The Basics of Blood Alcohol Testing by Gas Chromatography – Part I

The first stage of sample testing is the injection of the sample into the injector port. The sample is introduced into the injector port with a syringe or an exterior sampling device known as a pipette. The injector port is a hollow, glass-lined cylinder that is heated to 150-250°C which causes the sample to vaporize.

Most blood alcohol analyses are performed via headspace gas analysis as opposed to direct injection. A small amount (approximately 0.10 milliliters) of blood is diluted with water to about 1.0 milliliter in volume. The sample is placed in a sample vial and the cap is crimped and placed in an auto sampler with an incubating chamber set at a constant temperature. When the sample has reached equilibrium (when the rate of ethanol evaporation equals the rate of condensation in the sample vial), a sample of the gas above the liquid is withdrawn and injected into the gas chromatograph.

At this stage of the analysis Henry’s Law is in effect. Henry’s Law is a chemical principle that describes therelationship of volatile compounds in dilute aqueous solution at equilibrium.  Next, the vapors of the sample are carried into the column by a carrier gas.  The transportation of the vapors by the carrier gas, usually hydrogen, helium, or nitrogen is frequently referred to as the mobile phase. The carrier gas flows into the injector port, through the column, which is maintained by a temperature controlled oven, and then into the detector. The solutes travel through the column at a rate primarily determined by their physical properties, and the temperature and composition of the column. The column is packed or coated. This is commonly referred to as the stationary phase.

Written by Allen Trapp who is board certified by the National College for DUI Defense and the author of Georgia DUI Survival Guide Visit Website

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