GAS-CHROMATOGRAPHIC DETERMINATION OF ORGANIC-ACIDS FROM FRUIT JUICES BY COMBINED RESIN MEDIATED METHYLATION AND EXTRACTION IN SUPERCRITICALCARBON-DIOXIDE

A procedure in which anionic analytes, trapped on ion exchange resin, are simultaneously methylated and released using methyl iodide in eith er supercritical carbon dioxide or acetonitrile has been extended to p olyfunctional organic acids. The combined SFE methylation of fruit jui ce acids trapped onto ion exchange resin proceeds in good yield produc ing the methyl esters of fumaric, succinic, malic, tartaric, isocitric and citric acids which are readily separated by GC. Using this proced ure low concentrations of one acid can be detected and quantitated in the presence of very high concentrations of another. This new method d etects tartaric acid at levels of 10 ppm in juices containing 10 000 p pm citric acid. Quantitation was performed either by using GC-FID with triethyl citrate or diethyl tartrate as internal standards or with th e element specific calibration capability of the GC-AED. A simple new technique for the determination of citric/isocitric acid ratio is now available. Also, in contrast to HPLC methods, the identity of an analy te is readily confirmed by GC-MS. (C) 1997 Elsevier Science B.V.