Gas chromatography and isotope ratio mass spectrometry of Pinot Noir wine volatile compounds (δ13C) and solid residues (δ13C, δ15N) for the reassessment of vineyard water-status

Abstract

This paper describes a novel approach to reassess the water status in vineyards based on compound-specific isotope analysis (CSIA) of wine volatile organic compounds (δ¹³C_VOC/VPDB) and bulk carbon and nitrogen isotopes, and the C/N molar ratios of the wine solid residues (δ¹³C_SR/VPDB, δ¹⁵N_SR/Air-N2). These analyses link gas chromatography/combustion and elemental analysis to isotope ratio mass spectrometry (GC/C/IRMS, EA/IRMS). Field-grown cultivars of Pinot Noir grapevines were exposed during six growing seasons (2009-2014) to controlled soil water availability, while maintaining identical the other environmental variables and agricultural techniques. Wines were produced from the grapes by the same oenological protocol. This permitted for the assessment of the effects in the biochemistry of wines solely induced by the changes in the plant-soil water status. This mimicked the more recurrent and prolonged periods of soil water deficiency due to climate changes. Water stress in grapevine was assessed by the measurement of the predawn leaf water potential (Ψ_pd) and the stable carbon isotope composition of the berry sugars during harvest (must sugars). For quantitation purposes and the normalization of the measured stable carbon isotope ratios of the VOCs, the wine samples were spiked with three standard compounds with known concentration and δ¹³C_VPDB values. VOCs were extracted by liquid-liquid extraction and analyzed by gas chromatography/flame ionization detection (GC/FID), gas chromatography/mass spectrometry (GC/MS), and GC/C/IRMS. δ¹³C values were obtained for eighteen VOCs. The solid residues were obtained by freeze-drying wine aliquots and were analyzed for their C and N content and isotope composition by EA/IRMS. All the isotopic ratios (δ¹³C_SR, δ¹⁵N_SR, δ¹³C_VOC) are highly correlated with the Ψ_pd values, indicating that the proposed gas chromatography and isotope ratio mass spectrometry approach is a useful tool to assess the changes in the water status of grapevine cultivars in different terroirs. The combined analytical approach was used for the first time to complement the assessment of soil water availability effects on the grapevine. The δ¹³C values of the volatile compounds helped confirm (or establish) their main source(s) and biosynthetic pathway(s). Importantly, we also show for the first time that the combination of C/N and δ¹⁵N values of freeze-dried wines have an unexplored potential for the study of nitrogen dynamics in soil/grape/wine systems.

Keywords: Chromatography; Grapevine water status; Solid residues; Stable isotope ratio mass spectrometry; Volatile organic compounds; Wine analysis.