Quantitative determination of wine polyfunctional mercaptans at nanogram per liter level by gas chromatography–negative ion mass spectrometric analysis of their pentafluorobenzyl derivatives

Abstract

A fast method for the determination of aroma-powerful polyfunctional thiols at nanogram per liter level has been developed and applied to wine. A small volume of wine (6 mL) was extracted with 1.5 mL of benzene containing four internal standards. Pentafluorobenzyl derivatives of mercaptans were formed in the extract by adding small amounts (100 mg L−1) of pentafluorobenzyl bromide and a strong alkali: 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). After washing with a water:methanol (5:1) solution 0.5 M in HCl, 20 L of the extract was directly injected into a gas chromatograph. Derivatives were detected by negative ion mass spectrometry. The method makes it possible to simultaneously determine 2-furfurylthiol (2-furanmethanethiol) (FFT), 4-mercapto-4-methyl-2-pentanone (MP), 3-mercaptohexylacetate (MHA) and 3-mercaptohexanol (MH). Inconsistent results were obtained for 2-methyl-3-furanthiol (MF). Detection limits were 0.5 ng L−1 (FFT), 0.1 ng L−1 (MP), 0.6 ng L−1 (MHA) and 7 ng L−1 (MH), well below the corresponding odor detection thresholds. Method repeatability (10% < RSD < 17%) and linearity (0.98 < R2 < 0.999) were satisfactory. The linear range was more than 2 orders of magnitude wide, covering the natural range of occurrence of these compounds in wine, and the slopes of the standard addition plots from different wines were very similar. The different aspects of the method optimization are discussed.