especially in textiles, tessellations, tiles and physical wallpaper. | 1 | Crystallography |
Heteroepitaxial growth is classified into three primary growth modes-- Volmer–Weber (VW), Frank–van der Merwe (FM) and Stranski–Krastanov (SK).
In the VW growth regime, the epitaxial film grows out of 3D nuclei on the growth surface. In this mode, the adsorbate-adsorbate interactions are stronger than adsorbate-surface interactions, leading to island formation by local nucleation and the epitaxial layer is formed when the islands join.
In the FM growth mode, adsorbate-surface and adsorbate-adsorbate interactions are balanced, which promotes 2D layer-by-layer or step-flow epitaxial growth.
The SK mode is a combination of VW and FM modes. In this mechanism, the growth initiates in the FM mode, forming 2D layers, but after reaching a critical thickness, enters a VW-like 3D island growth regime.
Practical epitaxial growth, however, takes place in a high supersaturation regime, away from thermodynamic equilibrium. In that case, the epitaxial growth is governed by adatom kinetics rather than thermodynamics, and 2D step-flow growth becomes dominant. | 1 | Crystallography |
Antibody elution is the process of removing antibodies that are attached to their targets, such as the surface of red blood cells. Techniques include using heat, a freeze-thaw cycle, ultrasound, acids or organic solvents. No single method is best in all situations. | 0 | Chromatography + Titration + pH indicators |
In the isometric system, the most common types of twins are the Spinel Law (twin plane, parallel to an octahedron) <111>, where the twin axis is perpendicular to an octahedral face, and the Iron Cross <001>, which is the interpenetration of two pyritohedrons, a subtype of dodecahedron.
In the hexagonal system, calcite shows the contact twin laws {0001} and {0112}. Quartz shows the Brazil Law {1120}, and Dauphiné Law <0001>, which are penetration twins caused by transformation, and Japan Law {1122}, which is often caused by accidents during growth.
In the tetragonal system, cyclical contact twins are the most commonly observed type of twin, such as in rutile titanium dioxide and cassiterite tin oxide.
In the orthorhombic system, crystals usually twin on planes parallel to the prism face, where the most common is a {110} twin, which produces cyclical twins, such as in aragonite, chrysoberyl, and cerussite.
In the monoclinic system, twins occur most often on the planes {100} and {001} by the Manebach Law {001}, Carlsbad Law [001], Baveno Law {021} in orthoclase, and the Swallow Tail Twins (Manebach law) {001} in gypsum.
In the triclinic system, the most commonly twinned crystals are the feldspar minerals plagioclase and microcline. These minerals show the Albite and Pericline Laws.
The most common twin operations by crystal system are tabulated below. This list is not exhaustive, particularly for the crystal systems of lowest symmetry, such as the triclinic system. | 1 | Crystallography |
For over 150 years, scientists from all around the world have known about the crystallization of protein molecules.
In 1840, Friedrich Ludwig Hünefeld accidentally discovered the formation of crystalline material in samples of earthworm blood held under two glass slides and occasionally observed small plate-like crystals in desiccated swine or human blood samples. These crystals were named as haemoglobin, by Felix Hoppe-Seyler in 1864. The seminal findings of Hünefeld inspired many scientists in the future.
In 1851, Otto Funke described the process of producing human haemoglobin crystals by diluting red blood cells with solvents, such as pure water, alcohol or ether, followed by slow evaporation of the solvent from the protein solution. In 1871, William T. Preyer, Professor at University of Jena, published a book entitled Die Blutkrystalle (The Crystals of Blood), reviewing the features of haemoglobin crystals from around 50 species of mammals, birds, reptiles and fishes.
In 1909, the physiologist Edward T. Reichert, together with the mineralogist Amos P. Brown, published a treatise on the preparation, physiology and geometrical characterization of haemoglobin crystals from several hundreds animals, including extinct species such as the Tasmanian wolf. Increasing protein crystals were found.
In 1934, John Desmond Bernal and his student Dorothy Hodgkin discovered that protein crystals surrounded by their mother liquor gave better diffraction patterns than dried crystals. Using pepsin, they were the first to discern the diffraction pattern of a wet, globular protein. Prior to Bernal and Hodgkin, protein crystallography had only been performed in dry conditions with inconsistent and unreliable results. This is the first X‐ray diffraction pattern of a protein crystal.
In 1958, the structure of myoglobin (a red protein containing heme), determined by X-ray crystallography, was first reported by John Kendrew. Kendrew shared the 1962 Nobel Prize in Chemistry with Max Perutz for this discovery.
Now, based on the protein crystals, the structures of them play a significant role in biochemistry and translational medicine. | 1 | Crystallography |
The strictest form of order in a solid is lattice periodicity: a certain pattern (the arrangement of atoms in a unit cell) is repeated again and again to form a translationally invariant tiling of space. This is the defining property of a crystal. Possible symmetries have been classified in 14 Bravais lattices and 230 space groups.
Lattice periodicity implies long-range order: if only one unit cell is known, then by virtue of the translational symmetry it is possible to accurately predict all atomic positions at arbitrary distances. During much of the 20th century, the converse was also taken for granted – until the discovery of quasicrystals in 1982 showed that there are perfectly deterministic tilings that do not possess lattice periodicity.
Besides structural order, one may consider charge ordering, spin ordering, magnetic ordering, and compositional ordering. Magnetic ordering is observable in neutron diffraction.
It is a thermodynamic entropy concept often displayed by a second-order phase transition. Generally speaking, high thermal energy is associated with disorder and low thermal energy with ordering, although there have been violations of this. Ordering peaks become apparent in diffraction experiments at low energy. | 1 | Crystallography |
The Wigner–Seitz cell always has the same point symmetry as the underlying Bravais lattice. For example, the cube, truncated octahedron, and rhombic dodecahedron have point symmetry O, since the respective Bravais lattices used to generate them all belong to the cubic lattice system, which has O point symmetry. | 1 | Crystallography |
Crystal violet or gentian violet, also known as methyl violet 10B or hexamethyl pararosaniline chloride, is a triarylmethane dye used as a histological stain and in Gram's method of classifying bacteria. Crystal violet has antibacterial, antifungal, and anthelmintic (vermicide) properties and was formerly important as a topical antiseptic. The medical use of the dye has been largely superseded by more modern drugs, although it is still listed by the World Health Organization.
The name gentian violet was originally used for a mixture of methyl pararosaniline dyes (methyl violet), but is now often considered a synonym for crystal violet. The name refers to its colour, being like that of the petals of certain gentian flowers; it is not made from gentians or violets. | 0 | Chromatography + Titration + pH indicators |
The Martin Medal is an award given for outstanding contributions to the advancement of separation science. The award is presented by The Chromatographic Society, a UK-based organization promoting all aspects of chromatography and related separation techniques. The award is named after Professor Archer J.P Martin, who contributed to the invention of partition chromatography, and shared the 1952 Nobel Prize in Chemistry. | 0 | Chromatography + Titration + pH indicators |
The metric tensor is sometimes used for calculations involving the unit cell and is defined (in matrix form) as:
In two dimensions,
In three dimensions,
The distance between two points and in the unit cell can be determined from the relation:
The distance from the origin of the unit cell to a point within the unit cell can be determined from the relation:
The angle formed from three points , (apex), and within the unit cell can determined from the relation:
The volume of the unit cell, can be determined from the relation: | 1 | Crystallography |
#Rutile (TiO) frequently exhibits a prismatic or acicular growth habit. In the presence of alkali dopants or a solid-state ZrSiO dopant, rutile has been observed to crystallise from an anatase parent-phase in the form of abnormally large grains existing in a matrix of finer, equiaxed anatase or rutile grains.
# Alumina, AlO with silica and/or yttria dopants/impurities has been reported to exhibit undesirable AGG.
#BaTiO barium titanate with an excess of TiO is known to exhibit abnormal grain growth with profound consequences on piezoelectric performance.
#Tungsten carbide has been reported to exhibit AGG of faceted grains in the presence of a liquid cobalt-containing phase at grain boundaries
#Silicon nitride (SiN) may exhibit AGG depending on the size distribution of β-phase material in an α-SiN precursor. This type of grain growth is of importance in the toughening of silicon nitride materials
#Silicon carbide has been shown to exhibit improved fracture toughness as the result of AGG processes yielding elongated crack-tip/wake-bridging grains, with consequences for applications in ballistic armor. This enhancement of fracture toughness in ceramic materials via crack-bridging resulting from AGG is consistent with reported morphological effects on crack propagation in ceramics
#Strontium barium niobate, used for electro-optics and dielectric applications, is known to exhibit AGG with significant consequences on the electronic performance of the material
#Calcium titanate (CaTiO, perovskite) systems doped with BaO have been observed to exhibit AGG without the formation of liquid as the result of polytype interfaces between solid phases | 1 | Crystallography |
A slurry of resin, such as DEAE-Sephadex is poured into the column. The matrix that is used is insoluble with charged groups that are covalently attached. These charged groups are referred to as exchangers like cation and anion exchangers. After it settles, the column is pre-equilibrated in buffer before the protein mixture is applied. DEAE-Sephadex is a positively-charged slurry that will have electrostatic interactions with the negatively charged atoms, making them elute later than the positively-charged molecules in the interested sample. This is a separation technique used widely to discover specific proteins, or enzymes in the body. Unbound proteins are collected in the flow-through and/or in subsequent buffer washes. Proteins that bind to the positively charged resin are retained and can be eluted in one of two ways. First, the salt concentration in the elution buffer is gradually increased. The negative ions in the salt solution (e.g. Cl) compete with protein in binding to the resin. Second, the pH of the solution can be gradually decreased which results in a more positive charge on the protein, releasing it from the resin. Both of these techniques can displace the negatively charged protein which is then eluted into test tubes fractions with the buffer.
The separation of proteins will depend on the differences in total charge. Composition of ionizable side chain groups will determine the total charge of the protein at a particular pH. At the isoelectric point (pI), the total charge on the protein is 0 and it will not bind to the matrix. If the pH is above the pI, the protein will have a negative charge and bind to the matrix in an anion exchange column. The stability of the protein at values above or below the pI, will determine if an anion exchange column or cation exchange column should be used. If it is stable at pH values below the pI, the cation exchange column be used. If it is stable at pH values above the pI then the anion exchange column can be used. | 0 | Chromatography + Titration + pH indicators |
It can complement X-ray crystallography for studies of very small crystals (<0.1 micrometers), both inorganic, organic, and proteins, such as membrane proteins, that cannot easily form the large 3-dimensional crystals required for that process. Protein structures are usually determined from either 2-dimensional crystals (sheets or helices), polyhedrons such as viral capsids, or dispersed individual proteins. Electrons can be used in these situations, whereas X-rays cannot, because electrons interact more strongly with atoms than X-rays do. Thus, X-rays will travel through a thin 2-dimensional crystal without diffracting significantly, whereas electrons can be used to form an image. Conversely, the strong interaction between electrons and protons makes thick (e.g. 3-dimensional > 1 micrometer) crystals impervious to electrons, which only penetrate short distances.
