Fig. 4. GC/MS analysis of compound A mercapturic acids in human urine. ( A ) Selected-ion mode chromatograph ( m/z 236, 256, 196) of a diazomethane-derivatized extract of urine from a patient undergoing low-flow sevoflurane. The alkene and alkane mercapturic acid peaks at 35.2 and 35.5 min, respectively, were not present in urine obtained before anesthesia (not shown). ( B ) Mass spectrum of the 35.2-min peak in ( A ), identified as the alkene mercapturic acid N -acetyl- S -(1-fluoro-2-fluoromethoxy-2-(trifluorom-ethyl)vinyl)-L-cysteine methyl ester. ( C ) Mass spectrum of the 35.5-min peak in ( A ), identified as the alkane mercapturic acid N -acetyl- S -(1,1,3,3,3-pentafluoro-2-fluoromethoxypropyl)-L-cysteine methyl ester.