Determination of n-hexane metabolites by liquid chromatography/mass spectrometry. 2. Glucuronide-conjugated metabolites in untreated urine samples by electrospray ionization

A liquid chromatography atmospheric pressure electrospray mass spectrometry (ESI-LC/MS) system was evaluated for the identification and characterization of n-hexane conjugated metabolites (glucuronides) in untreated urine samples. Chromatography of glucuronides was obtained under ion-suppressed reversed-phase conditions, by using high-speed (3 cm, 3 microns) columns and formic acid (2 mM) as modifier in the mobile phase. The mass spectrometer was operated in negative ion (NI) mode. For the first time, four glucuronides were identified by ESI-LC/MS in untreated urine samples of rats exposed to n-hexane: 2-hexanol-glucuronide, 5-hydroxy-2-hexanone-glucuronide, 2,5-hexanediol-glucuronide and 4,5-dihydroxy-2-hexanone-glucuronide. Confirmation of the conjugated metabolites was obtained by LC/MS/MS experiments. Gas chromatography/mass spectrometry (GC/MS) and atmospheric pressure chemical ionization (APCI) LC/MS analyses were performed on the same samples. An integrated approach GC/MS-LC/MS for the semi-quantitative analysis of n-hexane glucuronides, whose standards are not commercially available, is discussed and proposed here. In order to understand the fate of the metabolites during sample pre-treatment, a study about the effects of enzymatic and acid hydrolysis on urine samples was conducted on glucuronides isolated by solid-phase extraction. Combined analyses by GC/MS and LC/MS enabled us to distinguish 'true' n-hexane metabolites from compounds resulting from sample treatment and handling (i.e. enzymatic and acid hydrolysis, extraction and GC injection).