Variations in abundance of the molecular ion of the derivatized cocaine metabolite benzoylecgonine.

Benzoylecgonine (BEG) is the principal urinary metabolite of cocaine. For forensic drug testing, the presence of BEG in urine, suggested by a positive result for a screening immunoassay, must be confirmed by quantitative gas chromatographic/mass spectrometric (GC/MS) analysis, i.e., stable isotope dilution with a deuterium-labeled internal standard. GC/MS criteria for positivity also require appropriate relative abundances of qualifier ions, including the molecular ion, but there is little published information on the observed absolute values for qualifier ion ratios or on the variability of these values. This lack of information creates uncertainty for laboratories initiating programs testing urine for drugs of abuse as to performance criteria for run acceptability and sample positivity. We have observed substantial variability (CV = 50%) in the abundance of the molecular ion of derivatized BEG relative to the base ion in reference materials. This variability can result in a high rate of repetition of runs and generate confusion in the defense of forensic drug-testing results. Normalization of the abundance of the molecular ion of derivatized BEG to that of the deuterium-labeled internal standard in the same sample greatly reduces the apparent variability in this measurement; it is also more reliable than the absolute value of the relative abundance of the molecular ion in determining run acceptability and positive or negative results for a sample.