One of the main difficulties in X-ray crystallography is determining phases in the diffraction pattern. Because of the complexity of X-ray lenses, it is difficult to form an image of the crystal being diffracted, and hence phase information is lost. Fortunately, electron microscopes can resolve atomic structure in real space and the crystallographic structure factor phase information can be experimentally determined from an image's Fourier transform. The Fourier transform of an atomic resolution image is similar, but different, to a diffraction pattern—with reciprocal lattice spots reflecting the symmetry and spacing of a crystal. Aaron Klug was the first to realize that the phase information could be read out directly from the Fourier transform of an electron microscopy image that had been scanned into a computer, already in 1968. For this, and his studies on virus structures and transfer-RNA, Klug received the Nobel Prize for chemistry in 1982. | 1 | Crystallography |
Any periodic tiling can be seen as a wallpaper. More particularly, we can consider as a wallpaper a tiling by identical tiles edge‑to‑edge, necessarily periodic, and conceive from it a wallpaper by decorating in the same manner every tiling element, and eventually erase partly or entirely the boundaries between these tiles. Conversely, from every wallpaper we can construct such a tiling by identical tiles edge‑to‑edge, which bear each identical ornaments, the identical outlines of these tiles being not necessarily visible on the original wallpaper. Such repeated boundaries delineate a repetitive surface added here in dashed lines.
Such pseudo‑tilings connected to a given wallpaper are in infinite number. For example image 1 shows two models of repetitive squares in two different positions, which have Another repetitive square has an We could indefinitely conceive such repetitive squares larger and larger. An infinity of shapes of repetitive zones are possible for this Pythagorean tiling, in an infinity of positions on this wallpaper. For example in red on the bottom right‑hand corner of image 1, we could glide its repetitive parallelogram in one or another position. In common on the first two images: a repetitive square concentric with each small square tile, their common center being a point symmetry of the wallpaper.
Between identical tiles edge‑to‑edge, an edge is not necessarily a segment of a straight line. On the top left‑hand corner of image 3, point C is a vertex of a repetitive pseudo‑rhombus with thick stripes on its whole surface, called pseudo‑rhombus because of a concentric repetitive rhombus constructed from it by taking out a bit of surface somewhere to append it elsewhere, and keep the area unchanged. By the same process on image 4, a repetitive regular hexagon filled with vertical stripes is constructed from a rhombic repetitive zone Conversely, from elementary geometric tiles edge‑to‑edge, an artist like M. C. Escher created attractive surfaces many times repeated. On image 2, the minimum area of a repetitive surface by disregarding colors, each repetitive zone in dashed lines consisting of five pieces in a certain arrangement, to be either a square or a hexagon, like in a proof of the Pythagorean theorem.
In the present article, a pattern is a repetitive parallelogram of minimal area in a determined position on the wallpaper. Image 1 shows two parallelogram‑shaped patterns — a square is a particular parallelogram —. Image 3 shows rhombic patterns — a rhombus is a particular parallelogram —.
On this page, all repetitive patterns (of minimal area) are constructed from two translations that generate the group of all translations under which the wallpaper is invariant. With the circle shaped symbol ⵔ of function composition, a pair like or generates the group of all translations that transform the Pythagorean tiling into itself. | 1 | Crystallography |
Thermal laser epitaxy (TLE) is a physical vapor deposition technique that utilizes irradiation from continuous-wave lasers to heat sources locally for growing films on a substrate. This technique can be performed under ultra-high vacuum pressure or in the presence of a background atmosphere, such as ozone, to deposit oxide films.
TLE operates at power densities between 10 – 10 W/cm, which results in evaporation or sublimation of the source material, with no plasma or high-energy particle species being produced. Despite operating at comparatively low power densities, TLE is capable of depositing many materials with low vapor pressures, including refractory metals, a process that is challenging to perform with molecular beam epitaxy. | 1 | Crystallography |
Gentian violet has antibacterial, antifungal, antihelminthic, antitrypanosomal, antiangiogenic, and antitumor properties. It is used medically for these properties, in particular for dentistry, and is also known as "pyoctanin" (or "pyoctanine"). It is commonly used for:
* Marking the skin for surgery preparation and allergy testing;
* Treating Candida albicans and related fungal infections, such as thrush, yeast infections, various types of tinea (ringworm, athlete's foot, jock itch);
* Treating impetigo; it was used primarily before the advent of antibiotics, but still useful to persons who may be allergic to penicillin.
In resource-limited settings, gentian violet is used to manage burn wounds, inflammation of the umbilical cord stump (omphalitis) in the neonatal period, oral candidiasis in HIV-infected patients and mouth ulcers in children with measles. | 0 | Chromatography + Titration + pH indicators |
* The analyst wishes to simplify the conduct of a variety of titrations by using one sensor for all. For example, a laboratory might conduct routinely acid/base, redox, complexometric, sulfate and chloride titrations. A single thermometric sensor in conjunction with an autosampler will enable all titrations to be performed in the same carousel load without having to change titration sensors. After preparation of the samples and placing in the carousel, the analyst assigns the appropriate thermometric method to the beaker position in the carousel.
* The titration environment is considered unsuitable for conventional titration sensors. For example, glass membrane pH electrodes must be kept adequately hydrated for proper operation. The use of such electrodes in substantially non-aqueous media as in the determination of trace acids in lipids and lubricating oils can lead to loss of performance as the membrane fouls and dehydrates, and/or if the reference junction is partly or completely blocked. It is often necessary to keep a number of electrodes cycling through a rejuvenation program in order to keep up with an analytical workload. Thermometric sensors have no electrochemical interaction with the titrating solution, and therefore can be used on a continuous basis with essentially no maintenance. Similarly, the potentiometric titration of sulfate with barium chloride in various industrial samples can lead to rapid degradation of the indicating barium ion selective electrode.
* A thermometric titration methodology which cannot be emulated using other types of titration sensors will deliver superior or results otherwise unobtainable by other techniques. Examples are the determination of fluoride by titration with boric acid, the analysis of orthophosphate by titration with magnesium ions, and the direct titration of aluminium with fluoride ions. | 0 | Chromatography + Titration + pH indicators |
The reciprocal to a simple hexagonal Bravais lattice with lattice constants and is another simple hexagonal lattice with lattice constants and rotated through 90° about the c axis with respect to the direct lattice. The simple hexagonal lattice is therefore said to be self-dual, having the same symmetry in reciprocal space as in real space. Primitive translation vectors for this simple hexagonal Bravais lattice vectors are | 1 | Crystallography |
MCP-RHEED is a system in which an electron beam is amplified by a micro-channel plate (MCP). This system consists of an electron gun and an MCP equipped with a fluorescent screen opposite to the electron gun. Because of the amplification, the intensity of the electron beam can be decreased by several orders of magnitude and the damage to the samples is diminished. This method is used to observe the growth of insulator crystals such as organic films and alkali halide films, which are easily damaged by electron beams. | 1 | Crystallography |
In 1925, Coward received a Rockefeller Fellowship to continue her studies and research on vitamin A in the Department of Agricultural Chemistry at the University of Wisconsin–Madison under Dr. Harry Steenbock. On her return to Britain, she was appointed head of the Nutrition Department of the Royal Pharmaceutical Society's pharmacological laboratories, the position which she remained until her retirement in 1950. In 1937 she was elected as an honorary member of the Pharmaceuticals Society. | 0 | Chromatography + Titration + pH indicators |
The principles involved can be understood by considering the most efficient way of packing together equal-sized spheres and stacking close-packed atomic planes in three dimensions. For example, if plane A lies beneath plane B, there are two possible ways of placing an additional atom on top of layer B. If an additional layer were placed directly over plane A, this would give rise to the following series:
:...ABABABAB...
This arrangement of atoms in a crystal structure is known as hexagonal close packing (hcp).
If, however, all three planes are staggered relative to each other and it is not until the fourth layer is positioned directly over plane A that the sequence is repeated, then the following sequence arises:
:...ABCABCABC...
This type of structural arrangement is known as cubic close packing (ccp).
The unit cell of a ccp arrangement of atoms is the face-centered cubic (fcc) unit cell. This is not immediately obvious as the closely packed layers are parallel to the {111} planes of the fcc unit cell. There are four different orientations of the close-packed layers. | 1 | Crystallography |
PED possesses many advantageous attributes that make it well suited to investigating crystal structures via direct methods approaches:
# Quasi-kinematical diffraction patterns: While the underlying physics of the electron diffraction is still dynamical in nature, the conditions used to collect PED patterns minimize many of these effects. The scan/de-scan procedure reduces ion channeling because the pattern is generated off of the zone axis. Integration via precession of the beam minimizes the effect of non-systematic inelastic scattering, such as Kikuchi lines. Few reflections are strongly excited at any moment during precession, and those that are excited are generally much closer to a two-beam condition (dynamically coupled only to the forward-scattered beam). Furthermore, for large precession angles, the radius of the excited Laue circle becomes quite large. These contributions combine such that the overall integrated diffraction pattern resembles the kinematical pattern much more closely than a single zone axis pattern.
# Broader range of measured reflections: The Laue circle (see Ewald sphere) that is excited at any given moment during precession extends farther into reciprocal space. After integration over multiple precessions, many more reflections in the zeroeth order Laue zone (ZOLZ) are present, and as stated previously, their relative intensities are much more kinematical. This provides considerably more information to input into direct methods calculations, improving the accuracy of phase determination algorithms. Similarly, more higher order Laue zone (HOLZ) reflections are present in the pattern, which can provide more complete information about the three-dimensional nature of reciprocal space, even in a single two-dimensional PED pattern.
# Practical robustness: PED is less sensitive to small experimental variations than other electron diffraction techniques. Since the measurement is an average over many incident beam directions, the pattern is less sensitive to slight misorientation of the zone axis from the optic axis of the microscope, and resulting PED patterns will generally still display the zone axis symmetry. The patterns obtained are also less sensitive to the thickness of the sample, a parameter with strong influence in standard electron diffraction patterns.
# Very small probe size: Because x-rays interact so weakly with matter, there is a minimum size limit of approximately 5 µm for single crystals that can be examined via x-ray diffraction methods. In contrast, electrons can be used to probe much smaller nano-crystals in a TEM. In PED, the probe size is limited by the lens aberrations and sample thickness. With a typical value for spherical aberration, the minimum probe size is usually around 50 nm. However, with Cs corrected microscopes, the probe can be made much smaller. | 1 | Crystallography |
Phenol red, sometimes labelled with a different name, such as "Guardex Solution #2", is used as a pH indicator in home swimming pool test kits.
