Chromatography

The development of chromatography

Types of chromatographic attraction

Industrial applications of chromatography

Resources

Chromatography refers to a variety of related laboratory techniques for separating mixtures of chemicals into their individual compounds. The basic principle of chromatography is that different compounds will stick to a solid surface, or dissolve in a film of liquid, to different degrees. The differing properties allow each compound to be retained in a distinctive way. As the sample is collected after passage through the chromatography column, the separated compounds can be individually collected.

When a gas or liquid containing a mixture of different compounds is made to flow over such a surface, the molecules of the various compounds can be prone to associate with the surface. The association can be on the basis of charge, with positively charged groups of the sample compound being attracted to negatively-charged surface groups. Or, in the case of a sample that is hydrophobic (water-hating), the association can be to partition away from the watery fluid. Sample compounds may even be attracted to specific proteins positioned on the surface of the chromatography column.

If the attraction between sample and surface is not too strong, a given molecule will become stuck and unstuck hundreds or thousands of times as it is swept along the surface. This repetition exaggerates even tiny differences in the various molecules, and they become spread out along the length of the chromatography column, as some compounds move more slowly than others. After a given time, the different compounds will have reached different places along the surface and will be physically separated from one another.

Using variations of this basic phenomenon, chromatographic methods have become an extremely powerful and versatile tool for separating and analyzing a vast variety of chemical compounds in quantities from picograms (10^-12 gram) to tons.

Chromatographic methods all share certain characteristics, although they differ in size, shape, and configuration. Typically, a stream of liquid or gas (the mobile phase) flows constantly through the column packed with a porous solid material (the stationary phase). A sample of the chemical mixture is injected into the mobile phase at one end of the column, and the compounds separate as they move along. The individual separated compounds can be removed one at a time as they exit (elute) from the column.

Because it usually does not alter the molecular structure of the compounds, chromatography can provide a non-destructive way to obtain pure chemicals from various sources. It works well on very large and very small scales; chromatographic processes are used both by scientists studying micrograms of a substance in the laboratory and by industrial chemists separating tons of material.

The technology of chromatography has advanced rapidly in the past few decades. It is now possible to obtain separation of mixtures in which the components are so similar they only differ in the way their atoms are oriented in space, in other words, they are isomers of the same compounds. It is also possible to obtain separation of a few parts per million of a contaminant from a mixture of much more concentrated materials.

The development of chromatography

The first paper on the subject of chromatography appeared in 1903, written by Mikhail Semyonovich Tsvet (1872-1919), a Russian-Italian biochemist, who also coined the term. Tsvet had managed to separate a mixture of plant pigments, including chlorophyll, on a column packed with finely ground calcium carbonate, using petroleumether as the mobile phase. As the colored mixture passed down the column, it separated into individual colored bands (the term chromatography comes from the Greek words chroma, meaning color, and graphein, meaning writing, or drawing). Although occasionally used by biochemists, chromatography as a science lagged until 1942, when A. J. P. Martin (1910–) and R. L. M. Synge (1914–1994) developed the first theoretical explanations for the chromatographic separation process. Although they eventually received the Nobel Prize in chemistry for this work, chromatography did not come into wide use until 1952, when Martin, this time working with A. T. James, described a way of using a gas instead of a liquid as the mobile phase and a highly viscous liquid coated on solid particles as the stationary phase.

Gas-liquid chromatography (now called gas chromatography) was an enormous advance. Eventually, the stationary phase could be chemically bonded to the solid support, which improved the temperature stability of the column’s packing. Gas chromatographs could then be operated at high temperatures, so even large molecules could be vaporized and would progress through the column without the stationary phase vaporizing and bleeding off. Additionally, since the mobile phase was a gas, the separated compounds were very pure; there was no liquid solvent to remove. Subsequent research on the technique produced many new applications.

The shapes of the columns themselves began to change. Originally vertical tubes an inch or so in diameter, columns began to get longer and thinner when it was found that this increased the efficiency of separation. Eventually, chemists were using coiled glass or fused silica capillary tubes less than a millimeter in diameter and many yards long. Capillaries cannot be packed, but they are so narrow that the stationary phase can simply be a thin coat on the inside of the column.

A somewhat different approach is the set of techniques known as planar or thin layer chromatography (TLC), in which no column is used. The stationary phase is thinly coated on a glass or plastic plate. A spot of sample is placed on the plate, and the mobile phase migrates through the stationary phase by capillary action (the movement of liquid molecules through the support from a region of higher density to a region of lower density).

