An evaluation of sample preparation techniques for the GC/MS analysis of urinary mercapturic acid conjugates

Recovery rates of four different techniques for the preparation of human urine samples spiked with N-acetyl-S-benzyl-L-cysteine (BMA) were compared at three different spiking levels. At concentrations of 1,000 ppm and 1 ppm in the urine, recoveries of BMA were greatest (80-96%) using an ion pair phase transfer technique and a C18 solid-phase extraction (C18) technique while an acidic ethyl acetate extraction method yielded 67-69% recoveries and a quaternary amine solid-phase extraction technique showed poor recoveries (5-7%). At 10 ppb, quantitative recovery could only be determined for the C18 technique due to interferences from samples prepared using the other three techniques. The results indicate that the C18 sample preparation technique followed by GC/MS analysis using stable isotopically labeled internal standards provides a rapid and accurate method for quantitation of mercapturic acids at low-ppb levels in the urine.