chromatography

chromatography

[kroh-muh-tog-ruh-fee]
chromatography, resolution of a chemical mixture into its component compounds by passing it through a system that retards each compound to a varying degree; a system capable of accomplishing this is called a chromatograph. The retarding system can be a surface adsorbant, such as silica, alumina, cellulose, or charcoal, capable of reversibly adsorbing the compounds (see adsorption). The earliest use of this technique, by the Russian botanist Mikhail Tsvett (c.1903), involved the separation of highly colored compounds, hence the name chromatography [Gr.,=color recording].

Column Chromatography

In column chromatography the adsorbant is packed into a column and a solution of the mixture is added at the top. An appropriate solvent is passed through the column, washing, or eluting, the compounds down the column. A polar substance that is adsorbed very tightly to the surface will be efficiently retarded by the column, while a nonpolar substance will elute (dissolve in the solvent) very rapidly. By varying the nature of the solid adsorbant and the eluting solvent, a wide variety of resolutions, even of very similar substances, can be carried out.

Gas Chromatography

The gas chromatograph (GC) is a system consisting of a liquid with a high boiling point impregnated on an inert solid support as the stationary phase and helium gas as the mobile phase. The stationary phase is packed into a thin metal column and helium gas is allowed to flow through it. The column is attached to an injection port, and the entire system is heated in an oven. A solution of the mixture is injected into the column through the injection port by means of a syringe and is immediately volatilized. The helium gas then sweeps the components out of the column and past a detector. The polarity of the compounds and their volatility determines how long they are retained by the column. When each component passes the detector, a peak is registered on a recorder. The relative quantities of the components can be determined from the relative areas under the peaks. By varying the polarity of the column and its temperature, many different resolutions can be carried out. Since the capacity of GC columns is very low, the gas chromatograph is used chiefly as an analytical tool, although it can be used for preparative purposes as well. Miniaturized GC instruments have been employed in space probes to analyze the atmospheres of other planets.

Liquid Chromatography

For compounds that cannot be volatilized readily, the liquid chromatograph (LC) can be used instead of the gas chromatograph. The stationary phase consists of a finely powdered solid adsorbant packed into a thin metal column and the mobile phase consists of an eluting solvent forced through the column by a high-pressure pump. The mixture to be analyzed is injected into the column and monitored by a detector. Many different LC packings and eluting solvents are available to achieve the desired resolution.

Gel-Permeation Chromatography

In gel-permeation chromatography, compounds are separated on the basis of their molecular size. Porous beads of the gel are packed into a column and the mixture is added at the top in an appropriate solvent. Large molecules move straight down the column, while small molecules stick in the pores and are retarded.

Ion-Exchange Chromatography

For compounds that can exist as ions, ion-exchange chromatography can be used to separate them from neutral or oppositely charged compounds. The mixture is added to a column packed with a porous, insoluble resin which has a negatively charged (anionic) group attached to it and an unattached, positively charged (cationic) counterion. A cation from the mixture will exchange with the positive counterion of the resin and will be retarded while neutral and anionic substances are not affected. Ion-exchange resins with exchangeable anions work in a similar manner.

Thin-Layer and Paper Chromatography

A layer of adsorbant also can be spread on a glass plate, instead of packed into a column, for analytical purposes. By means of a thin capillary tube, the plate is spotted with a solution of the mixture that is to be resolved, and the solvent is allowed to evaporate. An eluting solvent is then allowed to move up the plate by capillary action, drawing the components of the mixture along by varying degrees. The plate is developed by spraying it with an oxidizing agent, so that each component becomes charred and appears as a dark spot on the plate. The location and size of the spots serve to identify and measure the relative quantities of the components. As in column chromatography, polar substances will not elute as well and will remain nearer the bottom of the plate, while nonpolar substances will elute to the top. This process is called thin-layer chromatography (TLC). In paper chromatography a procedure similar to TLC is used except that the cellulose in the paper acts as the adsorbant.

