Forensic Alcohol The Science Behind DUI by Sloneker Mike

Author:Sloneker, Mike [Sloneker, Mike]
Language: eng
Format: epub
Publisher: Unknown
Published: 2012-12-10T16:00:00+00:00

The ethanol in the above chromatograms was identified properly and the value calculated according to the calibration curve. What could possibly be the problem? In figure 4.1A there are three bumps labeled A,B,C. The identity of these bumps is questioned at every trial. In order for the GC to identify a peak the following must happen:

• First, the retention time must match the retention time of a substance in its library. The library is built by analyzing different volatiles and determining the exact RT of each.

• Second, the "area" counts must be greater than a minimum threshold set by the analyst.

In the above chromatograms, the peaks labeled A,B and C were not identified by the instrument. I ticked the bumps myself in order to be able to point them out. The GC did not flag the peaks, either because the RTs were not in the instrument's library, or the area counts did not meet the minimum threshold. The attack by the defense in this case will be something like, "Something is obviously there, and we have no idea what since the GC did not identify it." The defense will begin to refer to these smaller peaks as contaminants. They will raise concerns that the contaminants might have caused errors in calculating the blood alcohol concentration.

In human blood more volatiles are expected than just ethanol. For instance, one might expect the breakdown product of ethanol, acetaldehyde, to be present. Heavy drinkers may show trace amounts of methanol as well91 . Regardless of being able to identify these substances, it can be said with certainty that they did not interfere with the calculation of ethanol because they did not co-elute with the ethanol peak. In chromatography, the term co-elute refers to different compounds that failed to separate and consequently came out of the column together. The peak formed under this circumstance looks very unusual and cannot be missed, even by a novice. Peak shapes becomes a very important indicator of GC performance and as such will be discussed briefly. Two items that need to be examined on the chromatogram to ascertain the reliability of GC are the baseline and the peak shape.

Bad Baselines

The baseline is the horizontal line that runs through the chromatogram, where nothing is being detected. It is a zero response of the flame ionization detector. This baseline should be smooth, as if drawn by a steady hand. Figure 4.3 is an example of several baseline oddities that will be discussed further.

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