Using experimental design and ¹³C satellites to optimize the measurement of the absolute concentration of ethanol in wine by ¹H NMR spectroscopy

Proton (¹H) NMR spectroscopy is routinely used for targeted food analysis. In alcoholic beverages, ethanol content is a key quality parameter. While ¹H NMR's inherently quantitative nature makes it ideal for absolute quantification of ethanol, challenges arise in complex matrices like wine. High ethanol concentrations, orders of magnitude larger than other constituents and co-solvents, can hinder accuracy due to phenomena such as radiation damping and intermolecular interactions. The present work investigates factors influencing absolute ethanol quantification by ¹H NMR, including ethanol concentration (0–40 % v/v), solvent for signal lock (D₂O vs. DMSO), pulse sequence (w/wo water suppression), NMR signal (main triplet, its ¹³C-satellite or the combination), quantification method (multivariate curve resolution vs. integration by raw sum), and sample type (red vs. white wine). FT-IR was employed as reference method for ethanol absolute quantification. Our results indicate that the highest accuracy for the absolute quantification of ethanol in ethanol-water solutions is achieved by measuring D₂O-containing ethanol solutions with water suppression, and raw sum for quantification of the ethanol main triplet and its satellites. As for wine samples, wine type affected the quantification the most (ASCA effect size = 60 %, p < 0.001). Based on our results, the best approach to obtain accurate absolute quantification of ethanol in wines samples entails the use of (1) an external standard with its satellites (i.e., TSP), (2) a correction factor k_ext, calculated from measurements of a known ethanol reference sample to compensate for misquantifications due to matrix effects, and (3) combining the intensities of the main triplet and its satellites for absolute quantification, in order to compensate for different ¹³C/¹²C ratios in wines. Using this method a correlation coefficient (R²) of 0.99 and RMSE of 0.23 % (v/v) was achieved for absolute quantification of ethanol in wine samples.