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.