Electrophoresis

Electrophoresis, like ion-exchange chromatography, can be used as an effective tool for analyzing mixtures of ions. A strip of paper or a column of polymeric gel, saturated with an electrolyte, is set up so that it spans two solutions containing electrodes. The mixture to be analyzed is spotted onto the paper or gel and the two electrodes are connected to a high-energy power source (about 5,000 volts). Positive ions will migrate in one direction and negative ions in the other. The greater the charge on the ion, the farther it will migrate. This method is especially useful for the resolution of mixtures of proteins.

Type of chromatography using as the stationary phase a thin layer (0.01 inch [0.25 mm]) of a special finely ground matrix (silica gel, alumina, or similar material) coated on a glass plate or incorporated in a plastic film. Solutions of the mixtures to be analyzed are spotted near one edge. Solutions of reference compounds are similarly applied. The edge of the plate is then dipped in a solvent. The solvent travels up the matrix by capillarity, moving the components of the samples at various rates because of their different degrees of attachment to the matrix and solubility in the developing solvent. The components, visible as separated spots, are identified by comparing the distances they have traveled with those of the known reference materials. TLC is useful for biological mixtures, especially lipids in animal or vegetable tissues and isoprenoids and essential oils found in flowers and other parts of plants. The matrices withstand strong solvents and developers better than the paper used in paper chromatography.

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Type of chromatography using filter paper or other special paper as the stationary phase. Spots of sample and reference materials are applied, usually as liquids, near one edge (or corner, for two-dimensional PC) of the paper. The edge of the paper is dipped in a solvent, which travels along it by capillarity, moving the components of the sample at rates depending on their relative solubilities in the solvent. In two-dimensional PC, the paper is then turned 90° and the new edge dipped in a different solvent. The components of the sample mixture, visible as separated spots, are identified by comparing the distances they have traveled with those of the known reference materials. PC is especially useful for complex mixtures of amino acids, peptides, carbohydrates, steroids, and many other organic compounds and inorganic ions.

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Type of chromatography with a gas mixture as the mobile phase. In a packed column, the packing or solid support (held in a tube) serves as the stationary phase (vapour-phase chromatography, or VPC) or is coated with a liquid stationary phase (gas-liquid chromatography, or GLC). In capillary columns, the stationary phase coats the walls of small-diameter tubes. The sample of gas or volatile liquid to be analyzed is injected into the inlet; its components move through with a carrier gas (usually hydrogen, helium, or argon) at rates influenced by their degree of interaction with the stationary phase. The temperature, nature of the stationary phase, and column length can be varied to improve separation. The gas stream issuing from the column's end may pass through a thermal conductivity detector or a flame ionization detector, where its properties are compared with those of known reference substances. GC is used to measure air pollutants, essential oils, gases or alcohol in blood, and composition of industrial process streams.

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Method first described in 1903 by Mikhail S. Tsvet for separating mixed chemical substances. Tsvet's neglected work, rediscovered in the 1930s, uses the different affinities of substances in a solution in a mobile phase (a moving stream of gas or liquid) for adsorption onto a stationary phase (a fine-grained solid, a sheet of filtering material, or a thin film of a liquid on a solid surface). Choices of materials for these phases allow enormous versatility for separating substances including biological fluids (e.g., amino acids, steroids, carbohydrates, pigments), chemical mixtures, and forensic samples. In the original technique, an organic solvent flowed through a column of powdered alumina (see aluminum), sodium carbonate, or even powdered sugar to separate mixed plant pigments. Among current adaptations are paper chromatography (PC), thin-layer chromatography (TLC), liquid chromatography (LC, including high-performance liquid chromatography, or HPLC), and gas chromatography (GC). Some remain laboratory techniques, but others (especially HPLC) can be used on an industrial scale. They require different methods for detecting and identifying the separated components, including colorimetry, spectrophotometry, mass spectrometry, and measurement of fluorescence, ionization potential, or thermal conductivity. A.J.P. Martin shared a 1952 Nobel Prize for developing LC and PC, and in his Nobel lecture announced the development (with his cowinner R.L.M. Synge and other colleagues) of GC.

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Chromatography (from Greek χρώμα:chroma, color and γραφειν:"graphein" to write) is the collective term for a family of laboratory techniques for the separation of mixtures. It involves passing a mixture dissolved in a "mobile phase" through a stationary phase, which separates the analyte to be measured from other molecules in the mixture and allows it to be isolated.