In the mid-1970s, interest in liquid mobile phases for column chromatography resurfaced when it was discovered that the efficiency of separation could be vastly improved by pumping the liquid through a short, packed column under pressure, rather than allowing it to flow slowly down a vertical column by gravity alone. High-pressure liquid chromatography, also called high performance liquid chromatography (HPLC), is now widely used in industry. A variation on HPLC is Supercritical Fluid Chromatography (SFC). Certain gases (carbon dioxide, for example), when highly pressurized above a certain temperature, become a state of matter intermediate between gas and liquid. These so-called supercritical fluids have unusual solubility properties, some of the advantages of both gases and liquids, and appear very promising for chromatographic use.

Most chemical compounds are not highly colored, as were the ones Tsvet used. A chromatographic separation of a colorless mixture would be fruitless if there were no way to tell exactly when each pure compound eluted from the column. All chromatographs thus must have a device attached for this purpose and some kind of recorder to capture the output of the detector—usually a chart recorder or its computerized equivalent. In gas chromatography, several kinds of detectors have been developed; the most common are the thermal conductivity detector, the flame ionization detector, and the electron capture detector. For HPLC, an ultraviolet detector is standardized to the concentration of the separated compound. The sensitivity of the detector is of special importance, and research has continually concentrated on increasing this sensitivity, because chemists often need to detect and quantify exceedingly small amounts of a material.

Within the last few decades, chromatographic instruments have been attached to other types of analytical instrumentation so that the mixture’s components can be identified as well as separated (this takes the concept of the “detector” to its logical extreme). Most commonly, this second instrument has been a mass spectrometer, which allows identification of compounds based on the masses of molecular fragments that appear when the molecules of a compound are broken up. Currently, chromatography as both science and practical tool is intensively studied, and several scientific journals are devoted exclusively to chromatographic research.

Types of chromatographic attraction

Absorption chromatography (the original type of chromatography) depends on physical forces such as dipole attraction to hold the molecules onto the surface of the solid packing. In gas chromatography and HPLC, however, the solubility of the mixture’s molecules in the stationary phase coating determines which ones progress through the column more slowly. Polarity can have an influence here as well. In gel filtration (also called size-exclusion or gel permeation) chromatography, the relative sizes of the molecules in the mixture determine which ones exit the column first. Large molecules flow right through; smaller ones are slowed down because they spend time trapped in the pores of the gel. Ion exchange chromatography depends on the relative strength with which ions are held to an ionic resin. Ions that are less strongly attached to the resin are displaced by more strongly attached ions. Hence the name ion exchange: one kind of ion is exchanged for another. This is the same principle upon which home water softeners operate. Affinity chromatography uses a stationary phase composed of materials that have been chemically altered. In this type of chromatography, the stationary phase is attached to a compound with a specific affinity for the desired molecules in the mobile phase. This process is similar to that of ion exchange chromatography and is used mainly for the recovery of biological compounds. Hydrophobic Interaction Chromatography is used for amino acids that do not carry a positive or negative charge. In this type of chromatography, the hydrophobic amino acids are attracted to the solid phase, which is composed of materials containing hydrophobic groups.

Chemists choose the mobile and stationary phases carefully because it is the relative interaction of the mixture’s compounds with those two phases that determines how efficient the separation can be. If the compounds have no attraction for the stationary phase at all, they will flow right through the column without separating. If the compounds are too strongly attracted to the stationary phase, they may stick permanently inside the column.

Industrial applications of chromatography

Chromatography of many kinds is widely used throughout the chemical industry. Environmental testing laboratories look for trace quantities of contaminants such as PCBs in waste oil and pesticides such as DDT in groundwater. The Environmental Protection Agency uses chromatography to test drinking water and to monitor air quality. Pharmaceutical companies use chromatography both to prepare large quantities of extremely pure materials, and also to analyze the purified compounds for trace contaminants.

A growing use of chromatography in the pharmaceutical industry is for the separation of chiral compounds (compounds that are mirror images of each other but which cannot be superimposed on one another). These compounds have molecules that differ slightly in the way their atoms are oriented in space. Although identical in almost every other way, including molecular weight, element composition, and physical properties, the two different forms—called optical isomers, or enantiomers—can have enormous differences in their biological activity. The compound thalidomide, for example, has two optical isomers. One causes birth defects when women take it early in pregnancy; the other isomer does not. Because this compound looks promising for the treatment of certain drug-resistant illnesses, it is important that the benign form be separated completely from the dangerous isomer.

