How Do the Police Test For Alcohol in My Blood?

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Keep in mind that a police-administered blood test creates an estimate of the amount of alcohol that is in your blood. Like all estimates, it can be accurate or inaccurate, depending upon the circumstance. The police crime labs use head-space gas chromatography with a flame ionization detector to analyze blood samples. It is a very well-respected and reliable method of testing blood for alcohol, but the people who work in the crime labs often have minimal education, poor training, inadequate facilities and unscientific laboratory protocols.

Blood is collected by a phlebotomist in two grey-top tubes which should contain a preservative and anticoagulant (grey-top tubes are normally used in medicine for glucose-testing). One tube is for the crime lab to test and the other is preserved for the subject’s independent testing purposes.

At the crime lab, one tube is opened and the contents are poured into a clean container. A measured amount of blood and a type of alcohol (usually n-propanol) are mixed together into a head-space vial, which is then crimp-sealed. The vial is placed into a rack called an auto-sampler with up to 123 other vials. Each vial is heated and rocked to allow volatile compounds to rise into the head-space above the liquid in the vial. When equilibrium of the ratio of volatile compounds in the liquid to the volatile compounds in the head-space is reached, the head-space gas is sampled.

A syringe pierces the crimp-seal and another gas (generally helium) is used to force the head-space gas out of the vial and into a hot injector port of a machine called a “gas chromatograph” (GC).

The head-space gas travels through a capillary column which is about 30 meters long and nearly as thin as a human hair. By chemical and mechanical means, the molecules traveling through the column are slowed down to different speeds, depending on the type of molecule. They elute (exit) the column at predictable times and are burned by a flame. The burning of the molecules creates ion energy which is measured by a flame ionization detector and converted by a computer formula to a blood alcohol estimate.

Head-space gas chromatography is a “separation science” and relies on the ability of the process to separate out various compounds and measure quantities so small that they cannot be seen with the human eye. Small mistakes in preparation of the sample will lead to large mistakes in the results.

Nesci & St. Louis is a criminal defense law firm in Tucson located at 216 North Main Avenue in Tucson and is solely responsible for the content of this blog. This blog is for informational purposes only and is not intended as legal advice. No attorney-client relationship is established by the use of this information. Laws and rules change rapidly and every effort to ensure the accuracy of this information has been made as of the date of this blog. © 2012