Direct accurate analysis of cysteinylated and glutathionylated precursors of 4-mercapto-4-methyl-2-pentanone and 3-mercaptohexan-1-ol in must by ultrahigh performance liquid chromatography coupled to mass spectrometry.

A direct method for the quantitative determination in grapes of four known precursors of the varietal aromas of the thiol character of wine has been optimized and validated. A small volume of centrifuged and filtered must is directly injected in the ultrahigh liquid performance chromatography coupled to mass spectrometry system (UHPLC-MS-MS). Must sugars and other early eluting polar compounds are diverted to the waste, not entering the ion source. Cysteinyl (CYS) and glutathionyl (GLU) precursors from 3-mercaptohexanol (3MH) and 4-mercapto-4-methylpentanone (4M4MP) are separated in 5 min rendering narrow peaks (W1/2<9s). No system performance degradation has been noticed in series of more than 200 injections. A standard addition procedure using the consecutive injection of a spiked sample made it possible to satisfactorily correct for matrix effects, with recoveries very close to 100% in all cases. Intermediate reproducibility was satisfactory (RSD between 1 and 10%), except for the glutathione-4-mercapto-4-methyl-2-pentanone (GLUMP) precursor, for which this value is around 20%. The limits of detection in real-must were below 1 μg L(-1) for the precursors of the 4M4MP and for cysteine-3-mercaptohexan-1-ol (CYSMH) and glutathione-3-mercaptohexan-1-ol (GLUMH), 2 and 7 μg L(-1), respectively, what is enough for the determination of these precursors in musts of any variety.