Gas Chromatography Systems

Abstract

Overview All forms of chromatography separate components of a mixture by taking advantage of the fact that each component has a different affinity for a particular media. Both liquid and gas chromatographs use this principle and employ two different media—a sorbent-packed column and a solvent (termed the stationary phase and the mobile phase)—to separate a mixture into its components. The relative affinity of each component of a mixture, to both the stationary phase and the mobile phase, causes them to pass through the column with a unique transit time. This is the basic principle of chromatography. In “Fundamentals of Liquid Chromatography” (BI&T, July/August 2012), we learned that chromatography systems are used to separate a complex mixture into its components for further examination or identification. The liquid chromatograph performs relatively low-temperature analyses of bodily fluids and is best suited for separating proteins and peptides, measuring toxins, and measuring drug levels in serum for both therapeutic monitoring and unknown drug identification. The gas chromatograph (or GC for short) operates in a comparable manner, performs similar analyses on volatile samples, and does an excellent job of separating mixtures with similar vapor pressures and chemical structures. Gas chromatographs are used to provide fatty acid profiles and blood alcohol determinations. A GC is the only instrument sensitive enough to detect low concentrations of volatile organic mixtures, such as the aromatics (benzene, toluene, xylene, etc.). A GC connected to a mass spectrometer is often referred to as a gas chromatograph-mass spectrometer or GCMS, and is used extensively for performing confirmatory tests for both therapeutic and street drugs. As a general rule, if the sample is not compromised or degraded by the processing temperature, it is suitable for analysis by a GC. Like its cousin the liquid chromatograph, the GC uses both a sorbent-packed column and a solvent. However, since the mobile phase is a gas, the stationary phase can be one of two types. The first is a solid sorbent packed into a tube, similar to that used in liquid chromatography. The other stationary phase employs a solid support with a nonvolatile liquid coating. As is the case in a liquid chromatograph, the relative affinity of each component of a mixture to both the sorbent and the solvent causes each component to exit the column at a particular time after sample injection. Using Kovat’s Retention Index, the retention time of the unknown is compared to retention times stored in a sample library to determine the component(s) of the sample.