Quantitative analysis of carisoprodol and meprobamate in whole blood using benzylcarbamate and deuterated meprobamate as internal standards.

Carisoprodol and meprobamate are frequently encountered drugs in impaired driving casework. Deuterated internal standards, although preferred, were not available until recently. Earlier published studies report the use of a variety of non-deuterated internal standards, many of which lack the chemical and physical similarities that are desired for quantitative analysis. Carisoprodol and meprobamate were determined in whole blood using solid-phase extraction and gas chromatography-mass spectrometry with benzylcarbamate and meprobamate-d(7) as internal standards. When benzylcarbamate was used as internal standard, the linear ranges for carisoprodol and meprobamate were 0-20 mg/L and 0-40 mg/L, respectively. The linear range increased to 100 mg/L when meprobamate-d(7) was used. Limits of detection for carisoprodol and meprobamate were 0.2 and 0.4 mg/L, respectively, regardless of the internal standard selection. The limit of quantitation for both drugs using either internal standard was 0.4 mg/L. Accuracies using benzylcarbamate and meprobamate-d(7) were 100-106% and 91-100%, respectively. Corresponding values for precision indicated intra-assay coefficients of variation of 2.6-4.3% for benzylcarbamate and 1.0-2.3% for meprobamate-d(7). No carryover was evident at 100 mg/L, the highest concentration tested, and no interferences were observed. Results indicated that either benzylcarbamate or meprobamate-d(7) is a suitable internal standard for quantitative determination of carisoprodol or meprobamate from whole blood.