Laboratory Analysis of DUI Blood Samples
How a crime lab analyzes DUI blood samples is a major issue. From the accreditation of the lab, to the preparation of the samples, to the instruments used in testing, there are numerous potential pitfalls. Below you will find a brief overview of the different ways blood samples are analyzed in DUI cases.
The most common form of blood laboratory analysis is called Gas Chromatography, a process in which a sample of blood is analyzed using a Gas Chromatograph instrument. Gas Chromatography is currently the most popular method for analyzing B.A.C. levels in a blood sample of a DUI suspect. There are two methods of Gas Chromatography analysis, headspace gas chromatography and direct injection chromatography. In addition to Gas Chromatography, another less commonly used method is called the Enzymatic Method.
Headspace Gas Chromatography
In Headspace Gas Chromatography, a nonvolatile solution is added to the blood sample. An internal standard is typically also added to the blood for quantitation purposes. A technician places the vial containing the blood sample into a closed container. The sample is then heated. Upon heating, a vapor is formed, filling the space (called the “headspace”) between the vial and the top of the container. Once the vapor reaches equilibrium, a sample of the vapor is taken and injected into a gas chromatograph instrument. This instrument tests the concentration of alcohol that was present in the headspace vapor.
Direct Injection Chromatography
Direct inject chromatography is also commonly used. In this testing procedure, instead of measuring the alcohol levels in the headspace vapor, the blood itself is directly injected into the chromatography instrument. This method is considered by some to be more reliable then the headspace method.
The Enzymatic method is not commonly encountered by DUI defense attorneys. It’s more technical and expensive. Typically, this method is only performed in highly sophisticated labrotories that are equiped with properly trained technicians and special machines.
Dichromate Method/Kozelka-Hines Method
Not specifically developed for testing ethyl alcohol, this method presents problems of mistaking other chemicals as alcohol, leading to a false reading.