Licensed in Pennsylvania and New Jersey
KnowWhere Your Defense Lies Gas Chromatography vs. Enzymatic Method
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. There are two most common methods used to analyze blood: gas chromatography and the enzymatic method. Gas Chromatography (GC) For forensic purposes, GC is the most widely used technique because it’s the most reliable. This method, also known as separation science, uses an instrument that separates mixtures of molecules based on their chemicals and/or physical properties. Individual substances, such as ethanol, are quantified by
liquid and headspace. Liquid GC measures the actual liquid sample introduced into the device, whereas headspace measures the vapor, making it the less precise and indirect analysis. Both liquid and headspace GC can measure volatile substances such as methanol, acetone, and two-propanol (isopropanol). 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 Chromograph? Chromatography produces a chromograph, which is a graph that shows each molecule as it makes contact with the detector within the device. Since different kinds of molecules burn or 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 are then compared to the chromograph for the unknown sample. If a peak appears Headspace GF operates under the principal of Henry’s Law, which states that within a sealed
While hospitals and laboratories conduct ethanol determinations, they typically perform tests on serum rather than whole blood. Hospitals use the enzymatic method instead of the GC method. The primary purpose of utilizing the enzymatic method is to obtain the quickest results. It can take up to 8 hours to obtain results for the GC method, whereas the enzymatic method will take approximately 20 minutes. However, the enzymatic method is less exact and accurate than the GC method, with an average deviation of 10–20 percent, 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 serum is the fluid portion of whole blood. This fluid 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 does. This is important because plasma and serum, with a water content of 95 and 97 percent, will contain more ethanol than whole blood, which is approximately 85 percent water. This is why there is a 10–15 percent difference between whole blood and serum. For more information on DUI defense in Pennsylvania and DWI defense in New Jersey, I encourage you to visit my blog at gambonelaw. com/blog, and check out my free download section.
measuring the size of peaks on a calibration
curve. There are two types of gas chromatography:
in the unknown sample at the same time that the peak appears in the known ethanol sample, then the unknown blood sample likewise contains ethanol. The area underneath the peak represents the concentration, which is critical in determining the accused’s blood-alcohol content (BAC). The area of the peak is then compared to three known ethanol standards, which are plotted on a graph known as the calibration curve.
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