Abstract
Nuclear magnetic resonance spectroscopy (NMR) is a high-tech analytical method that recently found its way into the field of wine analysis with special focus on wine authentication.1 NMR has several advantages including a strictly linear relationship between signal intensity and concentration, fast sample preparation, fast measurement and high repeatability. The biggest disadvantage of NMR in comparison to other analytical techniques is its relatively low sensitivity. Mostly untargeted NMR approaches are used and the obtained results lack cross-validation with established methods such as GC- and HPLC-MS.2 Very few studies for the quantification of wine analytes via NMR have been reported and focused on main wine analytes, such as diols, triols, main acids, amino acids and monomeric sugars present in the 100 to 10,000 ppm range.3,4 This study aims to develop a quantitative NMR based method for the compound class of polyphenols. Polyphenols (e.g. flavonoids such as catechin, benzoic acid derivatives such as gallic acid, cinnamic acid derivatives such as caffeic acid) are present in wine at concentration below 100 ppm (with some exceptions), therefore the wine needs to be concentrated before NMR measurement. An extraction method based on diethyl ether has been developed in this context. The novel method is easy to use and very robust, with a reliable standard deviation typically in the scale of 3% to 6% for the concentration of many analytes. The recovery rate of the method has been tested against an inhouse prepared standard reference material, emulating the composition of a typical Pinot Noir wine. Quantitative and near quantitative recovery rates (> 80 %) have been reached for many analytes.
A total of 20 Pinot Noir wines from South Tyrol, Italy, of different origins and, in some cases, different vintages from the same provenance, have been analysed to test the capacity of the method for the determination of significant differences. Additionally, the method was tested against an already established HPLC-MS methodology. Method parameters are evaluated to identify strengths and weaknesses of the new method. The method can be used in a variety of contexts such as control of authenticity and quality amongst others.