13C/12C isotope ratios of organic acids, glucose and fructose determined by HPLC-co-IRMS for lemon juices authenticity

High performance liquid chromatography linked to isotope ratio mass spectrometry via an interface allowing the chemical oxidation of organic matter (HPLC-co-IRMS) was used to simultaneously determine carbon 13 isotope ratio (δ13C) of organic acids, glucose and fructose in lime and lemon juices. Because of the significant difference between organic acids and sugars concentrations, the experimental protocol was optimised by applying a “current jump” to the IRMS device. The filament current is increased of 300 μA during elution in order to enhance IRMS sensitivity. Then, analysis were performed on 35 lemon and lime fruits from various geographical origins and squeezed in the laboratory. An overall average δ13C values of −25.40 ± 1.62‰, −23.83 ± 1.82‰ and −25.67 ± 1.72‰ is found for organic acids mixture mainly made up of citric acid, glucose and fructose, respectively. These authentic samples allowed the definition of a confidence domain to which have been confronted 30 commercial juices (24 “pure juices” and 6 coming from concentrate). Among these 30 samples, 10 present δ13C values outside the defined range revealing an added “C4” type organic acids or sugars, addition not specified on the label that is not in agreement with EU regulation.