Chromatography is used for quality control in the food industry, by separating and analyzing additives, vitamins, preservatives, proteins, and amino acids. It can also separate and detect contaminants such as aflatoxin, a cancer-causing chemical produced by a mold on peanuts. Chromatography can be used for various purposes, from finding drug compounds in urine or other body fluids to looking for traces of flammable chemicals in burned material from possible arson sites.

Resources

BOOKS

Grob, Robert L. and Eugene F. Barry. Modern Practice of Gas Chromatography. New York: Wiley-Interscience, 2004.

Miller, James M. Chromatography: Concepts and Contrasts. New York: Wiley-Interscience, 2004.

Poole, Colin F. The Essence of Chromatography. St. Louis: Elsevier Science, 2002.

Gail B. C. Marsella

Chromatography

Chromatography is a family of laboratory techniques for separating mixtures of chemicals into their individual compounds. The basic principle of chromatography is that different compounds will stick to a solid surface, or dissolve in a film of liquid, to different degrees.

To understand chromatography, suppose that all the runners in a race have sticky shoe soles, and that some runners have stickier soles than others. The runners with the stickier shoes will not be able to run as fast. All other things being equal, the runners will cross the finish line in the exact order of their shoe stickiness—the least sticky first and the stickiest last. Even before the race is over, they will spread out along the track in order of their stickiness.

Similarly, different chemical compounds will stick to a solid or liquid surface to varying degrees. When a gas or liquid containing a mixture of different compounds is made to flow over such a surface, the molecules of the various compounds will tend to stick to the surface. If the stickiness is not too strong, a given molecule will become stuck and unstuck hundreds or thousands of times as it is swept along the surface. This repetition exaggerates even tiny differences in the various molecules' stickiness, and they become spread out along the "track," because the stickier compounds move more slowly than the less-sticky ones do. After a given time, the different compounds will have reached different places along the surface and will be physically separated from one another. Or, they can all be allowed to reach the far end of the surface—the "finish line"—and be detected or measured one at a time as they emerge.

Using variations of this basic phenomenon, chromatographic methods have become an extremely powerful and versatile tool for separating and analyzing a vast variety of chemical compounds in quantities from picograms (10-12 gram) to tons.

Chromatographic methods all share certain characteristics, although they differ in size, shape, and configuration. Typically, a stream of liquid or gas (the mobile phase) flows constantly through a tube (the column) packed with a porous solid material (the stationary phase). A sample of the chemical mixture is injected into the mobile phase at one end of the column, and the compounds separate as they move along. The individual separated compounds can be removed one at a time as they exit (or "elute from") the column.

Because it usually does not alter the molecular structure of the compounds, chromatography can provide a non-destructive way to obtain pure chemicals from various sources. It works well on very large and very small scales; chromatographic processes are used both by scientists studying micrograms of a substance in the laboratory, and by industrial chemists separating tons of material.

The technology of chromatography has advanced rapidly in the past few decades. It is now possible to obtain separation of mixtures in which the components are so similar they only differ in the way their atoms are oriented in space , in other words, they are isomers of the same compounds. It is also possible to obtain separation of a few parts per million of a contaminant from a mixture of much more concentrated materials.

The development of chromatography

The first paper on the subject appeared in 1903, written by Mikhail Semyonovich Tsvet (1872-1919), a Russian-Italian biochemist, who also coined the word chromatography. Tsvet had managed to separate a mixture of plant pigments, including chlorophyll , on a column packed with finely ground calcium carbonate , using petroleum ether as the mobile phase. As the colored mixture passed down the column, it separated into individual colored bands (the term chromatography comes from the Greek words chroma, meaning color , and graphein, meaning writing, or drawing). Although occasionally used by biochemists, chromatography as a science lagged until 1942, when A. J. P. Martin (1910-2002) and R. L. M. Synge (1914-1994) developed the first theoretical explanations for the chromatographic separation process. Although they eventually received the Nobel Prize in chemistry for this work, chromatography did not come into wide use until 1952, when Martin, this time working with A. T. James, described a way of using a gas instead of a liquid as the mobile phase, and a highly viscous liquid coated on solid particles as the stationary phase.

