via Gas Chromatography:
Assessment of Data Reliability
Dominick A. Labianca
This particular case concerned blood-alcohol analysis via gas chromatography (GC), the preeminent analytical tool commonly used in the forensic science arena for the determination of alcohol in blood samples.
The focus was on the quantitative assessment of the data stemming from the analysis, which involved direct
injection of appropriately prepared liquid samples into the gas chromatograph.
However, a fundamental flaw of the analysis concerned the chromatographic profile—a term defined in the
following section—of one of the standard aqueous ethanol (ethyl alcohol, commonly referred to as “alcohol”
and symbolized by the formula, EtOH) solutions used in the analysis. That flaw affected the reliability of the
analytical results subsequently generated, and likely, contributed to the jury’s acquittal on all alcohol-related
charges in the case.
Overview of GC Analysis
To appreciate the significance of the flaw referred to above, consider first the analytical protocol characterizing the determination of BAC via GC. Central to this type of analysis is the inclusion of an internal
standard (IS) in all analytical samples, which minimizes various uncertainties, including those stemming
from sample injection, flow rate, and variations in column conditions.1 The analytical parameter involved
is the ratio, R-Std, of the peak area (PA) of analyte2 attributable to any standard (Std), or known, alcohol
solution (PA EtOH-Std) to the associated internal-standard peak area (PA IS-Std). Equation 1 (Eq 1)
defines this ratio, and the peak areas reflect the “chromatographic profile” referred to above.
A plot of R-Std versus the corresponding concentrations of the standard alcohol solutions used
([EtOH]-Std), would be expected to yield a “calibration curve.” A straight line passing through the origin
would characterize this curve, and all relevant analytical results would then be based upon this linearity
Associated with Eq 1 in any alcohol-based GC analysis is Eq 2, which represents R- Test Subject, the ratio
denoting the analytical parameter corresponding to the test subject’s blood sample.
Consistent with the critical feature of linearity cited above is Eq 3, which allows for the manual calculation of a test subject’s BAC from the product of two factors: first, any of the standard alcohol concentrations ([EtOH]-Std)—in the concentration unit, mg/dL, for the case under consideration—and, second, a
“comparison ratio”, R-Test Subject/R-Std. In effect, the comparison ratio simply represents the fraction of
R-Std denoted by R-Test Subject, so when that fraction is multiplied by [EtOH]-Std, according to Eq 3,
the test subject’s BAC is obtained, as shown below.