Development of stable isotope dilution assays for the quantitation of the food odorants hydrogen sulphide, methanethiol, ethanethiol, and propane-1-thiol and application to durian (Durio zibethinus L.) pulp

Stable isotope dilution assays were developed for the quantitation of the highly volatile odorants hydrogen sulphide, methanethiol, ethanethiol, and propane-1-thiol using sodium (S-34)sulphide, sodium (H-2(3))methanethiolate, (H-2(5))ethanethiol, and (H-2(7))propane-1-thiol as internal standards and 3-buten-2-one as derivatisation reagent. Applying sodium as the reducing agent, sodium (S-34)sulphide was synthesised from S-34(8) and sodium (H-2(3))methanethiolate from (H-2(6))dimethyl sulphide. The analytical approach included homogenisation of the food material with the added standards, derivatisation, solvent extraction, and solvent-assisted flavour evaporation (SAFE). Separate detection of analytes and standards was achieved by two-dimensional heart-cut gas chromatography-mass spectrometry using chemical ionisation and characteristic ions of the derivatives 4,4'-sulphanediyldi-2-butanone, 4-(methylsulphanyl)butan-2-one, 4-(ethylsulphanyl)butan-2-one, and 4-(propylsulphanyl)butan-2-one and their isotopically substituted analogues. Application of the approach to durian pulp resulted in high odour activity values (OAV = concentration/odour threshold) with ethanethiol (OAV 480000) being the most potent odorant, followed by methanethiol (OAV 45000), propane-1-thiol (OAV 6300), and hydrogen sulphide (OAV 330).