A generic method for the detection of covalent adducts to the cysteine-34 residue of human serum albumin (HSA) has been developed, based on an on-line combination of immunoaffinity chromatography for selective sample pre-treatment, solution phase digestion, liquid chromatography and tandem mass spectrometry. Selective anti-HSA antibodies immobilized on agarose were used for sample pre-concentration and purification of albumin from the chemically produced alkylated HSA. After elution, HSA and HSA adducts are mixed with pronase and directed to a reaction capillary kept at a digestion temperature of 70 degrees C. The digestion products were trapped on-line on a C18 SPE cartridge. The peptides were separated on a reversed-phase column using a gradient of organic modifier and subsequently detected using tandem mass spectrometry. Modified albumin samples consisted of synthetically alkylated HSA by the reactive metabolite of acetaminophen, N-acetyl-p-benzoquinoneimine (NAPQI), and using the alkylating agent 1-chloro-2,4-dinitrobenzene (CDNB) as reference. The resulting mixture of alkylated versus non-modified albumin has been applied to the on-line system, and alkylation of HSA is revealed by the detection of the modified marker tetra-peptide glutamine-cysteine-proline-phenylalanine (QCPF) adducts NAPQI-QCPF and CDNB-QCPF. Detection of alkylated species was enabled by the use of data comparison algorithms to distinguish between unmodified and modified HSA samples. The in-solution digestion proved to be a useful tool for enabling fast (less than 2 min) and reproducible on-line digestion of HSA. A detection limit of 1.5 micromol/L of modified HSA could be obtained by applying 10 microL of NAPQI-HSA sample.