Optimization of a procedure for the selective isolation of some powerful aroma thiols. Development and validation of a quantitative method for their determination in wine.

This paper deals with the isolation properties of four relevant wine mercaptans in two different separation systems and with the subsequent development, optimization and validation of an analytical procedure for their quantitative determination. 4-Methyl-4-mercaptopentanone (4M4MP), 2-furanmethanethiol (FFT), 3-mercaptohexanol (3MH) and 3-mercaptohexyl acetate (AMH) can be quantitatively extracted from 200 ml of wine in a bed packed with 1 g of LiChrolut EN resins. Fatty acids and diverse interfering compounds are removed by rinsing with an aqueous solution with a 40% of methanol and buffered at pH 7.2 with alpha,alpha,alpha-tris-(hydroxymethyl)-aminomethane (TRIS) 0.2 M. Mercaptans can be further isolated by extraction with an aqueous solution of 1 mM p-hydroxymercury benzoate. Best results were achieved with an extraction with a solution at pH 10.7 with 0.2 M 4-(2-hydroxyethyl)piperazine-1-ethanesulfonic acid (HEPES), further acidification to pH 7.5, release of thiols by the addition of dithioerythritol and extraction with 2 x 0.75 ml dichloromethane. The extract is concentrated to 200 and 20 microl are finally injected in a gas chromatograph-ion trap mass spectrometric system. Method detection limits are 0.8, 6, 15 and 5 ng l(-1) for 4M4MP, FFT, MH and AMH, respectively. The recovery in the sample pre-treatment was 76, 45, 75 and 95%, respectively. Repeatability and linearity are also satisfactory. Different critical aspects, such as the effect of air or the evaporation of the extract were also studied.