Derivatisation of varietal thiols with phenyl propiolate analogues for quantitation by LC/MS

Abstract

Varietal thiols are well established as key aroma compounds in Sauvignon blanc and many other white wines. The quantitation of these varietal thiols has proven to be difficult due to their volatile nature at low concentrations. Several methods of quantitation have been produced but could be improved upon due to the use of dangerous chemicals, long derivatisation times or the inability to quantitate all the intended varietal thiols. This research aimed to adapt a method produced by Herbst-Johnstone et al to utilise propiolates for the analysis of varietal thiols by LC/MS. The three derivatising agents explored during this project were phenyl propiolate, para-methoxy phenyl propiolate and para-nitro phenyl propiolate. The synthetic techniques used during this project resulted in moderate yields of the three propiolates. Subsequent reaction of the derivatising agents showed that thiols reacted withpara-nitro phenyl propiolate at the expected position but also at a second unexpected site. Due to this para-nitro phenyl propiolate was removed from further investigation. Initial analysis of the derivatised thiols showed that they could not be detected at the concentrations where the varietal thiols are normally present in wine. With this in mind, the analysis proceeded as a 'proof of concept' investigation using higher concentrations of varietal thiols. This final experimental allowed for the varietal thiols and their isotopically labelled internal standards to successfully undergo derivatisation when a spiked in Sauvignon blanc or model white wine were used. Following this, concentration of the derivatised thiols was achieved utilising solid phase extraction. These extracts were then analysed by LC/MS. The analysis of the three varietal thiols with each derivatising agent was used to produce calibration curves based on the concentration and response ratio of the thiol to its internal standard. These curves showed good correlation to the data with R2 values all near 1.0. While the data showed good correlation, the limit of quantitation calculated proved to be larger than desired for the effective use of the method in the quantitation of varietal thiols in white wine samples. Further research into the optimisation of this method is required in order to effectively quantitate varietal thiols in wine.