Simple method of methylation for gas chromatographic analysis of S-benzyl-N-acetylcysteine, a metabolite of toluene, in human urine.

A simplified method of methylation for the determination of urinary S-benzyl-N-acetylcysteine (benzylmercapturic acid, SBAC), a metabolite of toluene, by gas chromatography (GC) was developed. Acidified urine samples (pH 2) were extracted once with ethyl acetate and then derivatized to methyl ester (ME) using HCl-methanol. The optimum conditions for derivatization for SBAC and the internal standard (S-phenethyl-N-acetylcysteine) were reaction at 60 degrees C for 20 min. The SBAC-ME was measured by capillary GC. The calibration curve showed good linearity over the range of 0.2 to 5.0 mg/L (r = 0.986). This method was compared with a previously developed diazomethane methylation method for testing urine from subjects who had sniffed toluene. The values obtained by the two different methods were in good accordance. These results suggest that this technique for the methylation of SBAC by means of HCI-methanol is simpler and time-saving, thus making it feasible to determine SBAC and other mercapturic acids in urinary samples obtained from subjects who have been exposed to organic solvents.