Chromatography may be preparative or analytical. Preparative chromatography seeks to separate the components of a mixture for further use (and is thus a form of purification). Analytical chromatography normally operates with smaller amounts of material and seeks to measure the relative proportions of analytes in a mixture. The two are not mutually exclusive.

Explanation

An analogy which is sometimes useful is to suppose a mixture of bees and wasps passing over a flower bed. The wasps, being less attracted to the flowers than the bees, would fly past. The bees would be more attracted to the flowers than the wasps and they would stop flying to land and visit the flowers before moving on again. If one were to observe at a point past the flower bed, the wasps would pass first, followed by the bees. In this analogy, the bees and wasps represent the analytes to be separated, the flowers represent the stationary phase, and the mobile phase could be thought of as the air. The key to the separation is the differing affinities among analyte, stationary phase, and mobile phase. The observer could represent the detector used in some forms of analytical chromatography. A key point is that the detector need not be capable of discriminating between the analytes, since they have become separated before passing the detector.

History

The history of chromatography spans from the mid-19th century the 21st. Chromatography, literally "color writing", was used—and named— in the first decade of the 20th century, primarily for the separation of plant pigments such as chlorophyll. New forms of chromatography developed in the 1930s and 1940s made the technique useful for a wide range of separation processes and chemical analysis tasks, especially in biochemistry.

Some related techniques were developed in the 19th century (and even before), but the first true chromatography is usually attributed to Russian botanist Mikhail Semyonovich Tsvet, who used columns of calcium carbonate for separating plant pigments in the first decade of the 20th century during his research on chlorophyll.

Chromatography began to take its modern form following the work of Archer John Porter Martin and Richard Laurence Millington Synge in the 1940s and 1950s. They laid out the principles and basic techniques of partition chromatography, and their work spurred the rapid development of several lines of chromatography methods: paper chromatography, gas chromatography, and what would become known as high performance liquid chromatography. Since then, the technology has advanced rapidly. Researchers found that the principles underlying Tsvet's chromatography could be applied in many different ways, giving rise to the different varieties of chromatography described below. Simultaneously, advances continually improved the technical performance of chromatography, allowing the separation of increasingly similar molecules

Chromatography terms

  • The analyte is the substance that is to be separated during chromatography.
  • Analytical chromatography is used to determine the existence and possibly also the concentration of analyte(s) in a sample.
  • A bonded phase is a stationary phase that is covalently bonded to the support particles or to the inside wall of the column tubing.
  • A chromatogram is the visual output of the chromatograph. In the case of an optimal separation, different peaks or patterns on the chromatogram correspond to different components of the separated mixture.

Plotted on the x-axis is the retention time and plotted on the y-axis a signal (for example obtained by a spectrophotometer, mass spectrometer or a variety of other detectors) corresponding to the response created by the analytes exiting the system. In the case of an optimal system the signal is proportional to the concentration of the specific analyte separated.

  • A chromatograph is equipment that enables a sophisticated separation e.g. gas chromatographic or liquid chromatographic separation.
  • Chromatography is a physical method of separation in which the components to be separated are distributed between two phases, one of which is stationary (stationary phase) while the other (the mobile phase) moves in a definite direction.
  • The effluent is the mobile phase leaving the column.
  • An immobilized phase is a stationary phase which is immobilized on the support particles, or on the inner wall of the column tubing.
  • The mobile phase is the phase which moves in a definite direction. It may be a liquid (LC and CEC), a gas (GC), or a supercritical fluid (supercritical-fluid chromatography, SFC). A better definition: The mobile phase consists of the sample being separated/analyzed and the solvent that moves the sample through the column. In one case of HPLC the solvent consists of a carbonate/bicarbonate solution and the sample is the anions being separated. The mobile phase moves through the chromatography column (the stationary phase) where the sample interacts with the stationary phase and is separated.
  • Preparative chromatography is used to purify sufficient quantities of a substance for further use, rather than analysis.
  • The retention time is the characteristic time it takes for a particular analyte to pass through the system (from the column inlet to the detector) under set conditions. See also: Kovat's retention index
  • The sample is the matter analysed in chromatography. It may consist of a single component or it may be a mixture of components. When the sample is treated in the course of an analysis, the phase or the phases containing the analytes of interest is/are referred to as the sample whereas everything out of interest separated from the sample before or in the course of the analysis is referred to as waste.
  • The solute refers to the sample components in partition chromatography.
  • The solvent refers to any substance capable of solubilizing other substance, and especially the liquid mobile phase in LC.
  • The stationary phase is the substance which is fixed in place for the chromatography procedure. Examples include the silica layer in thin layer chromatography.

