Synthesizing a Novel Isotopically Labeled Standard for SIDA–SPE–GC–MS Analysis of γ-Nonalactone in Pinot Noir

A new study in the journal Analytical and Bioanalytical Chemistry focuses on γ-nonalactone, an aroma compound known as a linear aliphatic lactone that is found in wine and is associated with certain sweet aroma descriptors including coconut and stone fruit (1). For the first time, this research team said, γ-nonalactone was quantified in 12 commercial pinot noir wines from New Zealand with a stable isotope dilution assay (SIDA) that synthesized nonalactone, a novel isotopologue of γ-nonalactone. The end goal, according to the study, was also to find out for the first time if γ-nonalactone played an important role in the aroma profiles of this particular branch of pinot noir.

Die Essenz des Südtiroler Weinbaus: Pinot Noir mit Eichenfass im Weinberg | Image Credit: © PhotoArtBC - stock.adobe.com

γ-Nonalactone is an aroma compound that is widely recognized for its sweet, creamy, and coconut-like scent. It belongs to the family of lactones, which are cyclic esters commonly found in various natural products. γ-Nonalactone is naturally occurring and can be found in several fruits, including peaches, apricots, and pineapples, contributing to their distinctive aroma.

The authors, based at the University of Auckland, further sought an accurate and sensitive analysis of the wines from their home country by deploying solid-phase extraction (SPE) followed by gas chromatography coupled to mass spectrometry (GC–MS). The reason they gave for using an isotopically labeled standard (in this case, ²H₂¹³C₂-γ-nonalactone, with heptaldehyde as the starting material) instead of a surrogate internal standard is that the former would hypothetically behave in a similar way to the analyte of interest even in a variety of matrices (1).

γ-Nonalactone is known to have synergistic reactions with other aroma compounds, like fellow linear aliphatic lactones γ-decalactone, γ-dodecalactone, γ-octalactone, and γ-undecalactone (1). Given those interactions, the researchers wanted to see if γ-nonalactone might make contributions to the aroma of pinot noir even if it were found in levels below an odor detection threshold (ODT). So in this study, wine samples – which covered a range of regions, vintages, and price points – were described as being “spiked” at normal and elevated conditions during preparation, and the eventual MS analysis showed stability of ²H₂¹³C₂-γ-nonalactone.

In a model wine calibration, the study found, concentrations of γ-nonalactone ranged from 0 to 100 µg/L, and the researchers judged linearity (R² > 0.99), reproducibility (0.72%), and repeatability (0.38%) all to be excellent. When actually analyzed in the designed SPE–GC–MS sequence, γ-nonalactone concentrations were detected between 8.3 and 22.5 µg/L, with the upper end of those findings approaching the compound’s ODT (30 µg/L). Anecdotally, the researchers said, γ-nonalactone has been measured as high as 155 µg/L in pinot noir from Oregon, USA, and in such quantities may be associated with an undesirable aroma of prunes present in prematurely aged red wines (1).

Despite the samples, as mentioned, covering different New Zealand regions, as well as of different prices and vintages, the researchers found other factors were more likely than those to be central to the modulation of γ-nonalactone concentration (1). The researchers suggest further research into the biogenesis of γ-nonalactone during wine production as a marker of the compound’s variability in different samples.

Reference

(1) Miller, G.C.; Barker, D.; Pilkington, L.I.; Deed, R.C. Synthesis of a novel isotopically labelled standard for quantification of γ-nonalactone in New Zealand Pinot noir via SIDA–SPE–GC–MS. Anal. Bioanal. Chem. 2023. DOI: 10.1007/s00216-023-04789-2