Gas-liquid chromatography (now called gas chromatography) was an enormous advance. Eventually, the stationary phase could be chemically bonded to the solid support, which improved the temperature stability of the column's packing. Gas chromatographs could then be operated at high temperatures, so even large molecules could be vaporized and would progress through the column without the stationary phase vaporizing and bleeding off. Additionally, since the mobile phase was a gas, the separated compounds were very pure; there was no liquid solvent to remove. Subsequent research on the technique produced many new applications.

The shapes of the columns themselves began to change, too. Originally vertical tubes an inch or so in diameter, columns began to get longer and thinner when it was found that this increased the efficiency of separation. Eventually, chemists were using coiled glass or fused silica capillary tubes less than a millimeter in diameter and many yards long. Capillaries cannot be packed, but they are so narrow that the stationary phase can simply be a thin coat on the inside of the column.

A somewhat different approach is the set of techniques known as "planar" or "thin layer" chromatography (TLC), in which no column is used at all. The stationary phase is thinly coated on a glass or plastic plate. A spot of sample is placed on the plate, and the mobile phase migrates through the stationary phase by capillary action .

In the mid-1970s, interest in liquid mobile phases for column chromatography resurfaced when it was discovered that the efficiency of separation could be vastly improved by pumping the liquid through a short packed column under pressure , rather than allowing it to flow slowly down a vertical column by gravity alone. High-pressure liquid chromatography, also called high performance liquid chromatography (HPLC), is now widely used in industry. A variation on HPLC is Supercritical Fluid Chromatography (SFC). Certain gases (carbon dioxide , for example), when highly pressurized above a certain temperature, become a state of matter intermediate between gas and liquid. These "supercritical fluids" have unusual solubility properties, some of the advantages of both gases and liquids, and appear very promising for chromatographic use.

Most chemical compounds are not highly colored, as were the ones Tsvet used. A chromatographic separation of a colorless mixture would be fruitless if there were no way to tell exactly when each pure compound eluted from the column. All chromatographs thus must have a device attached, and some kind of recorder to capture the output of the detector—usually a chart recorder or its computerized equivalent. In gas chromatography, several kinds of detectors have been developed; the most common are the thermal conductivity detector, the flame ionization detector, and the electron capture detector. For HPLC, the UV detector is standardized to the concentration of the separated compound. The sensitivity of the detector is of special importance, and research has continually concentrated on increasing this sensitivity, because chemists often need to detect and quantify exceedingly small amounts of a material.

Within the last few decades, chromatographic instruments have been attached to other types of analytical instrumentation so that the mixture's components can be identified as well as separated (this takes the concept of the "detector" to its logical extreme). Most commonly, this second instrument has been a mass spectrometer, which allows identification of compounds based on the masses of molecular fragments that appear when the molecules of a compound are broken up. Currently, chromatography as both science and practical tool is intensively studied, and several scientific journals are devoted exclusively to chromatographic research.

Types of chromatographic attraction

Absorption chromatography (the original type of chromatography) depends on physical forces such as dipole attraction to hold the molecules onto the surface of the solid packing. In gas chromatography and HPLC, however, the solubility of the mixture's molecules in the stationary phase coating determines which ones progress through the column more slowly. Polarity can have an influence here as well. In gel filtration (also called size-exclusion or gel permeation) chromatography, the relative sizes of the molecules in the mixture determine which ones exit the column first. Large molecules flow right through; smaller ones are slowed down because they spend time trapped in the pores of the gel. Ion exchange chromatography depends on the relative strength with which ions are held to an ionic resin. Ions that are less strongly attached to the resin are displaced by more strongly attached ions. Hence the name ion exchange: one kind of ion is exchanged for another. This is the same principle upon which home water softeners operate. Affinity chromatography uses a stationary phase composed of materials that have been chemically altered. In this type of chromatography, the stationary phase is attached to a compound with a specific affinity for the desired molecules in the mobile phase. This process is similar to that of ion exchange chromatography, and is used mainly for the recovery of biological compounds. Hydrophobic Interaction Chromatography is used for amino acids that do not carry a positive or negative charge. In this type of chromatography, the hydrophobic amino acids are attracted to the solid phase, which is composed of materials containing hydrophobic groups.

