Development of a novel LC-MS/MS method for the detection of adulteration of South African sauvignon blanc wines with 3-alkyl-2-methoxypyrazines

[摘要] A method for the detection of adulteration of South African Sauvignon blancwines, by enrichment with foreign sources of 3-alkyl-2-methoxypyrazenes, isdescribed. The levels of three 3-alkyl-2-methoxypyrazenes (3-isobutyl-, 3-isopropyl- and 3-sec-butyl-2-methoxypyrazine) in South African Sauvignon blancwines were measured with liquid chromatography-mass spectrometry. Samplepreparation involved clean-up and pre-concentration by distillation followed bysolvent extraction of the distillate with dichloromethane. Extracts were acidifiedand concentrated by evaporation and finally reconstituted to a fixed volume toaffect quantitative pre-concentration of the samples. Sample extracts wereseparated with reversed phase liquid chromatography utilizing a phenyl-hexylseparation column. Residues were measured with liquid chromatography-massspectrometry utilizing a tandem quadrupole mass spectrometric detectoroperated in multiple reaction monitoring mode for optimal trace level quantitation.Atmospheric pressure chemical ionization was utilized as electrospray ionizationwas found to suffer from quenching effects attributed to the sample matrix.Qualitative information was obtained from the relevant molecular ions as well astwo secondary ion transitions (and one ion ratio) in each case. Recoveriesobtained by the extraction procedure were better than 90% with coefficient ofvariance of better than 10% at concentrations from 1 to 100 ng/L. The limit ofdetection of the method was 0.03 ng/L and the limit of quantification 0.10 ng/L forthe three analytes measured. The described LC-MS method is more sensitive forthe determination of 3-alkyl-2-methoxypyrazines in wine than GC methodsreported for the same purpose.From the experimental data, a set of parameters were established to discriminateadulterated South African Sauvignon blanc wines. It was demonstrated that the3-isobutyl-2-methoxypyrazine concentration, despite showing considerablevariance, was confined to a relatively narrow range spanning approximately twoorders of magnitude (0.20 to 22 ng/L). A clear indication of possible maximum values for this compound in South African Sauvignon blanc wines was obtainedfrom the analysis of a large number of samples (577), spanning most relevantwine producing regions and representing vintages 2003 to 2006. It was alsodemonstrated that South African Sauvignon blanc wines contain the major 3-alkyl-2-methoxypyrazenes in reasonably distinct relative amounts and that thesaid ratios of abundance may be used to elucidate authenticity. The expectedeffect of adulteration with green pepper extracts or some synthetic preparationson the 3-isobutyl-2-methoxypyrazine concentration as well as the relativeabundances were also determined by characterizing the corresponding profiles ingreen peppers and some synthetic flavor preparations. Two adulterated samplesin the dataset were identified by both outlined criteria. A limited number of winesof other cultivars were also analyzed. The results represent the most completeand accurate data on the 3-alkyl-2-methoxypyrazine content of South AfricanSauvignon blanc wines to date.A publication covering the work presented in this thesis is currently inpreparation.

[发布机构] Stellenbosch University