Gas chromatography

Gas-liquid chromatography also commonly referred to as GC is a separation technique first described in 1952 by James and Martin. In most GC column, the stationary phase is a liquid and the mobile phase is an inert gas, thus the name gasliquid chromatography. Typically, the stationary phase has a low vapor pressure so that at column temperature it can be considered nonvolatile. Introduction of the capillary column dramatically improved resolution of peaks in GC analysis. Resolution equivalent to several hundred thousand theoretical plates can be achieved using a fused silica capillary column, which was originally derived from the fiber-optic technology. Depending on the stationary phase composition, a GC column may have low polarity, intermediate polarity, or high polarity. Research is ongoing to develop new polymers for use as a stationary phase in GC column. Mayer-Helm reported development of a novel dimethylsiloxane-based copolymer for use as stationary phase in GC column (10). Microprocessor control of oven temperature and automatic sample injection techniques also enhanced both performance and ease of automation of GC technique in clinical laboratories. The sensitivity and specificity of GC analysis depends on the choice of detector. Mass spectrometry can be used in combination with a gas chromatograph, and mass spectrometry is capable of producing a mass spectrum of any compound coming out of the column of gas chromatograph. Nitrogen phosphorus detector is specific for nitrogen- and phosphorus-containing compounds and is very sensitive. Electron capture detector can detect any halogen-containing compounds. Flame ionization and thermal conductivity detectors are also used in GC. The major limitation of GC is that this technique can only be applied to volatile substances with relatively low molecular weights. Polar compounds cannot be analyzed by this technique. However, a relatively polar compound can be chemically converted to a nonpolar compound (derivatization) for analysis by GC.

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