Chemists choose the mobile and stationary phases carefully because it is the relative interaction of the mixture's compounds with those two phases that determines how efficient the separation can be. If the compounds have no attraction for the stationary phase at all, they will flow right through the column without separating. If the compounds are too strongly attracted to the stationary phase, they may stick permanently inside the column.

Industrial applications of chromatography

Chromatography of many kinds is widely used throughout the chemical industry. Environmental testing laboratories look for trace quantities of contaminants such as PCBs in waste oil, and pesticides such as DDT in groundwater . The Environmental Protection Agency uses chromatography to test drinking water and to monitor air quality. Pharmaceutical companies use chromatography both to prepare large quantities of extremely pure materials, and also to analyze the purified compounds for trace contaminants.

A growing use of chromatography in the pharmaceutical industry is for the separation of chiral compounds. These compounds have molecules that differ slightly in the way their atoms are oriented in space. Although identical in almost every other way, including molecular weight , element composition, and physical properties, the two different forms—called optical isomers, or enantiomers—can have enormous differences in their biological activity. The compound thalidomide , for example, has two optical isomers. One causes birth defects when women take it early in pregnancy; the other isomer does not. Because this compound looks promising for the treatment of certain drug-resistant illnesses, it is important that the benign form be separated completely from the dangerous isomer.

Chromatography is used for quality control in the food industry, by separating and analyzing additives, vitamins, preservatives, proteins , and amino acids. It can also separate and detect contaminants such as aflatoxin, a cancer-causing chemical produced by a mold on peanuts. Chromatography can be used for purposes as varied as finding drug compounds in urine or other body fluids, to looking for traces of flammable chemicals in burned material from possible arson sites.

See also Compound, chemical; Mixture, chemical.

Resources

books

Ebbing, Darrell. General Chemistry. 3d ed. Boston: Houghton Mifflin, 1990.

periodicals

Poole, F., and S.A. Schuette. Contemporary Practice of Chromatography Amsterdam: Elsevier, 1984.

Gail B. C. Marsella

Chromatography

Chromatography works by separating the individual parts of a mixture so that each one can be analyzed and identified. In the decades since its invention, the chromatograph has become an essential piece of equipment in bio-chemical laboratories. Using the analytical technique of chromatography, scientists can tell what chemical compounds are present in complex mixtures. These mixtures include such diverse things as smog, cigarette smoke, petroleum products, or even coffee aroma. Without chromatography, chemists might not have been able to synthesize proteins such as insulin or understand how plants use the sun's energy to make food.

The First Chromatograph

The first chromatograph was invented by Russian botanist Mikhail Semenovich Tsvett (1872-1919). While working in Poland, Tsvett was looking for a method of separating a mixture of plant pigments (tints) which are chemically very similar to each other. To isolate different types of chlorophyll, he trickled a mixture of dissolved pigments through a glass tube packed with calcium carbonate powder. As the solution washed downward, each pigment stuck to the powder with a different degree of strength, creating a series of colored bands. Each band of color represented a different substance. Tsvett referred to the colored bands as a chromatogram. He also suggested that the technique (now called adsorption chromatography) could be used to separate colorless substances.

Although Tsvett published a report of his work in the early 1900s, chemists paid very little attention to it. There were a few reasons for ignoring the work. First, the report was written in Russian, which few Western chemists of the time read. Second, the technique may have seemed too simple to chemists who were used to relying on lengthy extraction, crystallization, or distillation processes to separate mixtures. Within a few years, Tsvett's technique was rediscovered. The rediscovery was by the German organic chemist Richard Martin Willstatter (1872-1942), who was also studying chlorophyll. By introducing chromatography to Western European scientists, Willstatter helped establish one of the most versatile analytical techniques known to chemistry.

Ion-Exchange Chromatography

Chromatography was found to work on almost all kinds of mixtures, including colorless ones, just as Tsvett had predicted. Absorbing powders were discovered that perform better than calcium carbonate for separating ordinary molecules. Also, compounds known as "zeolites" were introduced to separate individual ions, or electrically charged particles, in a process called ion-exchange chromatography. American chemist Frank Harold Spedding adapted this technique to the separation of rare-earth metals. In the 1930s, synthetic resins were developed for complex ion-exchange processes. During World War II (1939-1945), life rafts were equipped with survival kits that contained resins for removing most salts from seawater.

