The National Association of Criminal Defense Lawyers’ amicus brief in Bullcoming v. New Mexico is an excellent resource for attorneys who want to understand gas chromatography measurements of blood alcohol concentration (BAC) and possible challenges to that technique. Gas Chromatography or Headspace Gas Chromatography is the typical method used in North Carolina when the State tests a blood sample in impaired driving cases or other instances in which a person’s alcohol consumption is at issue. (See Drug Chemistry Procedure Manual, p. 10 of 69 to read the State Crime Lab’s procedures.)
The brief explains that results arrived at through the gas chromatography process are by no means impervious to human error. To explain how error may enter into the procedure, the brief describes the procedure step by step in Section A, beginning with how the sample is loaded into the machine. The brief explains the purpose of standards and blanks that are run with the sample, how samples are taken from each vial, and what tests are performed on the samples. I won’t attempt to summarize the procedure here because the brief explains this complex procedure as clearly and concisely as possible.
Section B of the brief addresses the following four areas where analyst error could affect the results of the test: (1) sample preparation, (2) loading of vials into the machine, (3) selection of parameters for the test, and (4) interpretation of results. This is a potential roadmap for cross examination. The brief’s citations to scientific articles provide additional information and references. Section II lists actual cross examination questions that could be asked of a testifying analyst; the relevance of these questions is best understood when read in conjunction with the previous section.
Because gas chromatography, like all forensic science techniques, is not immune from human error, the accuracy of the results produced depends on the analyst who does the testing. The NACDL brief provides a clear explanation of the techniques used and areas where error may be introduced and is an excellent practical tool for attorneys trying cases where BAC is at issue.
UNC law student Giles Rhodenhiser contributed to this post.