Rapid mass spectrometry approaches for wine industry quality control applications

The work presented in this thesis investigates the utility of direct mass spectrometry (MS) approaches for wine industry quality control applications. Chapter 1 provides an insight into the evolution of direct mass spectrometry from the beginnings of mass spectrometry in 1913, to the development of proton transfer reaction mass spectrometry in 1995 and the breakthrough invention of desorption electrospray ionisation in 2004. The latter represents the birth of “ambient mass spectrometry” marking a new approach to mass spectrometry – the pursuit of direct, in situ analysis on unprocessed samples in their natural environment. High‐variability is a natural characteristic of grapes and oak‐aging processes and thus presents challenges from a wine quality perspective. These aspects of wine quality are introduced, highlighting how new analytical approaches based on direct mass spectrometry could be of benefit to the wine industry. Chapter 2, introduces a secondary electrospray ionization mass spectrometry (SESIMS) approach to monitor grape ripening by analysing volatile organic compounds (VOC’s) directly from intact berries of non‐Muscat grape cultivars (Pinot Noir, Chardonnay and Sauvignon Blanc). The method does not require sample preparation or concentration steps. Grape volatiles were tentatively identified based on soft ionisation and accurate mass, the related elemental composition and literature. Approximately 300 peaks were detected in positive ion mode, and fewer (70‐100) in negative ion mode. Peaks assigned to C13‐norisoprenoids and benzenoid derivatives have shown similar trends during ripening in previous studies using offline gas chromatography (GC) approaches. Considering the high sensitivity and speed of analysis, SESI‐MS holds promise as a tool for the rapid screening of grape volatiles that could be useful for a number of significant applications including ripeness monitoring and disease detection. Chapter 3, introduces a real‐time method to monitor the evolution of oak aromas during the oak toasting process. French and American oak wood boards were toasted in an oven at three different temperatures, while the process‐gas was continuously transferred to the inlet of a proton‐transfer‐reaction time‐of‐flight mass spectrometer for online monitoring. Oak wood aroma compounds important for their sensory contribution to oak‐aged wine, were tentatively identified based on soft ionisation, molecular mass and the literature. Their time‐intensity profiles revealed toasting process dynamics and illustrated in real‐time how different compounds evolve from the oak wood during toasting. The study demonstrated a new analytical approach for research on oak wood toasting which circumvents limitations associated with previous gas chromatography based approaches. In Chapter 4, a thermal desorption dielectric barrier discharge ionisation mass spectrometry system (TD‐DBDI‐MS) for rapid analysis of volatile organic compounds (VOC’s) from oak wood shavings was developed. Oak VOC’s were tentatively identified based on soft ionisation, accurate mass and the literature. Known oak wood VOC’s from different chemical classes were detected including furanic aldehydes, volatile phenols, phenolic aldehydes, oak lactones and norisoprenoids. 287 peaks were detected across all oak samples. Screening of oak VOC’s could be conducted with an analysis time of ten seconds. The system was also used to monitor the oak toasting process. Continuous infusion of an internal standard and appropriate dilution strategies demonstrated how ion suppression or enhancement phenomena could be minimised and accounted for in this ambient mass spectrometry approach. Finally, Chapter 5 of this thesis examines the findings of the three studies presented in Chapters 2, 3 and 4 and discusses directions for future research