Paper chromatography

The most dramatic advance in the history of chromatography took place in 1944. It was then that scientists discovered that a strip of porous (full of small holes) filter paper could substitute for the column of absorbing powder. The technique was called paper chromatography. A drop of the mixture to be separated is placed on the paper, then one edge is dipped into a solvent (a substance that dissolves). The solvent spreads across the paper, carrying the mixture's components with it.

When the components are finished spreading, the paper is dried and sprayed with a reagent that reveals a change in color. Because the components move at different speeds, they show up as distinct, physically separated spots that can be cut out with scissors and further analyzed. The paper method is a type of partition chromatography, which is based on differences in solubility (the measured rate at which one substance will dissolve in another) rather than differences in adsorption. One of its advantages is that it requires only a small sample of material.

Martin and Synge

Paper chromatography was invented by two British biochemists, Archer John Porter Martin (1910-) and Richard Laurence Millington Synge (1914-). In 1941 Martin and Synge began working together on proteins, which are made up of chains of amino acids. The duo was trying to characterize a particular protein by determining the precise numbers of each amino acid present. Amino acids are so similar to each other, however, that the problem of separating them had defeated a whole generation of biochemists. Martin and Synge's development of paper chromatography to solve this problem was an instant success. It worked not only on amino acids but also on various other mixtures. The two scientists were awarded the 1952 Nobel Prize in chemistry for their work.

Martin and Synge's research led to a number of other important scientific advances. After Synge determined the structure of an antibiotic peptide called "Gramicidin-S," Frederick Sanger (1918-) used paper chromatography to figure out the structure of the insulin molecule. He determined the number of amino acids in it as well as the order in which they occurred. Insulin is now used to control blood sugar levels in people afflicted with diabetes.

The same technique was used by Melvin Calvin (1911-) during the 1950s. Calvin discovered the complex series of reactions that enable green plants to convert solar energy into the chemical energy stored in food. Working with green algal (algae) cells, Calvin interrupted the photosynthetic process (process by which plants that contain chlorophyll use light to change carbon dioxide and water to carbohydrates) at different stages by plunging the cells into alcohol. Then he crushed them and separated their components via paper chromatography. Calvin was thus able to identify at least ten different intermediate products that had been created within a few seconds.

Paper chromatography was also used by Austrian-American biochemist Erwin Chargaff (1905-), who modified the technique to study the components of the nucleic acid molecule. His research revealed four components, or nitrogenous bases, that occur in pairs. British biochemists James Dewey Watson and Francis Harry Compton Crick later used these results to work out the structure of DNA (deoxyribonucleic acid).

Gas chromatography

In addition to inventing paper chromatography, Martin developed another technique called gas chromatography. The process allows chemists to separate mixtures of gases, or substances that can be vaporized or gasified by heat. Instead of a liquid solvent, helium gas is usually used to force the mixture through a column and separate the gaseous components. Martin and his colleague A. T. James first used gas chromatography to micro-analyze fatty acids.

The widespread acceptance of gas chromatography is unique in the laboratory instrumentation field. Today it is used in almost every branch of the chemical industry, particularly in the production of petrochemicals from oil and natural gas. One of the most common fixtures in biochemical laboratories is "GCMS" (gas chromatography, mass spectrometry) analytical equipment. This equipment uses gas chromatography to separate individual components from complex organic mixtures, then uses mass spectrometry to identify each component.

Thin-Layer Chromatography

Recently, chromatography has evolved into even more sophisticated analytical techniques. In thin-layer chromatography, for example, an alumina gel, silica gel, or other finely divided solid is spread onto a glass plate in a thin, uniform layer. This takes the place of filter paper in the chromatographic process. The technique is not only faster than paper chromatography, but it can also separate smaller quantities of pure components. It is often used in the pharmaceutical industry to isolate penicillin and other antibiotics.

Chromatography

Chromatography is a family of laboratory techniques for separating mixtures of chemicals into their individual compounds. The basic principle of chromatography is that different compounds will stick to a solid surface or dissolve in a film of liquid to different degrees. Chromatography is used extensively in forensics, from analyzing body fluids for the presence of illicit drugs , to fiber analysis, blood analysis from a crime scene, and at airports to detect residue from explosives .

