DETECTION OF ADDED L-ASCORBIC-ACID IN FRUIT JUICES BY ISOTOPE RATIO MASS-SPECTROMETRY

A method for the isotope effect free isolation of L-ascorbic acid from fruit juices has been developed. It consists of an anion exchange cleanup procedure and a subsequent liquid chromatography separation on a reversed phase column and implies a preceding reduction of L-dehydroascorbic acid. The recovery of L-ascorbic acid from model mixtures and spiked juices with this method was between 62 and 64%, but the delta(13)C value difference between the applied and recovered test compound did not exceed 0.2 parts per thousand. While commercial L-ascorbic acid has in general delta(13)C values near -11.3 parts per thousand, the mean delta(13)C value of vitamin C from authentic orange juices is -20.7 parts per thousand, and that of the corresponding sugar is -25.5 parts per thousand. These differences are the basis for the determination of an addition of synthetic L-ascorbic acid to fruit juices, and it is shown with spiked samples that less than 20% of added commercial product can be identified.