Chlorine can result in the bleaching of the dye in the absence of thiosulfate to inhibit the oxidizing chlorine. High levels of bromine can convert phenol red to bromophenol red (dibromophenolsulfonephthalein, whose lowered pK results in an indicator with a range shifted in the acidic direction – water at pH 6.8 will appear to test at 7.5). Even higher levels of bromine (>20 ppm) can result in the secondary conversion of bromophenol red to bromophenol blue with an even lower pK, erroneously giving the impression that the water has an extremely high pH despite being dangerously low. | 0 | Chromatography + Titration + pH indicators |
Alizarine Yellow R is produced by azo coupling of salicylic acid and diazonium derivative of 4-Nitroaniline | 0 | Chromatography + Titration + pH indicators |
Blood plasma is the liquid component of blood, which contains dissolved proteins, nutrients, ions, and other soluble components. In whole blood, red blood cells, white blood cells, and platelets are suspended within the plasma. The goal of plasma purification and processing is to extract specific materials that are present in blood, and use them for restoration and repair. There are several components that make up blood plasma, one of which is the protein albumin. Albumin is a highly water-soluble protein with considerable structural stability. It serves as a transportation device for materials such as hormones, enzymes, fatty acids, metal ions, and medicinal products. It is also used for therapeutic purposes, being essential in restoration and maintenance of circulating blood volume in imperative situations such as severe trauma or surgery. With little room for error, extremely pure samples that are lacking impurities needs to be at hand in good amount. Human blood plasma is important for the body so the nutrients etc. can be stored. | 0 | Chromatography + Titration + pH indicators |
* The advantages of an oxidation-reduction reactor in gas chromatography include
* The reactor ensures uniform sensitivity to most organic molecules, leading to consistent and reliable detection across a wide range of analytes.
* By eliminating the need for multiple calibrations and standards, the reactor increases the accuracy of quantification, reducing errors and enhancing the reliability of analytical results.
* Reduction in calibration requirements decreases the cost of ownership and saves time, making the analytical process more efficient.
* The reactor enables the quantification of complex mixtures even when standards are not available, provided retention times are known or can be estimated, thereby expanding the applicability of gas chromatography.
* Unlike traditional methanizers, which primarily convert CO and CO2, oxidation-reduction reactors can convert a broader range of organic compounds to methane, leading to a more comprehensive response and improved sensitivity for a wide variety of analytes.
* These reactors are more resistant to poisoning by compounds containing nitrogen and oxygen, ensuring consistent performance even in the presence of interfering substances.
* Compared to packed column versions of methanizers, oxidation-reduction reactors typically produce sharper peaks, enhancing resolution and improving the quality of chromatographic separation. | 0 | Chromatography + Titration + pH indicators |
In crystallography, a crystallographic point group is a three dimensional point group whose symmetry operations are compatible with a three dimensional crystallographic lattice. According to the crystallographic restriction it may only contain one-, two-, three-, four- and sixfold rotations or rotoinversions. This reduces the number of crystallographic point groups to 32 (from an infinity of general point groups). These 32 groups are one-and-the-same as the 32 types of morphological (external) crystalline symmetries derived in 1830 by Johann Friedrich Christian Hessel from a consideration of observed crystal forms.
In the classification of crystals, to each space group is associated a crystallographic point group by "forgetting" the translational components of the symmetry operations. That is, by turning screw rotations into rotations, glide reflections into reflections and moving all symmetry elements into the origin. Each crystallographic point group defines the (geometric) crystal class of the crystal.
The point group of a crystal determines, among other things, the directional variation of physical properties that arise from its structure, including optical properties such as birefringency, or electro-optical features such as the Pockels effect. | 1 | Crystallography |
The aforementioned hydrodynamic and hydrostatic instruments may be employed in a variety of ways, or modes of operation, in order to address the particular separation needs of the scientist. Many modes of operation have been devised to take advantage of the strengths and potentialities of the countercurrent chromatography technique. Generally, the following modes may be performed with commercially available instruments. | 0 | Chromatography + Titration + pH indicators |
Common sphere packings taken on by atomic systems are listed below with their corresponding packing fraction.
* Hexagonal close-packed (HCP): 0.74
* Face-centered cubic (FCC): 0.74 (also called cubic close-packed, CCP)
* Body-centered cubic (BCC): 0.68
* Simple cubic: 0.52
* Diamond cubic: 0.34
The majority of metals take on either the HCP, FCC, or BCC structure. | 1 | Crystallography |
Centrifugal partition chromatography is a special chromatographic technique where both stationary and mobile phase are liquid, and the stationary phase is immobilized by a strong centrifugal force. Centrifugal partition chromatography consists of a series-connected network of extraction cells, which operates as elemental extractors, and the efficiency is guaranteed by the cascade. | 0 | Chromatography + Titration + pH indicators |
Different methods to determine the equivalence point include:
;pH indicator: A pH indicator is a substance that changes color in response to a chemical change. An acid-base indicator (e.g., phenolphthalein) changes color depending on the pH. Redox indicators are also frequently used. A drop of indicator solution is added to the titration at the start; when the color changes the endpoint has been reached, this is an approximation of the equivalence point.
;Conductance: The conductivity of a solution depends on the ions that are present in it. During many titrations, the conductivity changes significantly. (For instance, during an acid-base titration, the HO and OH ions react to form neutral HO. This changes the conductivity of the solution.) The total conductance of the solution depends also on the other ions present in the solution (such as counter ions). Not all ions contribute equally to the conductivity; this also depends on the mobility of each ion and on the total concentration of ions (ionic strength). Thus, predicting the change in conductivity is harder than measuring it.
;Color change: In some reactions, the solution changes color without any added indicator. This is often seen in redox titrations, for instance, when the different oxidation states of the product and reactant produce different colors.
;Precipitation: If the reaction forms a solid, then a precipitate will form during the titration. A classic example is the reaction between Ag and Cl to form the very insoluble salt AgCl. Surprisingly, this usually makes it difficult to determine the endpoint precisely. As a result, precipitation titrations often have to be done as back titrations.
;Isothermal titration calorimeter: An isothermal titration calorimeter uses the heat produced or consumed by the reaction to determine the equivalence point. This is important in biochemical titrations, such as the determination of how substrates bind to enzymes.
;Thermometric titrimetry: Thermometric titrimetry is an extraordinarily versatile technique. This is differentiated from calorimetric titrimetry by the fact that the heat of the reaction (as indicated by temperature rise or fall) is not used to determine the amount of analyte in the sample solution. Instead, the equivalence point is determined by the rate of temperature change. Because thermometric titrimetry is a relative technique, it is not necessary to conduct the titration under isothermal conditions, and titrations can be conducted in plastic or even glass vessels, although these vessels are generally enclosed to prevent stray draughts from causing "noise" and disturbing the endpoint. Because thermometric titrations can be conducted under ambient conditions, they are especially well-suited to routine process and quality control in industry. Depending on whether the reaction between the titrant and analyte is exothermic or endothermic, the temperature will either rise or fall during the titration. When all analyte has been consumed by reaction with the titrant, a change in the rate of temperature increase or decrease reveals the equivalence point and an inflection in the temperature curve can be observed. The equivalence point can be located precisely by employing the second derivative of the temperature curve. The software used in modern automated thermometric titration systems employ sophisticated digital smoothing algorithms so that "noise" resulting from the highly sensitive temperature probes does not interfere with the generation of a smooth, symmetrical second derivative "peak" which defines the endpoint. The technique is capable of very high precision, and coefficients of variance (CV's) of less than 0.1 are common. Modern thermometric titration temperature probes consist of a thermistor which forms one arm of a Wheatstone bridge. Coupled to high resolution electronics, the best thermometric titration systems can resolve temperatures to 10K. Sharp equivalence points have been obtained in titrations where the temperature change during the titration has been as little as 0.001K. The technique can be applied to essentially any chemical reaction in a fluid where there is an enthalpy change, although reaction kinetics can play a role in determining the sharpness of the endpoint. Thermometric titrimetry has been successfully applied to acid-base, redox, EDTA, and precipitation titrations. Examples of successful precipitation titrations are sulfate by titration with barium ions, phosphate by titration with magnesium in ammoniacal solution, chloride by titration with silver nitrate, nickel by titration with dimethylglyoxime and fluoride by titration with aluminium (as KNaAlF) Because the temperature probe does not need to be electrically connected to the solution (as in potentiometric titrations), non-aqueous titrations can be carried out as easily as aqueous titrations. Solutions which are highly colored or turbid can be analyzed by thermometric without further sample treatment. The probe is essentially maintenance-free. Using modern, high precision stepper motor driven burettes, automated thermometric titrations are usually complete in a few minutes, making the technique an ideal choice where high laboratory productivity is required.
;Spectroscopy: Spectroscopy can be used to measure the absorption of light by the solution during the titration, if the spectrum of the reactant, titrant or product is known. The relative amounts of the product and reactant can be used to determine the equivalence point. Alternatively, the presence of free titrant (indicating that the reaction is complete) can be detected at very low levels. An example of robust endpoint detectors for etching of semiconductors is EPD-6, a system probing reactions at up to six different wavelengths.
;Amperometry: Amperometry can be used as a detection technique (amperometric titration). The current due to the oxidation or reduction of either the reactants or products at a working electrode will depend on the concentration of that species in solution. The equivalence point can then be detected as a change in the current. This method is most useful when the excess titrant can be reduced, as in the titration of halides with Ag. (This is handy also in that it ignores precipitates.) | 0 | Chromatography + Titration + pH indicators |
Although there are several commercially available universal pH indicators, most are a variation of a formula patented by Yamada in 1933. | 0 | Chromatography + Titration + pH indicators |
Much of the past research into the composition and toxicity of UCM hydrocarbons has been conducted by the Petroleum and Environmental Geochemistry Group (PEGG) at the University of Plymouth, UK. As well as the hydrocarbon UCM, oils also contain more polar compounds such as those containing oxygen, sulphur or nitrogen. These compounds can be very soluble in water and hence bioavailable to marine and aquatic organisms. Polar UCMs are present within produced waters from oil rigs and from oil sands processing. A polar UCM fraction extracted from North Sea oil produced water was reported to elicit hormonal disruption by way of both estrogen receptor agonist and androgen receptor agonist activity. Ongoing concern regarding the potential toxicity of components within Athabasca Oil Sands (Canada) tailings ponds has highlighted the need for identification of the compounds present. Until recently, such positive identification of individual so-called naphthenic acids from oil sands produced waters had so far eluded characterisation but recent research by PEGG presented at a SETAC conference in 2010 revealed that, using a new GCxGC-TOF-MS, it was possible to resolve and identify a range of new compounds within such highly complex extracts. One group of compounds found to be present were tricyclic diamondoid acids. These structures had previously not even been considered as naphthenic acids and suggests an unprecedented degree of biodegradation of some of the oil in the oil sands. | 0 | Chromatography + Titration + pH indicators |
By far the most common and perhaps standard form of ionization is electron ionization (EI). The molecules enter into the MS (the source is a quadrupole or the ion trap itself in an ion trap MS) where they are bombarded with free electrons emitted from a filament, not unlike the filament one would find in a standard light bulb. The electrons bombard the molecules, causing the molecule to fragment in a characteristic and reproducible way. This "hard ionization" technique results in the creation of more fragments of low mass-to-charge ratio (m/z) and few, if any, molecules approaching the molecular mass unit. Hard ionization is considered by mass spectrometrists as the employ of molecular electron bombardment, whereas "soft ionization" is charge by molecular collision with an introduced gas. The molecular fragmentation pattern is dependent upon the electron energy applied to the system, typically 70 eV (electronvolts). The use of 70 eV facilitates comparison of generated spectra with library spectra using manufacturer-supplied software or software developed by the National Institute of Standards (NIST-USA). Spectral library searches employ matching algorithms such as Probability Based Matching and dot-product matching that are used with methods of analysis written by many method standardization agencies. Sources of libraries include NIST, Wiley, the AAFS, and instrument manufacturers. | 0 | Chromatography + Titration + pH indicators |
In ion exchange chromatography, the Gibbs–Donnan effect is observed when the pH of the applied buffer and the ion exchanger differ, even up to one pH unit. For example, in anion-exchange columns, the ion exchangers repeal protons so the pH of the buffer near the column differs is higher than the rest of the solvent. As a result, an experimenter has to be careful that the protein(s) of interest is stable and properly charged in the "actual" pH.