When a gas or liquid containing a mixture of different compounds is made to flow over such a surface, the molecules of the various compounds will tend to stick to the surface. If the stickiness is not too strong, a given molecule will become stuck and unstuck hundreds or thousands of times as it is swept along the surface. This repetition exaggerates even tiny differences in the various molecules' stickiness, and they become spread out along the "track," because the stickier compounds move more slowly than the less sticky ones do. After a given time, the different compounds will have reached different places along the surface and will be physically separated from one another. Or, they can all be allowed to reach the far end of the separation surface and be detected or measured one at a time as they emerge.

Using variations of this basic phenomenon, chromatographic methods have become an extremely powerful and versatile tool for separating and analyzing a vast variety of chemical compounds in quantities from picograms (10^-12 gram) to tons.

Chromatographic methods all share certain characteristics, although they differ in size, shape, and configuration. Typically, a stream of liquid or gas (the mobile phase) flows constantly through a tube (the column) packed with a porous solid material (the stationary phase). A sample of the chemical mixture is injected into the mobile phase at one end of the column, and the compounds separate as they move along. The individual separated compounds can be removed one at a time as they exit (or "elute from") the column.

Because it usually does not alter the molecular structure of the compounds, chromatography can provide a non-destructive way to obtain pure chemicals from various sources. It works well on very large and very small scales; chromatographic processes are used both by scientists studying micrograms of a substance in the laboratory, and by industrial chemists separating tons of material.

The technology of chromatography has advanced rapidly in the past few decades. It is now possible to obtain separation of mixtures in which the components are so similar they only differ in the way their atoms are oriented in space, in other words, they are isomers of the same compounds. It is also possible to obtain separation of a few parts per million of a contaminant from a mixture of much more concentrated materials.

In gas-liquid chromatography (now called gas chromatography), the material that separates components is chemically bonded to the solid support, which improves the temperature stability of the column's packing. Gas chromatographs can be operated at high temperatures, so even large molecules can be vaporized and progress through the column without the stationary phase vaporizing and bleeding off. Additionally, since the mobile phase is a gas, the separated compounds are very pure; there is no liquid solvent to remove.

The shapes of chromatographic columns, originally vertical tubes an inch or so (2–3 cm) in diameter, became longer and thinner when it was found that this increased the efficiency of separation. Eventually, chemists were using coiled glass or fused silica capillary tubes less than a millimeter in diameter and many yards long. Capillaries cannot be packed, but they are so narrow that the stationary phase can simply be a thin coat on the inside of the column.

A somewhat different approach is the set of techniques known as "planar" or "thin layer" chromatography (TLC), in which no column is used at all. The stationary phase is thinly coated on a glass or plastic plate. A spot of sample is placed on the plate, and the mobile phase migrates through the stationary phase by capillary action.

In the mid-1970s, interest in liquid mobile phases for column chromatography resurfaced when it was discovered that the efficiency of separation could be vastly improved by pumping the liquid through a short packed column under pressure, rather than allowing it to flow slowly down a vertical column by gravity alone. High-pressure liquid chromatography, also called high performance liquid chromatography (HPLC), is now widely used in industry. A variation on HPLC is super-critical fluid chromatography (SFC). Certain gases (carbon dioxide, for example), when highly pressurized above a certain temperature, become a state of matter intermediate between gas and liquid. These "supercritical fluids" have unusual solubility properties, some of the advantages of both gases and liquids, and appear very promising for chromatographic use.

All chromatographs must have a detection device attached, and some kind of recorder to capture the output of the detector—usually a chart recorder or its computerized equivalent. In gas chromatography, several kinds of detectors have been developed; the most common are the thermal conductivity detector, the flame ionization detector, and the electron capture detector. For HPLC, the UV detector is standardized to the concentration of the separated compound. The sensitivity of the detector is of special importance, and research has continually concentrated on increasing this sensitivity, because chemists often need to detect and quantify exceedingly small amounts of a material.

Within the last few decades, chromatographic instruments have been attached to other types of analytical instrumentation so that the mixture's components can be identified as well as separated (this takes the concept of the "detector" to its logical extreme). Most commonly, this second instrument has been a mass spectrometer, which allows identification of compounds based on the masses of molecular fragments that appear when the molecules of a compound are broken up.

