Synthesis of a novel isotopically labelled standard for quantification of γ-nonalactone in New Zealand Pinot noir via SIDA-SPE-GC-MS

Abstract

γ-Nonalactone is a linear aliphatic lactone ubiquitous in wine, associated with coconut, sweet, and stone fruit aroma descriptors. Little research has been conducted looking at the importance of this compound to New Zealand (NZ) wine aroma. ²H₂¹³C₂-γ-Nonalactone, a novel isotopologue of γ-nonalactone, was synthesised in this work for use in a stable isotope dilution assay (SIDA) for quantification of γ-nonalactone in NZ Pinot noir wines for the first time. Synthesis was carried out using heptaldehyde as the starting material, and ¹³C atoms and ²H atoms were introduced via Wittig olefination and deuterogenation steps, respectively. The suitability of this compound as an internal standard was demonstrated by spiking model wine at normal and elevated conditions during sample preparation, with subsequent analysis via mass spectrometry showing stability of ²H₂¹³C₂-γ-nonalactone. A model wine calibration, with concentrations of γ-nonalactone from 0 to 100 µg L^-1, was shown to have excellent linearity (R² > 0.99), reproducibility (0.72%), and repeatability (0.38%). Twelve NZ Pinot noir wines, representative of a range of NZ Pinot noir-producing regions, prices, and vintages, were analysed by solid-phase extraction-gas chromatography-mass spectrometry (SPE-GC-MS). The concentrations of γ-nonalactone ranged from 8.3 to 22.5 µg L^-1, the latter of which was close to the odour detection threshold of this compound. These findings provide a basis for further research into γ-nonalactone and its impact on NZ Pinot noir aroma and provide a robust method for the quantification of this compound in Pinot noir.