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  • October 7th, 2010

    Gas Chromatography Drug Testing

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    Gas Chromatography Drug Testing

    Gas Chromatography (GC) is the technique of separating various elements of a mixture. It can be used with only those mixtures and compounds that do not decompose when vaporized. Gas chromatography is typically used for separating various components of a mixture, determining the relative amounts of components in such mixtures, prepare pure compounds from a mixture – this is called preparative chromatography, testing the purity of a particular substance and identifying a compound / element. The last application is the reason why GC is used in drug testing.

    In GC there is mobile or a moving phase – an inert or inactive gas like helium or nitrogen. Next, there is a stationary phase comprising of a very thin film, of microscopic dimensions, of a liquid or polymer deposited on the inner surface of an inert solid. This solid is either glass or metal. Different stationary phases are coated inside the solid. The gaseous elements react with the stationary phase and get eluted on it. Different gaseous elements are eluted for different durations of time called as retention time. A comparison of these retention times helps identify / separate the element.

    GC is also called as Gas Liquid Partition Chromatography (GLPC) and Vapor Phase Chromatography (VPC). Technically GLPC is the most precise terms that can be used to describe this process. GC / GLPC is somewhat similar to Fractional Distillation as both are used for separating elements inside a mixture / compound based on differences in boiling point / vapor pressure.

    The GC is used as a confirmation test for those drug tests in which the person, who has tested positive, appeals for a repeat of the test. The reason why GC is not used as a first drug test is its expensive nature. Samples that gave a false positive test in the initial drug test do test negative during such a confirmation test. Usually most of the drug testing laboratories preserve the sample that tested positive for some months or years in case there are lawsuits due to disputed results. Only those samples that test positive in the initial and the confirmation test are reported as positive.

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