Determination of free and glucuronated hexane metabolites without prior hydrolysis by liquid- and gas-chromatography coupled with mass spectrometry

Since n-hexane metabolites are excreted as glucuronide conjugates, most con ventional analytical procedures require preliminary hydrolysis, yielding to the 'total' 2,5-hexanedione (2,5-HD), but also giving rise to a number of artifacts. The whole pattern of n-hexane metabolites, both conjugated and u nconjugated, as well as different methods of sample pretreatment have been evaluated by hyphenated techniques (liquid chromatography-mass spectrometry (LC-MS) and gas chromatography-mass spectrometry (GC-MS)). Aliquots of uri ne from rats exposed to n-hexane underwent enzymatic or acid hydrolysis or both; whereas one aliquot was applied to LC-MS, dichloromethane extracts we re analyzed by GC-MS. In untreated urine, four glucuronides (-G) were ident ified and characterized by LC-MS: 2-hexanol-G, 5-hydroxy-2-hexanone-G, 4,5- dyhydroxy-2-hexanone-G, and 2,5-hexanediol-G. 'Free' 2,5-HD was detectable in non-hydrolyzed samples by both GC- and LC-MS. Whereas enzymatic hydrolys is did not increase the amount of 2,5-HD, acid hydrolysis led to increase 2 ,5-HD in variable amount and produced gamma-valerolactone as a result of a complete transformation of 4,5-dihydroxy-2-hexanone-G and the partial conve rsion from 5-hydroxy-2-hexanone-G. Further experiments showed that both 5-h ydroxy-2-hexanone-G and 4,5-dihydroxy-2-hexanone-G, isolated by solid-phase extraction and hydrolyzed, yield comparable amount of 2,5-HD and gamma-val erolactone. In samples treated by acid hydrolysis, GC-MS only does not allo w to understand the true source of 'total' 2,5-HD, which may be produced no t only from 4,5-dihydroxy-2-hexanone-G but also from the more abundant 5-hy droxy-2-hexanone-G, which thus represents the main source of analytical art ifacts. 'Free' 2,5-HD seems to be both suitable from an analytical point of view and meaningful for biological monitoring purposes, provided that conj ugate metabolites are rapidly removed from the body leading to a negligible neurotoxic risk. (C) 1999 Published by Elsevier Science Ireland Ltd. All r ights reserved.