l(+)-tartaric acid of grape origin: Definition of threshold limits for the 13C/12C and 18O/16O stable isotope ratios and validation of the isotopic method through an interlaboratory study.

l(+)-tartaric acid is one of the most used additives in the oenological practices. Its properties make it useful as preservative and acidulant in grape-derived products. This additive can be obtained through the extraction from grapes, from other fruits or through synthesis from petrochemical sources. In Europe, only l(+)-tartaric acid from grapes can be added to wine products, but the price difference between the forms could lead to the fraudulent use of the cheaper synthetic l(+)-tartaric acid. In other continents where such limitation is not applied, the clear commercial identification of the products is still desirable. Stable isotope analysis of carbon (δ¹³C) and oxygen (δ¹⁸O) proved to be a robust and effective method in discriminating between synthetic and grape-derived l(+)-tartaric acid. In this study, to define the threshold limits of δ¹³C (above -24.8 ‰) and δ¹⁸O (above +25.6 ‰) of grape-derived l(+)-tartaric acid, a database consisting of 81 authentic natural samples was created. Moreover, the validation of the isotopic method through an interlaboratory study involving nine European and extra-European laboratories, was provided. The repeatability and the reproducibility standard deviation of the method were calculated.