This effect comes as a result of two similarly charged particles, one from the resin and one from the solution, failing to distribute properly between the two sides; there is a selective uptake of one ion over another. For example, in a sulphonated polystyrene resin, a cation exchange resin, the chlorine ion of a hydrochloric acid buffer should equilibrate into the resin. However, since the concentration of the sulphonic acid in the resin is high, the hydrogen of HCl has no tendency to enter the column. This, combined with the need of electroneutrality, leads to a minimum amount of hydrogen and chlorine entering the resin. | 0 | Chromatography + Titration + pH indicators |
The simple cubic Bravais lattice, with cubic primitive cell of side , has for its reciprocal a simple cubic lattice with a cubic primitive cell of side (or in the crystallographer's definition). The cubic lattice is therefore said to be self-dual, having the same symmetry in reciprocal space as in real space. | 1 | Crystallography |
In the RHEED setup, only atoms at the sample surface contribute to the RHEED pattern. The glancing angle of incident electrons allows them to escape the bulk of the sample and to reach the detector. Atoms at the sample surface diffract (scatter) the incident electrons due to the wavelike properties of electrons.
The diffracted electrons interfere constructively at specific angles according to the crystal structure and spacing of the atoms at the sample surface and the wavelength of the incident electrons. Some of the electron waves created by constructive interference collide with the detector, creating specific diffraction patterns according to the surface features of the sample. Users characterize the crystallography of the sample surface through analysis of the diffraction patterns. Figure 2 shows a RHEED pattern. Video 1 depicts a metrology instrument recording the RHEED intensity oscillations and deposition rate for process control and analysis.
Two types of diffraction contribute to RHEED patterns. Some incident electrons undergo a single, elastic scattering event at the crystal surface, a process termed kinematic scattering. Dynamic scattering occurs when electrons undergo multiple diffraction events in the crystal and lose some of their energy due to interactions with the sample. Users extract non-qualitative data from the kinematically diffracted electrons. These electrons account for the high intensity spots or rings common to RHEED patterns. RHEED users also analyze dynamically scattered electrons with complex techniques and models to gather quantitative information from RHEED patterns. | 1 | Crystallography |
CrysTBox offers tools for automated processing of diffraction patterns and high-resolution transmission electron microscope images. Since the tools employ algorithms of artificial intelligence and computer vision, they are designed to require minimal operator effort providing higher accuracy compared to manual evaluation. Four analytical tools can be used to index diffraction patterns, measure lattice constants (distances and angles), sample thickness etc. Despite the high level of automation, the user is able to control the whole process and perform individual steps manually if needed. | 1 | Crystallography |
Macle is a term used in crystallography. It is a crystalline form, twin-crystal or double crystal (such as chiastolite). It is crystallographic twin according to the spinel twin law and is seen in octahedral crystals or minerals such as diamond and spinel. The twin law name comes from the fact that is commonly observed in the mineral spinel.
Macle is an old French word, a heraldic term for a voided lozenge (one diamond shape within another). Etymologically the word is derived from the Latin macula meaning spot, mesh, or hole. | 1 | Crystallography |
Order strengthening brought about from the interaction of dislocations with ordered precipitates, forming anti-phase boundaries as dislocations move throughout the crystal, can lead to significant increases in strength and creep resistance. For this reason, order strengthening is often exploited for high-temperature creep resistant superalloys used in turbine blades.
Antiphase domains carry with them a surface energy penalty when compared to the perfect lattice due to their chemical disorder, and the presence of these boundaries impedes dislocation motion throughout the crystal, leading to increased strength under shear stress. Figure 3 below shows the process of an edge dislocation propagating through an ordered particle. As the dislocation moves throughout the particle, lattice planes are displaced from their equilibrium configuration, and A-A bonds and B-B bonds are formed throughout the slip plane. This forms a higher energy state than when compared to the equilibrium A-B bonding configuration, and the change in energy is called the anti-phase boundary energy (APBE). This can increase the degree of strengthening created from precipitation hardening, making it more difficult for cutting to occur, and instead increasing the likelihood of Orowan bowing around the precipitate.
<br />
Figure 3: The process of an edge dislocation moving through an ordered precipitate. In (a), the perfectly ordered particle is shown. In (b), the dislocations has moved through part of the particle. In (c), the dislocation exits the precipitate, leading to an increase in surface energy from increased surface area and a higher-energy bonding configuration.
Order strengthening is often characterized by a ratio of the attractive anti-phase boundary energy (APBE) to the repulsive dislocation energy(Gb): . The degree of order strengthening depends on both this ratio and whether the alloy is in the early or late stages of precipitation. When is low, the trailing dislocation moves far behind the leading dislocations, leading to separate cutting of precipitates as seen in Figure 4a. Alternatively, when is high, the trailing dislocation follows close behind the leading dislocation, leading to common cutting as seen in Figure 4b. During the early stages of precipitation, the increase in shear stress can be expressed as:
for low or
for high where G is the shear modulus, f is the volume fraction of precipitates, r is the radius of the precipitate, and b is the burgers vector of the dislocation.
In the later stages of precipitation, the analogous expressions are:
for low or
for high .
<br />
Figure 4: Dislocation motion around precipitates. | 1 | Crystallography |
In microbiology, methyl red is used in the methyl red test (MR test), used to identify bacteria producing stable acids by mechanisms of mixed acid fermentation of glucose (cf. Voges–Proskauer test).
The MR test, the "M" portion of the four IMViC tests, is used to identify enteric bacteria based on their pattern of glucose metabolism. All enterics initially produce pyruvic acid from glucose metabolism. Some enterics subsequently use the mixed acid pathway to metabolize pyruvic acid to other acids, such as lactic, acetic, and formic acids. These bacteria are called methyl-red positive and include Escherichia coli and Proteus vulgaris. Other enterics subsequently use the butylene glycol pathway to metabolize pyruvic acid to neutral end products. These bacteria are called methyl-red-negative and include Serratia marcescens and Enterobacter aerogenes. | 0 | Chromatography + Titration + pH indicators |
Anthocyanins are approved for use as food colorants in the European Union, Australia, and New Zealand, having colorant code E163. In 2013, a panel of scientific experts for the European Food Safety Authority concluded that anthocyanins from various fruits and vegetables have been insufficiently characterized by safety and toxicology studies to approve their use as food additives. Extending from a safe history of using red grape skin extract and blackcurrant extracts to color foods produced in Europe, the panel concluded that these extract sources were exceptions to the ruling and were sufficiently shown to be safe.
Anthocyanin extracts are not specifically listed among approved color additives for foods in the United States; however, grape juice, red grape skin and many fruit and vegetable juices, which are approved for use as colorants, are rich in naturally occurring anthocyanins. No anthocyanin sources are included among approved colorants for drugs or cosmetics. When esterified with fatty acids, anthocyanins can be used as a lipophilic colorant for foods. | 0 | Chromatography + Titration + pH indicators |
In materials science, the Burgers vector, named after Dutch physicist Jan Burgers, is a vector, often denoted as , that represents the magnitude and direction of the lattice distortion resulting from a dislocation in a crystal lattice. | 1 | Crystallography |
Weak affinity chromatography (WAC) is an affinity chromatography technique for affinity screening in drug development. WAC is an affinity-based liquid chromatographic technique that separates chemical compounds based on their different weak affinities to an immobilized target. The higher affinity a compound has towards the target, the longer it remains in the separation unit, and this will be expressed as a longer retention time. The affinity measure and ranking of affinity can be achieved by processing the obtained retention times of analyzed compounds. Affinity chromatography is part of a larger suite of techniques used in chemoproteomics based drug target identification.
The WAC technology is demonstrated against a number of different protein targets – proteases, kinases, chaperones and protein–protein interaction (PPI) targets. WAC has been shown to be more effective than established methods for fragment based screening. | 0 | Chromatography + Titration + pH indicators |
Similar to yttrium, rare-earth metals from Gd to Lu can form REBSi-type boride. The first such compound was synthesized by solid-state reaction and its structure was deduced as YB. X-ray powder diffraction (XRD) and electron diffraction indicated that YB has an orthorhombic structure with lattice constants a = 1.66251(9), b = 1.76198 and c = 0.94797(3) nm. The space group was assigned as P222. Because of the close similarity in lattice constants and space group, one might expect that YB has the γ-AlB-type orthorhombic structure whose lattice constants and space group are a = 1.6573(4), b = 1.7510(3) and c = 1.0144(1) nm and P222. YB decomposes at ~1750 °C without melting that hinders growth of single crystals from the melt. Small addition of silicon made YB to melt without decomposition, and so enabled single-crystal growth from the melt and single-crystal structure analysis.
The structure analysis indicated that YBSi has not the γ-AlB-type lattice but a rare orthorhombic crystal structure (space group: Pbam, No. 55) with lattice constants of a = 1.674(1) nm, b = 1.7667(1) nm and c = 0.9511(7) nm. There are 58 independent atomic sites in the unit cell. Three of them are occupied by either B or Si atoms (mixed-occupancy sites), one is a Si bridge site and one is Y site. From the remaining 53 boron sites, 48 form icosahedra and 5 are bridging sites. Atomic coordinates and site occupancies are summarized in table III.
The boron framework of YBSi consists of five B icosahedra (I1–I5) and a BSi polyhedron shown in figure 8a. An unusual linkage is depicted in figure 8b, where two B-I5 icosahedra connect via two B atoms of each icosahedron forming an imperfect square. The boron framework of YBSi can be described as a layered structure where two boron networks (figures 9a,b) stack along the z-axis. One boron network consists of 3 icosahedra I1, I2 and I3 and is located in the z = 0 plane; another network consists of the icosahedron I5 and the BSi polyhedron and lies at z = 0.5. The icosahedron I4 bridges these networks, and thus its height along the z-axis is 0.25.
The I4 icosahedra link two networks along the c-axis and therefore form an infinite chain of icosahedra along this axis as shown in figure 10. The unusually short distances (0.4733 and 0.4788 nm) between the neighboring icosahedra in this direction result in the relatively small c-axis lattice constant of 0.95110(7) nm in this compound – other borides with a similar icosahedral chain have this value larger than 1.0 nm. However, the bonding distances between the apex B atoms (0.1619 and 0.1674 nm) of neighboring I4 icosahedra are usual for the considered metal borides.
