Measurement of uncertainty for blood alcohol concentration by headspace gas chromatography

ABSTRACT Measurement of uncertainty has recently become more relevant to the science of forensic toxicology for a variety of reasons. From accreditation to court testimony, measurement of uncertainty is being requested to accompany case results. The Toxicology Bureau of the New Mexico Department of Health has conducted a study to estimate a reasonable uncertainty for the mass concentration of ethanol, methanol, acetone, and isopropanol (g/100 mL) determined through dual capillary column headspace gas chromatography. In this measurement of uncertainty for BAC, alcohol encompasses ethanol, methanol, acetone, and isopropanol, and all four analytes are included in the final expanded uncertainty. One expanded uncertainty value incorporates all four analytes, simplifies the calculation, and streamlines the reporting process of the estimation of error. Three major groups were used in the estimation of the blood alcohol concentration expanded uncertainty: GC calibration adjustment slope, GC analytical, and certified reference material. The proportional standard uncertainties for these sources were combined using root-sum-squares to yield a combined standard uncertainty of uc = ±2.39% for the blood alcohol concentration method. The combined standard uncertainty was expanded to a confidence level of 95% with a coverage factor of 2. The final expanded uncertainty for the blood alcohol concentration method was U = ±4.8%.