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Comparing Gas Chromatography with Enzymatic Assay

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Expert Witness Knowledge Can Affect Your DUI Case

According to Okorie Okorocha, “When hiring a forensic toxicology expert witness, you need a qualified professional who really knows what they’re talking about.” The equipment and methods by which we test for the presence of toxins are complex and delicate. Sloppy lab work and sample taking, or external factors like humidity or air pressure can easily skew results. This is why a forensic toxicologist must be rigorous and critical in their testing.

In the incident of a DUI (driving under the influence), an expert witness can make all the difference to the case, whether they are representing the defendant or prosecuting them. In a previous post, I looked at the difference between breath and blood testing for BAC (blood alcohol content). In this post, I’ll discuss two of the most common methods of blood testing in more detail: enzymatic assay, and gas chromatography.

What is Enzymatic Assay?
An enzyme assay is a method of testing for BAC by measuring the rates of enzyme reactions. Put simply, an enzyme is added to the prepared blood sample; the enzyme reacts in the presence of ethanol, and we measure the products of this reaction (what remains after the enzyme has done its work). This method was developed as a ‘quick and dirty’ means of testing for BAC. However, there are a few serious problems with enzyme assays:

  • It is an indirect method – we test for the products of the reaction, not the ethanol itself – and therefore cannot be entirely accurate.
  • There is a high margin of error (about 25%) due to the testing instrument alone.
  • Other natural compounds in the blood can sometimes react in the same way as ethanol, resulting in a false positive.
  • Enzyme assays separate the blood sample, and test only the serum or plasma component of the blood, which can yield a higher BAC than if the ‘whole blood’ sample were tested.

For these reasons, an enzymatic assay is never an accurate measure of BAC, and should not be taken as thorough evidence in a DUI case.

What is Gas Chromatography?
Gas chromatography (GC) is a method of separating a sample into its various components for individual analysis. The sample is heated, the components are then vaporized and mixed with a carrier gas, and then pass through a column which is lined with silica or polymers that interact with these individual components. Each component – also known as a volatile organic compound, or VOC – reacts with the lining differently. Some bind with it tightly and pass through the column slowly, whereas others bind loosely and pass through quickly. A computer measures each VOC as it leaves the column, and produces a graph that indicates which chemicals were present in the sample.

For the purposes of testing blood alcohol content, GC is a far more accurate and direct method for detecting the presence of ethanol in a blood sample. It can also account for other VOCs that may appear as ethanol in an enzyme assay, and can use the whole blood sample instead of just the serum or plasma. However, for the sake of scientific rigor and accuracy, it’s usually best to opt for dual column confirmation – running the same sample through another GC to confirm the VOCs measured.

By analyzing the accuracy of the methods used to test for BAC, an expert witness can make or break a DUI case. For more information about how forensic testing methods can affect your DUI case, contact Okorie Okorocha for expert advice.

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