Another unusual feature of YBSi is the 100% occupancy of the Y site. In most icosahedron-based metal borides, metal sites have rather low site occupancy, for example, about 50% for YB and 60–70% for REAlB. When the Y site is replaced by rare-earth elements, REBSi can have an antiferromagnetic-like ordering because of this high site occupancy. | 1 | Crystallography |
Redox titrations are based on a reduction-oxidation reaction between an oxidizing agent and a reducing agent. A potentiometer or a redox indicator is usually used to determine the endpoint of the titration, as when one of the constituents is the oxidizing agent potassium dichromate. The color change of the solution from orange to green is not definite, therefore an indicator such as sodium diphenylamine is used. Analysis of wines for sulfur dioxide requires iodine as an oxidizing agent. In this case, starch is used as an indicator; a blue starch-iodine complex is formed in the presence of excess iodine, signalling the endpoint.
Some redox titrations do not require an indicator, due to the intense color of the constituents. For instance, in permanganometry a slight persisting pink color signals the endpoint of the titration because of the color of the excess oxidizing agent potassium permanganate. In iodometry, at sufficiently large concentrations, the disappearance of the deep red-brown triiodide ion can itself be used as an endpoint, though at lower concentrations sensitivity is improved by adding starch indicator, which forms an intensely blue complex with triiodide. | 0 | Chromatography + Titration + pH indicators |
Dye-ligand affinity chromatography is one of the Affinity chromatography techniques used for protein purification of a complex mixture. Like general chromatography, but using dyes to apply on a support matrix of a column as the stationary phase that will allow a range of proteins with similar active sites to bind to, refers to as pseudo-affinity. Synthetic dyes are used to mimic substrates or cofactors binding to the active sites of proteins which can be further enhanced to target more specific proteins. Follow with washing, the process of removing other non-target molecules, then eluting out target proteins out by changing pH or manipulate the salt concentration. The column can be reused many times due to the stability of immobilized dyes. It can carry out in a conventional way by using as a packed column, or in high-performance liquid chromatography (HPLC) column. | 0 | Chromatography + Titration + pH indicators |
The column is a glass or plastic cylinder packed with beads of resin and filled with buffer solution. It is normally mounted vertically with the buffer flowing downward from top to bottom. A glass frit at the bottom of the column retains the resin beads in the column while allowing the buffer and dissolved proteins to exit. | 0 | Chromatography + Titration + pH indicators |
In geometry and crystallography, a Bravais lattice, named after , is an infinite array of discrete points generated by a set of discrete translation operations described in three dimensional space by
where the n are any integers, and a are primitive translation vectors, or primitive vectors, which lie in different directions (not necessarily mutually perpendicular) and span the lattice. The choice of primitive vectors for a given Bravais lattice is not unique. A fundamental aspect of any Bravais lattice is that, for any choice of direction, the lattice appears exactly the same from each of the discrete lattice points when looking in that chosen direction.
The Bravais lattice concept is used to formally define a crystalline arrangement and its (finite) frontiers. A crystal is made up of one or more atoms, called the basis or motif, at each lattice point. The basis may consist of atoms, molecules, or polymer strings of solid matter, and the lattice provides the locations of the basis.
Two Bravais lattices are often considered equivalent if they have isomorphic symmetry groups. In this sense, there are 5 possible Bravais lattices in 2-dimensional space and 14 possible Bravais lattices in 3-dimensional space. The 14 possible symmetry groups of Bravais lattices are 14 of the 230 space groups. In the context of the space group classification, the Bravais lattices are also called Bravais classes, Bravais arithmetic classes, or Bravais flocks. | 1 | Crystallography |
The simplest approach is to connect two commercial columns in series, which is termed a “tandem column”. Another approach is “biphasic column”, by packing two stationary phases separately in two ends of the same column. The third approach is to homogenize two or more different types of stationary phases in a single column, which is termed a “hybrid column” or “mixed-bed column”. | 0 | Chromatography + Titration + pH indicators |
LC–MS is frequently used in drug development because it allows quick molecular weight confirmation and structure identification. These features speed up the process of generating, testing, and validating a discovery starting from a vast array of products with potential application. LC–MS applications for drug development are highly automated methods used for peptide mapping, glycoprotein mapping, lipodomics, natural products dereplication, bioaffinity screening, in vivo drug screening, metabolic stability screening, metabolite identification, impurity identification, quantitative bioanalysis, and quality control. | 0 | Chromatography + Titration + pH indicators |
The Wyckoff positions are named after Ralph Walter Graystone Wyckoff, an American X-ray crystallographer who authored several books in the field. His 1922 book, The Analytical Expression of the Results of the Theory of Space Groups, contained tables with the positional coordinates, both general and special, permitted by the symmetry elements. This book was the forerunner of International Tables for X-ray Crystallography, which first appeared in 1935. | 1 | Crystallography |
In chemistry, isomorphism has meanings both at the level of crystallography and at a molecular level. In crystallography, crystals are isomorphous if they have identical symmetry and if the atomic positions can be described with a set of parameters (unit cell dimensions and fractional coordinates) whose numerical values differ only slightly.
Molecules are isomorphous if they have similar shapes. The coordination complexes tris(acetylacetonato)iron (Fe(acac)) and tris(acetylacetonato)aluminium (Al(acac)) are isomorphous. These compounds, both of D symmetry have very similar shapes, as determined by bond lengths and bond angles. Isomorphous compounds give rise to isomorphous crystals and form solid solutions. Historically, crystal shape was defined by measuring the angles between crystal faces with a goniometer. Whereas crystals of Fe(acac) are deep red and crystals of Al(acac) are colorless, a solid solution of the two, i.e. FeAl(acac) will be deep or pale pink depending on the Fe/Al ratio, x.
Double sulfates, such as Tutton's salt, with the generic formula MM(SO).6HO, where M is an alkali metal and M is a divalent ion of Mg, Mn, Fe, Co, Ni, Cu or Zn, form a series of isomorphous compounds which were important in the nineteenth century in establishing the correct atomic weights of the transition elements. Alums, such as KAl(SO).12HO, are another series of isomorphous compounds, though there are three series of alums with similar external structures, but slightly different internal structures. Many spinels are also isomorphous.
In order to form isomorphous crystals two substances must have the same chemical formulation (i.e., atoms in the same ratio), they must contain atoms which have corresponding chemical properties and the sizes of corresponding atoms should be similar. These requirements ensure that the forces within and between molecules and ions are approximately similar and result in crystals that have the same internal structure. Even though the space group is the same, the unit cell dimensions will be slightly different because of the different sizes of the atoms involved. | 1 | Crystallography |
The heat of neutralization of a fully dissociated acid with a fully dissociated base is approximately –56kJ/mol. The reaction is thus strongly exothermic, and is an excellent basis for a wide range of analysis in industry. An advantage for the industrial analyst is that the use of stronger titrants (1 to 2 mol/L) permits a reduction in the amount of sample preparation, and samples can often be directly and accurately dispensed into the titration vessel prior to titration. | 0 | Chromatography + Titration + pH indicators |
The ÄKTA explorer was the first high end FPLC (Fast Protein Liquid Chromatography) system that was developed for Life Science research by the Swedish company Pharmacia in 1994. Its purpose was to simplify and automatize protein purification. It was followed by a line of similar devices (the "Äkta" line). The product line name was transferred together with the sale of Pharmacia first to Amersham and then to GE Healthcare. Although protein purification is possible with a large range of chromatographic devices, the Äkta line represents together with BioRad's NGC line the only devices that were specifically designed for this purpose both from the hardware and software perspective. Main users of these devices are the pharmaceutical industry and academic researchers. | 0 | Chromatography + Titration + pH indicators |
There has been a growing interest in the application of IC in the analysis of pharmaceutical drugs. IC is used in different aspects of product development and quality control testing. For example, IC is used to improve stabilities and solubility properties of pharmaceutical active drugs molecules as well as used to detect systems that have higher tolerance for organic solvents. IC has been used for the determination of analytes as a part of a dissolution test. For instance, calcium dissolution tests have shown that other ions present in the medium can be well resolved among themselves and also from the calcium ion. Therefore, IC has been employed in drugs in the form of tablets and capsules in order to determine the amount of drug dissolve with time. IC is also widely used for detection and quantification of excipients or inactive ingredients used in pharmaceutical formulations. Detection of sugar and sugar alcohol in such formulations through IC has been done due to these polar groups getting resolved in ion column. IC methodology also established in analysis of impurities in drug substances and products. Impurities or any components that are not part of the drug chemical entity are evaluated and they give insights about the maximum and minimum amounts of drug that should be administered in a patient per day. | 0 | Chromatography + Titration + pH indicators |
4-Nitrophenol can be used as a pH indicator. A solution of 4-nitrophenol appears colorless below pH 5.4 and yellow above pH 7.5. This color-changing property makes this compound useful as a pH indicator.
The yellow color of the 4-nitrophenolate form (or 4-nitrophenoxide) is due to a maximum of absorbance at 405 nm (ε = 18.3 to 18.4 mM cm in strong alkali). In contrast, 4-nitrophenol has a weak absorbance at 405 nm (ε = 0.2 mM cm).
The isosbestic point for 4-nitrophenol/4-nitrophenoxide is at 348 nm, with ε = 5.4 mM cm. | 0 | Chromatography + Titration + pH indicators |
Chemical additives are small chemical compounds that are added to the crystallization process to increase the yield of crystals. The role of small molecules in protein crystallization had not been well thought of in the early days since they were thought of as contaminants in most case. Smaller molecules crystallize better than macromolecules such as proteins, therefore, the use of chemical additives had been limited prior to the study by McPherson. However, this is a powerful aspect of the experimental parameters for crystallization that is important for biochemists and crystallographers to further investigate and apply. | 1 | Crystallography |
Anthocyanins may have a protective role in plants against extreme temperatures. Tomato plants protect against cold stress with anthocyanins countering reactive oxygen species, leading to a lower rate of cell death in leaves. | 0 | Chromatography + Titration + pH indicators |
The frit fast atom bombardment (FAB) and continuous flow-FAB (CF-FAB) interfaces were developed in 1985 and 1986 respectively. Both interfaces were similar, but they differed in that the first used a porous frit probe as connecting channel, while CF-FAB used a probe tip. From these, the CF-FAB was more successful as a LC–MS interface and was useful to analyze non-volatile and thermally labile compounds. In these interfaces, the LC effluent passed through the frit or CF-FAB channels to form a uniform liquid film at the tip. There, the liquid was bombarded with ion beams or high energy atoms (fast atoms). For stable operation, the FAB based interfaces were able to handle liquid flow rates of only 1–15 μl and were also restricted to microbore and capillary columns. In order to be used in FAB MS ionization sources, the analytes of interest had to be mixed with a matrix (e.g., glycerol) that could be added before or after the separation in the LC column. FAB based interfaces were extensively used to characterize peptides, but lost applicability with the advent of electrospray based interfaces in 1988. | 0 | Chromatography + Titration + pH indicators |
The Zener ratio is only applicable to cubic crystals. To overcome this limitation, a Universal Elastic Anisotropy Index (AU) was formulated from variational principles of elasticity and tensor algebra. The AU is now used to quantify the anisotropy of elastic crystals of all classes. | 1 | Crystallography |
If the SAD is taken from one a or a few single crystals, the diffractogram depicts a regular pattern of bright spots. Since the diffraction pattern can be seen as a two-dimensional projection of reciprocal crystal lattice, the pattern can be used to measure lattice constants, specifically the distances and angles between crystallographic planes. The lattice parameters are typically distinctive for various materials and their phases which allows to identify the examined material or at least differentiate between possible candidates.
