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GAS CHROMATOGRAPHY STATIONARY AND MOBILE PHASES

المؤلف:  Max M. Houck، Jay A. Siegel

المصدر:  Fundamentals of Forensic Science

الجزء والصفحة:  13

2026-07-05

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GAS CHROMATOGRAPHY STATIONARY AND MOBILE PHASES

The most versatile chromatography method is called gas chromatography (GC), sometimes called gas–liquid chromatography. Many people call the method simply GC. Modern GC is called capillary GC because the stationary phase is contained within a very narrow, hollow tube that is made of glass or a synthetic polymer and is often coated with a plastic to add strength. The inside of the column contains the stationary phase, a thick, high boiling, viscous liquid, that has the consistency of molasses. The stationary phase forms a thin coating on the inside of the capillary. The columns in capillary GCs can be up to 30 m long or on rare occasions, even longer. The develop ment of capillary columns in the late 1970s was a huge improvement in GC because it resulted in great increases in column efficiencies and separation of even similar sub stances. The older generation of GC columns was glass or copper tubes usually from 1/8″ to 1/4″ (or sometimes even wider) in diameter and generally 1 or 2 m long. They were filled with tiny, polymeric spheres that were coated with the same kind of stationary phase described above for capillary columns. These were called packed columns. They were orders of magnitude less efficient in separating complex mixtures. The mobile phase in GC is an inert gas. The most commonly used gas is helium. Nitrogen is also used but gives inferior results. The best gas is hydrogen but it is more expensive than helium and is quite flammable. If there is a leak in the system, an explosion or fire can result. In a gas chromatograph, the mobile phase is forced through the stationary phase under pressure. Since the analytes are carried through the stationary phase by the gaseous mobile phase, they too have to be in a vapor state. This results in one of the limitations of GC. The analytes have to be heated until they vaporize. GC stationary phases are only stable to about 350 °C so the analyte must be able to be converted to a vapor (at least partially) at this temperature. In addition, the analytes must be thermally stable at such temperatures. Some substances decompose at high temperatures and would not be suitable for GC analysis. Forensically important substances that are not stable at high temperatures include many explosives. The presence of the inert gas as the mobile phase purges the entire system of oxygen so there is no chance for analytes to burn when they are heated inside the instrument.