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.
What is a ?
Chromatography produces a , which is a graph which shows each molecule as it makes contact with the detector within the device. Since different kinds of molecules burn (ionize) at different rates. The graph will have a series of peaks between the time it takes for a peak to appear. The known samples of ethanol then compared to the for the unknown sample. If a peak appears in the unknown sample at the same time that the peak appears as of known ethanol sample, then the unknowns blood sample likewise contains ethanol. The area underneath the peak represents the concentration which is critical in determining the accused’s blood alcohol content or BAC). The area of the peak is then compared to 3 known ethanol standards which are plotted on a graph known as the calibration curve.
Enzymatic Method (, Hospital Analysis)
While hospitals and laboratories conduct ethanol determinations, they typically perform tests on serum rather than whole blood. Hospitals use the enzymatic method as opposed to the GC method and it is the most common chemical process in hospital laboratories. The main purpose of utilizing the enzymatic method as opposed to GC is to obtain the quickest results. It can take up to 8 hours to obtain results from the GC method, whereas the enzymatic method will take approximately 20 minutes. The enzymatic method is less exact and accurate than the GC method with an average deviation of 10-20% as well as a lack of specificity for isopropyl and butyl alcohols. If the prosecution intends to introduce evidence of BAC from the enzymatic method it must use a conversion factor since this method tests serum as opposed to whole blood.
Serum vs. Whole Blood
The difference between serum and whole blood is that the serum is the fluid portion of whole blood which remains after the whole blood has clotted and the clot is removed (typically by centrifugation). Because plasma and serum contain no cellular solids, they contain a relatively greater content of water than whole blood. This is important because plasma and serum, with a water content of 95 and 97%, will contain more methanol than whole blood, which is approximately 85% water. This is why there is a 10-15% difference between whole blood and serum.
For more information on DUI defense in Pennsylvania and DWI defense in New Jersey, I encourage you to keep reading my blog and visit my free download section.