Quantification of whisky congeners by ¹H NMR spectroscopy

Whisky is a complex mixture made up of thousands of compounds originating in different stages of its production. Analysis of whisky congeners is critical to our understanding of the manufacturing process, quality control, and the detection of counterfeit products. The current chromatographic methods have a long analysis time, can require milliliters of sample and may not detect all required compounds in a single analysis. We have demonstrated that the majority of the whisky congeners of interest can be analyzed using ¹H NMR spectroscopy in a single session using 500 μL of sample with the addition of 100 μL of buffer. We addressed two issues with this application of NMR: sensitivity and complexity of spectra. The sensitivity issues were solved by using a highly sensitive 600 MHz instrument equipped with a cryoprobe. To achieve consistent quantitative analysis of overlapping signals, Chenomx software was used. This allowed successful determination of the absolute concentration of 13 of the 21 studied whisky congeners with an average relative difference from nominal concentration of 6.4% and a standard deviation of 5.0%. Some compounds such as iso-amyl acetate and n-butanol were not accurately quantifiable due to their low concentration and overlapping peaks with those of more concentrated compounds. Scopoletin, lactose, sucrose, and maltose were not detectable in whisky samples, but they were accurately quantified in model mixtures. At higher concentrations, these compounds could be accurately quantified in whisky samples. Overlap of glucose and fructose signals led to >10% deviations from nominal concentration values. The limits of quantification (LOQ) and limits of detection (LOD) for each analyte were determined, with the LOD varying between 10 and 20 μM for the major volatile congeners, 1 to 5 μM for maturation related congeners, and 10 to 30 μM for carbohydrates.