Techniques by chromatographic bed shape

Column chromatography

Column chromatography is a separation technique in which the stationary bed is within a tube. The particles of the solid stationary phase or the support coated with a liquid stationary phase may fill the whole inside volume of the tube (packed column) or be concentrated on or along the inside tube wall leaving an open, unrestricted path for the mobile phase in the middle part of the tube (open tubular column). Differences in rates of movement through the medium are calculated to different retention times of the sample.

In 1978, W. C. Still introduced a modified version of column chromatography called flash column chromatography (flash). The technique is very similar to the traditional column chromatography, except for that the solvent is driven through the column by applying positive pressure. This allowed most separations to be performed in less than 20 minutes, with improved separations compared to the old method. Modern flash chromatography systems are sold as pre-packed plastic cartridges, and the solvent is pumped through the cartridge. Systems may also be linked with detectors and fraction collectors providing automation. The introduction of gradient pumps resulted in quicker separations and less solvent usage.

In expanded bed adsorption, a fluidized bed is used, rather than a solid phase made by a packed bed. This allows omission of initial clearing steps such as centrifugation and filtration, for culture broths or slurries of broken cells.

Planar Chromatography

Planar chromatography is a separation technique in which the stationary phase is present as or on a plane. The plane can be a paper, serving as such or impregnated by a substance as the stationary bed (paper chromatography) or a layer of solid particles spread on a support such as a glass plate (thin layer chromatography).

Paper Chromatography

Paper chromatography is a technique that involves placing a small dot of sample solution onto a strip of chromatography paper. The paper is placed in a jar containing a shallow layer of solvent and sealed. As the solvent rises through the paper it meets the sample mixture which starts to travel up the paper with the solvent. Different compounds in the sample mixture travel different distances according to how strongly they interact with the paper. This paper is made of cellulose, a polar molecule, and the compounds within the mixture travel farther if they are non-polar. More polar substances bond with the cellulose paper more quickly, and therefore do not travel as far. This process allows the calculation of an Rf value and can be compared to standard compounds to aid in the identification of an unknown substance.

Thin layer chromatography

Thin layer chromatography (TLC) is a widely-employed laboratory technique and is similar to paper chromatography. However, instead of using a stationary phase of paper, it involves a stationary phase of a thin layer of adsorbent like silica gel, alumina, or cellulose on a flat, inert substrate. Compared to paper, it has the advantage of faster runs, better separations, and the choice between different adsorbents. Different compounds in the sample mixture travel different distances according to how strongly they interact with the adsorbent. This allows the calculation of an Rf value and can be compared to standard compounds to aid in the identification of an unknown substance. For even better resolution and to allow for quantitation, high-performance TLC can be used.

Techniques by physical state of mobile phase

Gas chromatography

Gas chromatography (GC), also sometimes known as Gas-Liquid chromatography, (GLC), is a separation technique in which the mobile phase is a gas. Gas chromatography is always carried out in a column, which is typically "packed" or "capillary" (see below) .

Gas chromatography (GC) is based on a partition equilibrium of analyte between a solid stationary phase (often a liquid silicone-based material) and a mobile gas (most often Helium). The stationary phase is adhered to the inside of a small-diameter glass tube (a capillary column) or a solid matrix inside a larger metal tube (a packed column). It is widely used in analytical chemistry; though the high temperatures used in GC make it unsuitable for high molecular weight biopolymers or proteins (heat will denature them), frequently encountered in biochemistry, it is well suited for use in the petrochemical, environmental monitoring, and industrial chemical fields. It is also used extensively in chemistry research.

Liquid chromatography

Liquid chromatography (LC) is a separation technique in which the mobile phase is a liquid. Liquid chromatography can be carried out either in a column or a plane. Present day liquid chromatography that generally utilizes very small packing particles and a relatively high pressure is referred to as high performance liquid chromatography (HPLC).

