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.
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, 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.
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.
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
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 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).
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 (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.