LC-MS/MS procedure for the simultaneous determination of N-acetyl-S-(1-naphthyl)cysteine and N-acetyl-S-(2-napthyl)cysteine in human urine.

•Simultaneous determination of different mercapturic acids of naphthalene and benzene in human urine by LC–MS/MS.•Acidic preparation of urine guarantees an efficient transfer from pre-mercapturic acids into stable aromatic structures.•Online-SPE with C18-RAM phase provides a comfortable and effective extraction of all analytes.•Use of structurally matched isotope-labelled internal standards supports high reliability of the methods.