Even though the SAD-based analyses were not considered quantitative for a long time, computer tools brought accuracy and repeatability allowing to routinely perform accurate measurements of interplanar distances or angles on appropriately calibrated microscopes. Tools such as CrysTBox are capable of automated analysis achieving sub-pixel precision.
If the sample is tilted against the electron beam, diffraction conditions are satisfied for different set of crystallographic planes yielding different constellation of diffraction spots. This allows to determine the crystal orientation, which can be used for instance to set up the orientation needed for particular experiment, to determine misorientation between adjacent grains or crystal twins. Since different sample orientations provide different projections of the reciprocal lattice, they provide an opportunity to reconstruct the three-dimensional information lost in individual projections. A series of diffractograms varying in tilt can be acquired and processed with diffraction tomography analysis in order to reconstruct an unknown crystal structure.
SAD can also be used to analyze crystal defects such as stacking faults. | 1 | Crystallography |
Desalting spin columns are widely available with various volumes and MWCO limits:
* [http://www.piercenet.com/cat/desalting-columns Thermo Scientific Pierce Products]
* [http://www.bio-rad.com/en-us/category/bio-spin-micro-bio-spin-size-exclusion-columns Bio-Rad Laboratories, Inc.]
* [https://www.cytivalifesciences.com/en/us/shop/chromatography/prepacked-columns/desalting-and-buffer-exchange/ Cytiva ] | 0 | Chromatography + Titration + pH indicators |
After the discovery of x-rays by Wilhelm Röntgen in 1895, and of the principles of X-ray diffraction by Laue and the Bragg family, it took several decades for the benefits of diffraction imaging to be fully recognized, and the first useful experimental techniques to be developed. The first systematic reports of laboratory topography techniques date from the early 1940s. In the 1950s and 1960s, topographic investigations played a role in detecting the nature of defects and improving crystal growth methods for germanium and (later) silicon as materials for semiconductor microelectronics.
For a more detailed account of the historical development of topography, see J.F. Kelly – "A brief history of X-ray diffraction topography".
From about the 1970s on, topography profited from the advent of synchrotron x-ray sources which provided considerably more intense x-ray beams, allowing to achieve shorter exposure times, better contrast, higher spatial resolution, and to investigate smaller samples or rapidly changing phenomena.
Initial applications of topography were mainly in the field of metallurgy, controlling the growth of better crystals of various metals. Topography was later extended to semiconductors, and generally to materials for microelectronics. A related field are investigations of materials and devices for X-ray optics, such as monochromator crystals made of Silicon, Germanium or Diamond, which need to be checked for defects prior to being used. Extensions of topography to organic crystals are somewhat more recent.
Topography is applied today not only to volume crystals of any kind, including semiconductor wafers, but also to thin layers, entire electronic devices, as well as to organic materials such as protein crystals and others. | 1 | Crystallography |
* Orbifold signature:
* Coxeter notation: [4,4]
* Lattice: square
* Point group: D
* The group p4g has two centres of rotation of order four (90°), which are each other's mirror image, but it has reflections in only two directions, which are perpendicular. There are rotations of order two (180°) whose centres are located at the intersections of reflection axes. It has glide reflections axes parallel to the reflection axes, in between them, and also at an angle of 45° with these.
A p4g pattern can be looked upon as a checkerboard pattern of copies of a square tile with 4-fold rotational symmetry, and its mirror image. Alternatively it can be looked upon (by shifting half a tile) as a checkerboard pattern of copies of a horizontally and vertically symmetric tile and its 90° rotated version. Note that neither applies for a plain checkerboard pattern of black and white tiles, this is group p4m (with diagonal translation cells).
;Examples of group p4g | 1 | Crystallography |
Many of the crystallographic point groups share the same internal structure. For example, the point groups , 2, and m contain different geometric symmetry operations, (inversion, rotation, and reflection, respectively) but all share the structure of the cyclic group C. All isomorphic groups are of the same order, but not all groups of the same order are isomorphic. The point groups which are isomorphic are shown in the following table:
This table makes use of cyclic groups (C, C, C, C, C), dihedral groups (D, D, D, D), one of the alternating groups (A), and one of the symmetric groups (S). Here the symbol " × " indicates a direct product. | 1 | Crystallography |
Phenolphthalein has been used for over a century as a laxative, but is now being removed from over-the-counter laxatives over concerns of carcinogenicity. Laxative products formerly containing phenolphthalein have often been reformulated with alternative active ingredients: Feen-a-Mint switched to bisacodyl, and Ex-Lax was switched to a senna extract.
Thymolphthalein is a related laxative made from thymol.
Despite concerns regarding its carcinogenicity based on rodent studies, the use of phenolphthalein as a laxative is unlikely to cause ovarian cancer. Some studies suggest a weak association with colon cancer, while others show none at all.
Phenolphthalein is described as a stimulant laxative. In addition, it has been found to inhibit human cellular calcium influx via store-operated calcium entry (SOCE, see ) in vivo. This is effected by its inhibiting thrombin and thapsigargin, two activators of SOCE that increase intracellular free calcium.
Phenolphthalein has been added to the European Chemicals Agency's candidate list for substance of very high concern (SVHC). It is on the IARC group 2B list for substances "possibly carcinogenic to humans".
The discovery of phenolphthalein's laxative effect was due to an attempt by the Hungarian government to label genuine local white wine with the substance in 1900. Phenolphthalein did not change the taste of the wine and would change color when a base is added, making it a good label in principle. However, it was found that ingestion of the substance led to diarrhea. Max Kiss, a Hungarian-born pharmacist residing in New York, heard about the news and launched Ex-Lax in 1906. | 0 | Chromatography + Titration + pH indicators |
Acidic solutions of fluoride (including hydrofluoric acid) can be determined by a simple thermometric titration with boric acid.
: B(OH) + 3F + 3H ↔ BF + 3HO
The titration plot illustrated in Figure 19 shows that the endpoint is quite rounded, suggesting that the reaction might not proceed to stoichiometric equilibrium. However, since the regions of the temperature curve immediately before and after the endpoint are quite linear, the second derivative of this curve (representing the intersection of tangents) will accurately locate the endpoint. Indeed, excellent precision can be obtained with this titration, with a CV of less than 0.1. | 0 | Chromatography + Titration + pH indicators |
In a crystal, a superstructure manifests itself through additional reflections in diffraction patterns, e.g., in low energy electron diffraction (LEED) or X-ray diffraction experiments. Often a set of weak diffraction spots appears between the stronger spots belonging to what is referred to as the substructure. In some cases a phase transition occurs, e.g., at higher temperatures, where the superstructure disappears and the material reverts to the simpler substructure. Not all compounds exhibit a superstructure.
The superspots in diffraction patterns represent a modulation of the substructure that causes the inherent translation symmetry of the (substructure) lattice to be violated slightly or the size of the repeat motif of the structure to be increased. One could speak of symmetry breaking of the translation symmetry of the lattice, although rotational symmetry may be lost simultaneously. | 1 | Crystallography |
The Bragg condition is correct for very large crystals. Because the scattering of X-rays and neutrons is relatively weak, in many cases quite large crystals with sizes of 100 nm or more are used. While there can be additional effects due to crystal defects, these are often quite small. In contrast, electrons interact thousands of times more strongly with solids than X-rays, and also lose energy (inelastic scattering). Therefore samples used in transmission electron diffraction are much thinner. Typical diffraction patterns, for instance the Figure, show spots for different directions (plane waves) of the electrons leaving a crystal. The angles that Braggs law predicts are still approximately right, but in general there is a lattice of spots which are close to projections of the reciprocal lattice that is at right angles to the direction of the electron beam. (In contrast, Braggs law predicts that only one or perhaps two would be present, not simultaneously tens to hundreds.) With low-energy electron diffraction where the electron energies are typically 30-1000 electron volts, the result is similar with the electrons reflected back from a surface. Also similar is reflection high-energy electron diffraction which typically leads to rings of diffraction spots.
With X-rays the effect of having small crystals is described by the Scherrer equation. This leads to broadening of the Bragg peaks which can be used to estimate the size of the crystals. | 1 | Crystallography |
Though the terms equivalence point and endpoint are often used interchangeably, they are different terms. Equivalence point is the theoretical completion of the reaction: the volume of added titrant at which the number of moles of titrant is equal to the number of moles of analyte, or some multiple thereof (as in polyprotic acids). Endpoint is what is actually measured, a physical change in the solution as determined by an indicator or an instrument mentioned above.
