The Reliability of a Solid-Phase Extraction System for the Analysis of Benzoylecgonine in Urine

Gas chromatographic—mass spectrometric (GC—MS) analysis for benzoylecgonine (BE), a metabolite of cocaine, requires an initial extraction from urine. Although liquid—liquid extraction methods are frequently used, solid-phase extraction (SPE) may be preferable for obtaining reliable results and clean chromatograms. We describe a 12-month study that evaluates the accuracy, precision, variability between analysts, variability between column lots, and cleanliness of BE extracts using SPE columns followed by GC—MS analysis. The overall mean for a control urine sample prepared at 150 ng/mL is 151 ng/mL (N=293) with a standard deviation of 8.59 and a coefficient of variation (CV) of 5.7%. Within-run precision (measured as CV) at 75,150, and 2000 ng/mL is 4.0,1.8, and 0.8%, respectively. Mean results from 10 different analysts vary a maximum of 4.6% from the overall mean of 151 ng/mL, and the CV for 9 out of 10 analysts is 7.0% or less. The CV for the remaining analyst is 10.4%. Quantitative results from nine different lots of SPE columns fluctuate 3.3% from the overall mean of 151 ng/mL, and the CV varies from 3.5 to 6.2%. GC—MS chromatograms following SPE are significantly cleaner (i.e., reduced baseline signal and no interfering peaks) than those from two types of liquid—liquid extractions