Improved solid-phase extraction procedure for the isolation and in-sorbent pentafluorobenzyl alkylation of polyfunctional mercaptans: Optimized procedure and analytical applications

A fast method for the determination of aroma-powerful polyfunctional thiols at ng L−1 level has been developed. Mercaptans are selectively retained in a 50 mg solid-phase extraction cartridge and derivatization takes place in the same cartridge at room temperature (25 °C) in 20 min by adding small amounts of pentafluorobenzyl bromide (PFBBr) and a strong alkali: 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). The corresponding derivatives are further eluted and determined by gas chromatography–negative ion mass spectrometry (GC–NCI-MS). Isolation, derivatization, clean-up, elution and calibration conditions were examined. Carrying the reaction in the cartridge makes it possible to use water and non-polar reagents simultaneously, to avoid large volumes of toxic solvent, and to eliminate the excess of reagent. This was last accomplished by the reaction with mercaptoglycerol and further rinsing with a hydromethanolic solution. The method makes it possible to simultaneously determine 2-methyl-3-furanthiol (MF), 2-furfurylthiol (2-furanmethanethiol) (FFT), 4-mercapto-4-methyl-2-pentanone (MP) (as its methoxime), 3-mercaptohexylacetate (MHA) and 3-mercaptohexanol (MH). Absolute limits of detection were 0.2 (MF), 0.1 (FFT), 0.1 (MP), 0.3 (MHA) and 2 (MH) ng L−1. Repeatability (1% < RSD < 20%) and linearity (0.978 < R2 < 0.999) were satisfactory. Problems with matrix effects were solved by the use of deuterated analogues for MP, MHA and MH and by avoiding the oxidation of analytes and standards via addition of cysteine and EDTA. The different aspects of the method optimization are discussed.