There is a slight difference between the endpoint and the equivalence point of the titration. This error is referred to as an indicator error, and it is indeterminate. | 0 | Chromatography + Titration + pH indicators |
The previously described work of Martin and Synge impacted the development of the previously known column chromatography and inspired new forms of chromatography such as countercurrent distribution, paper chromatography, and gas-liquid chromatography which is more commonly known as gas chromatography. The modification of silica gel stationary phase led to many creative ways of modifying stationary phases in order to influence the separation characteristics. The most notable modification was the chemical bonding of alkane functional groups to silica gel to produce reversed-phase media. The original problem that Martin and Synge encountered with devising an instrument that would employ two free-flowing liquid phases was solved by Lyman C. Craig in 1944, and commercial counter-current distribution instruments were used for many important discoveries. The introduction of paper chromatography was an important analytical technique which gave rise to thin-layer chromatography. Finally, gas-liquid chromatography, a fundamental technique in modern analytical chemistry, was described by Martin with coauthors A. T. James and G. Howard Smith in 1952. | 0 | Chromatography + Titration + pH indicators |
A Chromatography column is a device used in chromatography for the separation of chemical compounds. A chromatography column contains the stationary phase, allowing the mobile phase to pass through it. Chromatography columns of different types are used in both gas and liquid chromatography. | 0 | Chromatography + Titration + pH indicators |
Thorin (also called thoron or thoronol) is an indicator used in the determination of barium, beryllium, lithium, uranium and thorium compounds. Being a compound of arsenic, it is highly toxic. | 0 | Chromatography + Titration + pH indicators |
Gas chromatography-mass spectrometry (GC-MS) is a two-dimensional chromatography technique that combines the separation technique of gas chromatography with the identification technique of mass spectrometry. GC-MS is the single most important analytical tool for the analysis of volatile and semi-volatile organic compounds in complex mixtures. It works by first injecting the sample into the GC inlet where it is vaporized and pushed through a column by a carrier gas, typically helium. The analytes in the sample are separated based upon their interaction with the coating of the column, or the stationary phase, and the carrier gas, or the mobile phase. The compounds eluted from the column are converted into ions via electron impact (EI) or chemical ionization (CI) before traveling through the mass analyzer. The mass analyzer serves to separate the ions on a mass-to-charge basis. Popular choices perform the same function but differ in the way that they accomplish the separation. The analyzers typically used with GC-MS are the time-of-flight mass analyzer and the quadrupole mass analyzer. After leaving the mass analyzer, the analytes reach the detector and produce a signal that is read by a computer and used to create a gas chromatogram and mass spectrum. Sometimes GC-MS utilizes two gas chromatographers in particularly complex samples to obtain considerable separation power and be able to unambiguously assign the specific species to the appropriate peaks in a technique known as GCxGC-(MS). Ultimately, GC-MS is a technique utilized in many analytical laboratories and is a very effective and adaptable analytical tool. | 0 | Chromatography + Titration + pH indicators |
Solid-phase epitaxy (SPE) is a transition between the amorphous and crystalline phases of a material. It is usually produced by depositing a film of amorphous material on a crystalline substrate, then heating it to crystallize the film. The single-crystal substrate serves as a template for crystal growth. The annealing step used to recrystallize or heal silicon layers amorphized during ion implantation is also considered to be a type of solid phase epitaxy. The impurity segregation and redistribution at the growing crystal-amorphous layer interface during this process is used to incorporate low-solubility dopants in metals and silicon. | 1 | Crystallography |
Stable isotope mass spectrometry is conducted in the Department of Geography, and was recently used by the Landmark Trust to determine very precisely the age of the timber from Llwyn Celyn farmhouse to the year 1420. | 0 | Chromatography + Titration + pH indicators |
The name "gentian violet" (or Gentianaviolett in German) is thought to have been introduced by the German pharmacist Georg Grübler, who in 1880 started a company in Leipzig that specialized in the sale of staining reagents for histology. The gentian violet stain marketed by Grübler probably contained a mixture of methylated pararosaniline dyes. The stain proved popular and in 1884 was used by Hans Christian Gram to stain bacteria. He credited Paul Ehrlich for the aniline-gentian violet mixture. Grüblers gentian violet was probably very similar, if not identical, to Lauths methyl violet, which had been used as a stain by Victor André Cornil in 1875.
Although the name gentian violet continued to be used for the histological stain, the name was not used in the dye and textile industries. The composition of the stain was not defined and different suppliers used different mixtures. In 1922, the Biological Stain Commission appointed a committee chaired by Harold Conn to look into the suitability of the different commercial products. In his book Biological Stains, Conn describes gentian violet as a "poorly defined mixture of violet rosanilins".
The German ophthalmologist Jakob Stilling is credited with discovering the antiseptic properties of gentian violet. He published a monograph in 1890 on the bactericidal effects of a solution that he christened "pyoktanin", which was probably a mixture of aniline dyes similar to gentian violet. He set up a collaboration with E. Merck & Co. to market "Pyoktanin caeruleum" as an antiseptic.
In 1902, Drigalski and Conradi found that although crystal violet inhibited the growth of many bacteria, it has little effect on Bacillus coli (Escherichia coli) and Bacillus typhi (Salmonella typhi), which are both gram-negative bacteria. A much more detailed study of the effects of Grübler's gentian violet on different strains of bacteria was published by John Churchman in 1912. He found that most gram-positive bacteria (tainted) were sensitive to the dye, while most gram-negative bacteria (not tainted) were not, and observed that the dye tended to act as a bacteriostatic agent rather than a bactericide. | 0 | Chromatography + Titration + pH indicators |
High performance affinity chromatography (HPAC) works by passing a sample solution through a column packed with a stationary phase that contains an immobilized biologically active ligand. The ligand is in fact a substrate that has a specific binding affinity for the target molecule in the sample solution. The target molecule binds to the ligand, while the other molecules in the sample solution pass through the column, having little or no retention. The target molecule is then eluted from the column using a suitable elution buffer.
This chromatographic process relies on the capability of the bonded active substances to form stable, specific, and reversible complexes thanks to their biological recognition of certain specific sample components. The formation of these complexes involves the participation of common molecular forces such as the Van der Waals interaction, electrostatic interaction, dipole-dipole interaction, hydrophobic interaction, and the hydrogen bond. An efficient, biospecific bond is formed by a simultaneous and concerted action of several of these forces in the complementary binding sites. | 0 | Chromatography + Titration + pH indicators |
Two dimensional space has the same number of crystal systems, crystal families, and lattice systems. In 2D space, there are four crystal systems: oblique, rectangular, square, and hexagonal. | 1 | Crystallography |
An acid–base titration is a method of quantitative analysis for determining the concentration of Brønsted-Lowry acid or base (titrate) by neutralizing it using a solution of known concentration (titrant). A pH indicator is used to monitor the progress of the acid–base reaction and a titration curve can be constructed.
This differs from other modern modes of titrations, such as oxidation-reduction titrations, precipitation titrations, & complexometric titrations. Although these types of titrations are also used to determine unknown amounts of substances, these substances vary from ions to metals.
Acid–base titration finds extensive applications in various scientific fields, such as pharmaceuticals, environmental monitoring, and quality control in industries. This method's precision and simplicity makes it an important tool in quantitative chemical analysis, contributing significantly to the general understanding of solution chemistry. | 0 | Chromatography + Titration + pH indicators |
The retention factor (R) may be defined as the ratio of the distance travelled by the solute to the distance travelled by the solvent. It is used in chromatography to quantify the amount of retardation of a sample in a stationary phase relative to a mobile phase. R values are usually expressed as a fraction of two decimal places.
* If R value of a solution is zero, the solute remains in the stationary phase and thus it is immobile.
* If R value = 1 then the solute has no affinity for the stationary phase and travels with the solvent front.
For example, if a compound travels 9.9 cm and the solvent front travels 12.7 cm, the R value = (9.9/12.7) = 0.779 or 0.78. R value depends on temperature and the solvent used in experiment, so several solvents offer several R values for the same mixture of compound. A solvent in chromatography is the liquid the paper is placed in, and the solute is the ink which is being separated. | 0 | Chromatography + Titration + pH indicators |
The use of displacement chromatography is rather limited, and is mostly used for preparative chromatography. The basic principle is based on a molecule with a high affinity for the chromatography matrix (the displacer) which is used to compete effectively for binding sites, and thus displace all molecules with lesser affinities.
There are distinct differences between displacement and elution chromatography. In elution mode, substances typically emerge from a column in narrow, Gaussian peaks. Wide separation of peaks, preferably to baseline, is desired in order to achieve maximum purification. The speed at which any component of a mixture travels down the column in elution mode depends on many factors. But for two substances to travel at different speeds, and thereby be resolved, there must be substantial differences in some interaction between the biomolecules and the chromatography matrix. Operating parameters are adjusted to maximize the effect of this difference. In many cases, baseline separation of the peaks can be achieved only with gradient elution and low column loadings. Thus, two drawbacks to elution mode chromatography, especially at the preparative scale, are operational complexity, due to gradient solvent pumping, and low throughput, due to low column loadings. Displacement chromatography has advantages over elution chromatography in that components are resolved into consecutive zones of pure substances rather than "peaks". Because the process takes advantage of the nonlinearity of the isotherms, a larger column feed can be separated on a given column with the purified components recovered at significantly higher concentration. | 0 | Chromatography + Titration + pH indicators |
The fourth shell includes programs on representation theory of space and point groups. [http://www.cryst.ehu.es/rep/repres.html REPRES] constructs little group and full group irreducible representations for a given space group and a k-vector; [http://www.cryst.ehu.es/rep/correl.html CORREL] deals with the correlations between the irreducible representations of group-subgroup related space groups. The program [http://www.cryst.ehu.es/rep/point.html POINT] lists character tables of crystallographic point groups, Kronecker multiplication tables of their irreducible representations and further useful symmetry information. | 1 | Crystallography |
A close packed unit cell, both face-centered cubic and hexagonal close packed, can form two different shaped holes. Looking at the three green spheres in the hexagonal packing illustration at the top of the page, they form a triangle-shaped hole. If an atom is arranged on top of this triangular hole it forms a tetrahedral interstitial hole. If the three atoms in the layer above are rotated and their triangular hole sits on top of this one, it forms an octahedral interstitial hole. In a close-packed structure there are 4 atoms per unit cell and it will have 4 octahedral voids (1:1 ratio) and 8 tetrahedral voids (1:2 ratio) per unit cell. The tetrahedral void is smaller in size and could fit an atom with a radius 0.225 times the size of the atoms making up the lattice. An octahedral void could fit an atom with a radius 0.414 times the size of the atoms making up the lattice. An atom that fills this empty space could be larger than this ideal radius ratio, which would lead to a distorted lattice due to pushing out the surrounding atoms, but it cannot be smaller than this ratio. | 1 | Crystallography |
* Cost of reactor and replacements (each replacement unit costs ~ $6000)
* Addition of dead volume causes an increase in peak broadening depending on the GC column flow rates and molecule types (5-10% broadening is typical)
** Heteroatoms and oxidation of Polyarc transfer lines increase this broadening
* Susceptible to sulfur, silicon, and halogen poisoning
* Requires constant feed of hydrogen
* Cannot be regenerated in-house; must be shipped back to the manufacturer for a replacement
* Cannot be used with cryogenic oven temperatures, or for GCxGC
* Poor response for species containing C-F bonds
* May contribute to power overload on older GC models, preventing the use of other heated components such as valve boxes or auxiliary detectors | 0 | Chromatography + Titration + pH indicators |
TLC is a useful tool for reaction monitoring. For this, the plate normally contains a spot of starting material, a spot from the reaction mixture, and a co-spot (or cross-spot) containing both. The analysis will show if the starting material disappeared and if any new products appeared. This provides a quick and easy way to estimate how far a reaction has proceeded. In one study, TLC has been applied in the screening of organic reactions. The researchers react an alcohol and a catalyst directly in the co-spot of a TLC plate before developing it. This provides quick and easy small-scale testing of different reagents.
