Comparing aroma profiles of whisky by SPME-GCxGC-TOF MS

Description:
Over 1000 compounds from a wide range of chemical classes are known to contribute to the aroma of whisky, including alcohols, phenolics, fatty acids, esters, lactones and aldehydes. It is important to be able to confidently identify these volatiles, for quality control and authentication purposes, as well as in the engineering of new aromas. Solid-phase micro-extraction (SPME) coupled with gas chromatography-mass spectrometry (GC–MS) is commonly used for such aroma profiling studies, but it typically results in complex chromatograms with multiple co-elutions, meaning that important trace compounds may be masked by higher-loading constituents. Deconvolution algorithms may be applied, but the more co-elutions that occur and the more similar the co-eluting compounds, the more difficult it can be for these algorithms to be effective. Comprehensive two-dimensional gas chromatography coupled with time-of-flight mass spectrometry (GCxGC-TOF MS) is an ideal alternative, as the enhanced separation capacity allows the entire composition to be screened in a single analysis, with confident identification of compounds that would ordinarily co-elute. However, this increased separation capacity results in data-rich chromatograms which must then be distilled into meaningful conclusions. Here, we apply automated untargeted comparisons using a novel tile-based approach. Firstly, chromatographic alignment accounts for any retention time drift over the course of the study. The software then splits the raw chromatograms into small tiles and sums the signal for each m/z channel in each tile. This information is collated and compared for every sample, allowing the significant differences to be highlighted. In this study, we perform headspace sampling of multiple brands of whisky using solid phase microextraction (SPME) followed by flow-modulated GCxGC-TOF MS, with tile-based chemometrics for fast and simple identification of chemical markers of quality.