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The page below is a sample from the LabCE course Drug Testing Methods in the Clinical Toxicology Laboratory. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Gas Chromatography

The prepared sample or extract is injected into the heated injection port of a gas chromatograph where the solvent is vaporized under high temperature. With a constant carrier gas (an inert gas such as hydrogen or helium) flow, the solvent and the solutes (including target analytes) dissolved in the solvent are swept onto the chromatographic column that is housed inside the GC oven. The solvent will elute very quickly from the column, but the compounds that were dissolved in the solvent begin to elute at different rates depending on their affinity for the stationary phase bonded to the inside of the column. The rates at which they elute depend on many physical and chemical properties.
Examples of physical properties are column length, column diameter, stationary phase film thickness, column temperature, column flow, volatility of the compound, and the size of the compound.
Examples of chemical properties are polarity of the stationary phase and polarity of the compound.
The time it takes for individual compounds to elute from the column is termed the retention time. Compounds are identified based on retention time.