Absorption chromatography (the original type of chromatography) depends on physical forces such as dipole attraction to hold the molecules onto the surface of the solid packing. In gas chromatography and HPLC, however, the solubility of the mixture's molecules in the stationary phase coating determines which ones progress through the column more slowly. Polarity can have an influence here as well. In gel filtration (also called size-exclusion or gel permeation) chromatography, the relative sizes of the molecules in the mixture determine which ones exit the column first. Large molecules flow right through; smaller ones are slowed down because they spend time trapped in the pores of the gel. Ion exchange chromatography depends on the relative strength with which ions are held to an ionic resin. Ions that are less strongly attached to the resin are displaced by more strongly attached ions. Hence the name ion exchange: one kind of ion is exchanged for another. This is the same principle upon which home water softeners operate. Affinity chromatography uses a stationary phase composed of materials that have been chemically altered. In this type of chromatography, the stationary phase is attached to a compound with a specific affinity for the desired molecules in the mobile phase. This process is similar to that of ion exchange chromatography, and is used mainly for the recovery of biological compounds. Hydrophobic interaction chromatography is used for amino acids that do not carry a positive or negative charge. In this type of chromatography, the hydrophobic amino acids are attracted to the solid phase, which is composed of materials containing hydrophobic groups.

Chemists choose the mobile and stationary phases carefully because it is the relative interaction of the mixture's compounds with those two phases that determines how efficient the separation can be. If the compounds have no attraction for the stationary phase at all, they will flow right through the column without separating. If the compounds are too strongly attracted to the stationary phase, they may stick permanently inside the column.

see also Analytical instrumentation; Gas chromatograph-mass spectrometer.

Chromatography

Chromatography is the process of separating mixtures of chemicals into individual components as a means of identification or purification. It derives from the Greek words chroma, meaning color, and graphy, meaning writing. The word was coined in 1906 by the Russian chemist Mikhail Tsvett who used a column to separate plant pigments. Currently chromatography is applied to many types of separations far beyond those of just color separations. Common chromatographic applications include gas-liquid chromatography (GC), liquid-solid chromatography (LC), thin layer chromatography (TLC), ion exchange chromatography, and gel permeation chromatography (GPC). All of these methods are invaluable in analytical environmental chemistry , particularly GC, LC, and GPC.

The basic principle of chromatography is that different compounds have different retentions when passed through a given medium. In a chromatographic system, one has a mobile phase and a stationary phase. The mixture to be separated is introduced in the mobile phase and passed through the stationary phase. The compounds are selectively retained by the stationary phase and move at different rates which allows the compounds to be separated.

In gas chromatography, the mobile phase is a gas and the stationary phase is a liquid fixed to a solid support. Liquid samples are first vaporized in the injection port and carried to the chromatographic column by an inert gas which serves as the mobile phase. The column contains the liquid stationary phase, and the compounds are separated based on their different vapor pressures and their different affinities for the stationary phase. Thus different types of separations can be optimized by choosing different stationary phases, and by altering the temperature of the column. As the compounds elute from the end of the column, they are detected by one of a number of methods that have specificity for different chemical classes.

Liquid chromatography consists of a liquid mobile phase and a solid stationary phase. There are two general types of liquid chromatography: column chromatography and high pressure liquid chromatography (HPLC). In column chromatography, the mixture is eluted through the column containing stationary packing material by passing successive volumes of solvents or solvent mixtures through the column. Separations result as a function of both chemical-solvent interactions as well as chemical-stationary phase interactions. Often this technique is used in a preparative manner to remove interferences from environmental sample extracts. HPLC refers to specific instruments designed to perform liquid chromatography under very high pressures to obtain a much greater degree of resolution. The column outflow is passed through a detector and can be collected for further processing if desired. Detection is typically by ultraviolet light or fluorescence.

A variation of column chromatography is gel permeation chromatography (GPC), which separates chemicals based on size exclusion. The column is packed with porous spheres, which allow certain size chemicals to penetrate the spheres and excludes larger sizes. As the sample mixture traverses the column, larger molecules move more quickly and elute first while smaller molecules require longer elution times. An example of this application in environmental analyses is the removal of lipids (large molecules) from fish tissue extracts being analyzed for pesticides (small molecules).

[Deborah L. Swackhammer ]

RESOURCES

BOOKS

McNair, H. M., and E. J. Bonelli. Basic Gas Chromatography. Palo Alto, CA: Varian Instruments, 1969.

Peters, D. G., J. M. Hayes, and G. M. Hieftje. Chemical Separations and Measurements. Philadelphia: Saunders, 1974.