A large percentage of our criminal defense practice in Pennsylvania and New Jersey involves DUI and DWI. In Pennsylvania, blood evidence analysis is a critical element in any case involving or pertaining to drunk driving and related felony offenses, which include aggravated assault by DUI and homicide by DUI. I have written previous articles and a book on DUI defense but this blog article focuses on gas chromatography and the enzymatic method, which are the 2 most common methods used to analyze blood.
Gas Chromatography (GC)
For forensic purposes, gas chromatography (GC) is the most widely used technique as it is the most reliable. This method, also known as separation science, uses an instrument that separates mixtures of molecules based upon their chemical and/or physical properties. Individual substances, such as ethanol (AKA ethyl alcohol), are quantified by measuring the size of peaks on a calibration curve. There are actually 2 types of gas chromatography — liquid and headspace. Liquid GC measures the actual liquid sample introduced into the device, whereas headspace measures the vapor. Headspace is therefore, less exact and an indirect analysis. Both liquid and headspace GC can measure volatile substances such as methanol, acetone, and 2-propanol (isopropanol).
Headspace GC operates under the principal of Henry’s Law (read my article on which basically states that within a sealed vessel a volatile substance present in the vapor above the liquid will exist in the same concentration as the respective liquid below it. The major difference between Headspace GC and Liquid GC is that with the Headspace method only volatile substances are analyzed so the potential universe of interference is drastically limited.