Quantitative determination of N-acetyl(-L-)cysteine derivatives in human urine by tandem mass spectrometry.

Collision-induced dissociation (CID) methods are described for the quantification of nanogram per millilitre (ppb) concentrations of 2-acetamido-3-(3'-hydroxypropylthio)-propanoic acid (I) and 2-acetamido-3-phenylthiopropanoic acid (II) in human urine extracts. I and II are potential detoxification products of acrolein and benzene in conjugation with N-acetyl(-L-)cysteine derived from glutathione. We have studied the potential of tandem mass spectrometry (MS/MS) under electron impact (EI) and chemical ionization (CI) conditions as a confirmatory screening technique for these compounds. Our main goals were high selectivity and low detection limits along with little or no sample clean-up. The effects of the mode of ionization and of collision conditions on the CID spectra have been investigated. Direct insertion probe without any derivatization or short-column gas chromatographic separation techniques are used. Total instrument and data analysis time is about 15 min for direct insertion probe MS/MS and about 30 min for short-column gas chromatography (GC)/MS/MS. Detection limits are: direct insertion probe MS/MS (EI mode), 50 ppb (100 pg) for compound I; short-column GC/MS/MS (EI mode), 1.5 ppb (5 pg) for compound II; and short-column GC/MS/MS (CI mode), 0.6 ppb (2 pg) for the methyl ester of compound II. Results are compared with non-mass spectrometric methods. The MS/MS methods were applied for the determination of I (EI mode) and II (CI mode) in urinary samples of a smoker and eight non-smokers. After smoking, the urinary levels of I and II were elevated, whereas no increase was observed after experimental passive smoking.