As any forensic scientist knows, human error can account for inaccurate information, resulting in a flawed interpretation. In cases involving drunken driving, where the blood alcohol content (BAC) of the driver is requested, defense attorneys have been known to challenge what’s called the headspace analysis.
Headspace gas chromatography (GC) is a way of separating volatile material from a liquid or solid prior to analysis. This sounds straightforward enough, but there are a number of process control issues where analyst error could affect the results, thereby providing the defense attorney ammunition for a wounding cross examination. One critical area facing scrutiny is the sample collection and preparation itself.
BAC Analysis Begins Long Before Headspace Analysis
Determining the ethanol levels in BAC analysis by headspace gas chromatography begins long before the instrument analyzes the sample. The analyst needs to ensure that the identifying information on the sample corresponds to its packaging. He or she also should look for conditions that could affect the reliability of the analysis, such as leaks, blood volume, clotting and signs of fermentation.
Clotting will lead to a false high result because the standards that the unknown are compared against are designed to test for whole blood and not the serum resulting from clotting. Fermentation can lead to a false high result because ethanol can actually be created in the sample after it is withdrawn from the defendant.
Unequal Temperature of Samples Can Cause Problems
The specimens and standards must be allowed to reach room temperature. Unequal temperature may lead to unequal sampling volumes. If the volume of the defendant’s blood sample is different from the known ethanol standards, then the results will be invalid. All the vials must contain the same volume of liquid for the test to work properly. In addition, the specimens should be rocked or gently inverted to assure the sample is homogenous.
If the vials are vigorously shaken, this can cause bubbles in the sample which will increase the volume of the sample and lead to invalid results. The analyst must then properly prepare the vials that go into the auto-sampler. The analyst introduces the blood and the external standards into the separate 20 milliliter (ml) headspace vials, using a properly calibrated pipette.
Pipetting Fraught with Potential for Error
Also using a pipette, he or she introduces a precise volume of internal standard into every vial, with the exception of the water blank vials. The manual pipetting of these solutions in such small volumes is inherently fraught with the potential for error. The analyst must exercise extreme care in order to ensure consistent samplings of both the specimen and the internal standard. Consistency is critical to precision and reproducibility.
That’s just the first step in a long chain of evidence process from taking of a BAC sample at the time of arrest to the time of prosecution. A lot can happen in the lab and during subsequent steps, such as the pipetting technique used, loading of the vials into the machine, selection of parameters for the test and final interpretation of the results that can point to a tainted sample.
BAC Test Errors Offers DUI Defense
A good defense attorney can create a viable road map of BAC test errors and lead a jury to believe that the blood sample was contaminated and therefore prejudicial to his client. A good analyst will be able to document the quality process controls in place and the specifics of how each vial was prepared. He must also be trained to be aware that sample preparations may affect a test result. It is the lack of this awareness that a skillful criminal defense attorney can expose on cross-examination.
Want to learn more about headspace analysis? Read this blog to find out how it can be instrumental when testing for contaminants in soft drinks.
I want to thank you for the information, and I want to ask ¿which is the RSD of Headspace Analysis? I mean, how big is the mean margin of error of the analysis for BAC measure?