In the HPLC technique, the sample is forced through a column that is packed with irregularly or spherically shaped particles or a porous monolithic layer (stationary phase) by a liquid (mobile phase) at high pressure. HPLC is historically divided into two different sub-classes based on the polarity of the mobile and stationary phases. Technique in which the stationary phase is more polar than the mobile phase (e.g. toluene as the mobile phase, silica as the stationary phase) is called normal phase liquid chromatography (NPLC) and the opposite (e.g. water-methanol mixture as the mobile phase and C18 = octadecylsilyl as the stationary phase) is called reversed phase liquid chromatography (RPLC). Ironically the "normal phase" has fewer applications and RPLC is therefore used considerably more.

Specific techniques which come under this broad heading are listed below. It should also be noted that the following techniques can also be considered fast protein liquid chromatography if no pressure is used to drive the mobile phase through the stationary phase. See also Aqueous Normal Phase Chromatography.

Affinity chromatography

Affinity chromatography is based on selective non-covalent interaction between an analyte and specific molecules. It is very specific, but not very robust. It is often used in biochemistry in the purification of proteins bound to tags. These fusion proteins are labelled with compounds such as His-tags, biotin or antigens, which bind to the stationary phase specifically. After purification, some of these tags are usually removed and the pure protein is obtained.

Supercritical fluid chromatography

Supercritical fluid chromatography is a separation technique in which the mobile phase is a fluid above and relatively close to its critical temperature and pressure.

Techniques by separation mechanism

Ion exchange chromatography

Ion exchange chromatography utilizes ion exchange mechanism to separate analytes. It is usually performed in columns but the mechanism can be benefited also in planar mode. Ion exchange chromatography uses a charged stationary phase to separate charged compounds including amino acids, peptides, and proteins. In conventional methods the stationary phase is an ion exchange resin that carries charged functional groups which interact with oppositely charged groups of the compound to be retained. Ion exchange chromatography is commonly used to purify proteins using FPLC.

Size exclusion chromatography

Size exclusion chromatography (SEC) is also known as gel permeation chromatography (GPC) or gel filtration chromatography and separates molecules according to their size (or more accurately according to their hydrodynamic diameter or hydrodynamic volume). Smaller molecules are able to enter the pores of the media and, therefore, take longer to elute, whereas larger molecules are excluded from the pores and elute faster. It is generally a low resolution chromatography technique and thus it is often reserved for the final, "polishing" step of a purification. It is also useful for determining the tertiary structure and quaternary structure of purified proteins, especially since it can be carried out under native solution conditions.

Special techniques

Reversed-phase chromatography

Reversed-phase chromatography is an elution procedure used in liquid chromatography in which the mobile phase is significantly more polar than the stationary phase.

Two-dimensional chromatography

In some cases, the chemistry within a given column can be insufficient to separate some analytes. It is possible to direct a series of unresolved peaks onto a second column with different physico-chemical (Chemical classification) properties. Since the mechanism of retention on this new solid support is different from the first dimensional separation, it can be possible to separate compounds that are indistinguishable by one-dimensional chromatography.

Simulated Moving-Bed Chromatography

Pyrolysis gas chromatography

Fast protein liquid chromatography

Fast protein liquid chromatography (FPLC) is a term applied to several chromatography techniques which are used to purify proteins. Many of these techniques are identical to those carried out under high performance liquid chromatography, however use of FPLC techniques are typically for preparing large scale batches of a purified product.

Countercurrent chromatography

Countercurrent chromatography (CCC) is a type of liquid-liquid chromatography, where both the stationary and mobile phases are liquids. It involves mixing a solution of liquids, allowing them to settle into layers and then separating the layers.

Chiral chromatography

Chiral chromatography involves the separation of stereoisomers. In the case of enantiomers, these have no chemical or physical differences apart from being three dimensional mirror images. Conventional chromatography or other separation processes are incapable of separating them. To enable chiral separations to take place, either the mobile phase or the stationary phase must themselves be made chiral, giving differing affinities between the analytes. Chiral chromatography HPLC columns (with a chiral stationary phase) in both normal and reversed phase are commercially available.

See also

References

External links

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