Compound characterization with TLC is also possible and is similar to reaction monitoring. However, rather than spotting with starting material and reaction mixture, it is with an unknown and a known compound. They may be the same compound if both spots have the same R and look the same under the chosen visualization method. However, co-elution complicates both reaction monitoring and characterization. This is because different compounds will move to the same spot on the plate. In such cases, different solvent mixtures may provide better separation. | 0 | Chromatography + Titration + pH indicators |
In reversed-phase (e.g. aqueous mobile phase) elution, the aqueous phase is used as the mobile phase with a less polar stationary phase. In countercurrent chromatography the same solvent system may be used in either normal or reversed phase mode simply by switching the direction of mobile phase flow through the column. | 0 | Chromatography + Titration + pH indicators |
Phases can also be inferred by using a process called molecular replacement, where a similar molecules already-known phases are grafted onto the intensities of the molecule at hand, which are observationally determined. These phases can be obtained experimentally from a homologous molecule or if the phases are known for the same molecule but in a different crystal, by simulating the molecules packing in the crystal and obtaining theoretical phases. Generally, these techniques are less desirable since they can severely bias the solution of the structure. They are useful, however, for ligand binding studies, or between molecules with small differences and relatively rigid structures (for example derivatizing a small molecule). | 1 | Crystallography |
In geometry, the dyakis dodecahedron /ˈdʌɪəkɪsˌdəʊdɪkəˈhiːdrən/ or diploid is a variant of the deltoidal icositetrahedron with pyritohedral symmetry, transforming the kite faces into chiral quadrilaterals. It is the dual of the cantic snub octahedron. It can be constructed by enlarging 24 of the 48 faces of the disdyakis dodecahedron, and is inscribed in the dyakis dodecahedron, thus it exists as a hemihedral form of it with indices {hkl}. It can be constructed into two non regular pentagonal dodecahedra, the pyritohedron and the tetartoid. The transformation to the pyritohedron can be made by combining two adjacent trapezoids that share a long edge together into one hexagon face, which is a pyritohedral pentagon with an extra vertex added. The edges that bend at it can be combined and the vertex removed to finally get the pentagon. The transformation to the tetartoid can be made by enlarging 12 of the dyakis dodecahedron's 24 faces.
Since the quadrilaterals are chiral and non-regular, the dyakis dodecahedron is a non-uniform polyhedron, a type of polyhedron that isnt vertex transitive and doesnt have regular polygon faces. Since it is face-transitive, it is an isohedron. The name diploid derives from the Greek word διπλάσιος (diplásios), meaning twofold since it has 2-fold symmetry along its 6 octahedral vertices. It has the same number of faces, edges and vertices as the deltoidal icositetrahedron as they are topologically identical. | 1 | Crystallography |
Anthocyanins are thought to be subject to physiochemical degradation in vivo and in vitro. Structure, pH, temperature, light, oxygen, metal ions, intramolecular association, and intermolecular association with other compounds (copigments, sugars, proteins, degradation products, etc.) generally are known to affect the color and stability of anthocyanins. B-ring hydroxylation status and pH have been shown to mediate the degradation of anthocyanins to their phenolic acid and aldehyde constituents. Indeed, significant portions of ingested anthocyanins are likely to degrade to phenolic acids and aldehyde in vivo, following consumption. This characteristic confounds scientific isolation of specific anthocyanin mechanisms in vivo. | 0 | Chromatography + Titration + pH indicators |
The mobile phase or eluent is a solvent or a mixture of solvents used to move the compounds through the column. It is chosen so that the retention factor value of the compound of interest is roughly around 0.2 - 0.3 in order to minimize the time and the amount of eluent to run the chromatography. The eluent has also been chosen so that the different compounds can be separated effectively. The eluent is optimized in small scale pretests, often using thin layer chromatography (TLC) with the same stationary phase, using solvents of different polarity until a suitable solvent system is found. Common mobile phase solvents, in order of increasing polarity, include hexane, dichloromethane, ethyl acetate, acetone, and methanol. A common solvent system is a mixture of hexane and ethyl acetate, with proportions adjusted until the target compound has a retention factor of 0.2 - 0.3. Contrary to common misconception, methanol alone can be used as an eluent for highly polar compounds, and does not dissolve silica gel.
There is an optimum flow rate for each particular separation. A faster flow rate of the eluent minimizes the time required to run a column and thereby minimizes diffusion, resulting in a better separation. However, the maximum flow rate is limited because a finite time is required for the analyte to equilibrate between the stationary phase and mobile phase, see Van Deemter's equation. A simple laboratory column runs by gravity flow. The flow rate of such a column can be increased by extending the fresh eluent filled column above the top of the stationary phase or decreased by the tap controls. Faster flow rates can be achieved by using a pump or by using compressed gas (e.g. air, nitrogen, or argon) to push the solvent through the column (flash column chromatography).
The particle size of the stationary phase is generally finer in flash column chromatography than in gravity column chromatography. For example, one of the most widely used silica gel grades in the former technique is mesh 230 – 400 (40 – 63 µm), while the latter technique typically requires mesh 70 – 230 (63 – 200 µm) silica gel.
A spreadsheet that assists in the successful development of flash columns has been developed. The spreadsheet estimates the retention volume and band volume of analytes, the fraction numbers expected to contain each analyte, and the resolution between adjacent peaks. This information allows users to select optimal parameters for preparative-scale separations before the flash column itself is attempted. | 0 | Chromatography + Titration + pH indicators |
Tandem mass spectrometry (Tandem MS or MS/MS) uses two mass analyzers in sequence to separate more complex mixtures of analytes. The advantage of tandem MS is that it can be much faster than other two-dimensional methods, with times ranging from milliseconds to seconds. Because there is no dilution with solvents in MS, there is less probability of interference, so tandem MS can be more sensitive and have a higher signal-to-noise ratio compared to other two-dimensional methods. The main disadvantage associated with tandem MS is the high cost of the instrumentation needed. Prices can range from $500,000 to over $1 million. Many form of tandem MS involve a mass selection step and a fragmentation step. The first mass analyzer can be programmed to only pass molecules of a specific mass-to-charge ratio. Then the second mass analyzer can fragment the molecule to determine its identity. This can be especially useful for separating molecules of the same mass (i.e. proteins of the same mass or molecular isomers). Different types of mass analyzers can be coupled to achieve varying effects. One example would be a TOF-Quadrupole system. Ions can be sequentially fragmented and/or analyzed in a quadrupole as they leave the TOF in order of increasing m/z. Another prevalent tandem mass spectrometer is the quadrupole-quadrupole-quadrupole (Q-Q-Q) analyzer. The first quadrupole separates by mass, collisions take place in the second quadrupole, and the fragments are separated by mass in the third quadrupole. | 0 | Chromatography + Titration + pH indicators |
After successful formation of a stable nucleus, a growth stage ensues in which free particles (atoms or molecules) adsorb onto the nucleus and propagate its crystalline structure outwards from the nucleating site. This process is significantly faster than nucleation. The reason for such rapid growth is that real crystals contain dislocations and other defects, which act as a catalyst for the addition of particles to the existing crystalline structure. By contrast, perfect crystals (lacking defects) would grow exceedingly slowly. On the other hand, impurities can act as crystal growth inhibitors and can also modify crystal habit. | 1 | Crystallography |
In geometry and crystallography, the Laves graph is an infinite and highly symmetric system of points and line segments in three-dimensional Euclidean space, forming a periodic graph. Three equal-length segments meet at 120° angles at each point, and all cycles use ten or more segments. It is the shortest possible triply periodic graph, relative to the volume of its fundamental domain. One arrangement of the Laves graph uses one out of every eight of the points in the integer lattice as its points, and connects all pairs of these points that are nearest neighbors, at distance . It can also be defined, divorced from its geometry, as an abstract undirected graph, a covering graph of the complete graph on four vertices.
named this graph after Fritz Laves, who first wrote about it as a crystal structure in 1932. It has also been called the K crystal, (10,3)-a network, diamond twin, triamond, and the srs net. The regions of space nearest each vertex of the graph are congruent 17-sided polyhedra that tile space. Its edges lie on diagonals of the regular skew polyhedron, a surface with six squares meeting at each integer point of space.
Several crystalline chemicals have known or predicted structures in the form of the Laves graph. Thickening the edges of the Laves graph to cylinders produces a related minimal surface, the gyroid, which appears physically in certain soap film structures and in the wings of butterflies. | 1 | Crystallography |
The compound has uses ranging from medicine to laboratory syntheses of chemically similar compounds. o-Cresophthalein has been used to derive polyamides and polyimides, colorimetrically estimate calcium in serum, and predict amount of time to wait before blood collection after a patient receives gadodiamide. | 0 | Chromatography + Titration + pH indicators |
The extraction cells consist of hollow bodies with inlets and outlets of liquid connection. The cells are first filled with the liquid chosen to be the stationary phase. Under rotation, the pumping of the mobile phase is started, which enters the cells from the inlet. When entering the flow of mobiles phase forms small droplets according to the Stokes' law, which is called atomization. These droplets fall through the stationary phase, creating a high interface area, which is called the extraction. At the end of the cells, these droplets unite due to the surface tension, which is called settling.
When a sample mixture is injected as a plug into the flow of mobile phase the compounds of the mixtures elute according to their partition coefficients:
Centrifugal partition chromatography requires only a biphasic mixture of solvents, so by varying the constitution of the solvent system it is possible to tune the partition coefficients of different compounds so that separation is guaranteed by the high selectivity. | 0 | Chromatography + Titration + pH indicators |
To carry out metal cation titrations using EDTA, it is almost always necessary to use a complexometric indicator to determine when the end point has been reached. Common indicators are organic dyes such as Fast Sulphon Black, Eriochrome Black T, Eriochrome Red B, Patton Reeder, or Murexide. Color change shows that the indicator has been displaced (usually by EDTA) from the metal cations in solution when the end point has been reached. Thus, the free indicator (rather than the metal complex) serves as the endpoint indicator. | 0 | Chromatography + Titration + pH indicators |
* Coupled substitutions are common in the silicate minerals where substitutes for in tetrahedral sites.
For example, when a plagioclase feldspar solid solution series forms, albite (Na Al SiO) can change to anorthite (Ca AlSiO) by having replace . However, this leaves a negative charge that has to be balanced by the (coupled) substitution of for .
* Despite being nicknamed fool's gold, pyrite is sometimes found in association with small quantities of gold. Gold and arsenic occur as a coupled substitution in the pyrite structure. In the Carlin–type gold deposits, arsenian pyrite contains up to 0.37% gold by weight.
* The possible replacement of by in Corundum.
* and in Haematite
* → Diopside (MgCaSiO) → Jadeite: (NaAlSiO or
* 2 → 2 As in the Spinel groups
* The site being filled to maintain charge does not have to be a substitution. It can also involve filling a site that is normally vacant in order to achieve charge balance. For example, in the amphibole mineral Tremolite - (Ca(MgFe)SiO(OH)), replaces then can go into a site that is normally vacant to maintain charge balance. This new mineral would then be edenite a variety of hornblende.
* Bityite’s structure consists of a coupled substitution it exhibits between the sheets of polyhedra; the coupled substitution of beryllium for aluminium within the tetrahedral sites allows a single lithium substitution for a vacancy without any additional octahedral substitutions. The transfer is completed by creating a tetrahedral sheet composition of SiBeAl. The coupled substitution of lithium for vacancy and the beryllium for the tetrahedral aluminium maintains all the charges balanced; thereby, resulting in the trioctahedral end member for the margarite sub-group of the phyllosilicate group.
* Ferrogedrite is related to anthophyllite amphibole and gedrite through coupled substitution of (Al, Fe) for (Mg, Fe, Mn) and Al for Si. | 1 | Crystallography |