Determination of Oxalates in Cork Stoppers by HPLC with Electrochemical Detection

¹ Laboratório de Toxicologia, 4Laboratório de Bromatologia. Faculdade de Farmacia. Universidade do Porto. 4000 Porto. Portugal.

For oxalate quantification in cork stoppers an automated HPLC method using an ion exchange organic acid column and an electrochemical detector was performed. The study was conducted on cork stoppers in three steps of their manufacture: untreated cork stopper (corresponding to natural cork), whitened stopper, and final stage (ready for commercial delivery). The oxalate determinations may be performed in a linear range from 0.1 µg/mL (detection limit) to 20 µg/mL. The identification of the chromatographic peak corresponding to oxalic acid was proved by the specific action of oxalate oxidase. Satisfactory recovery for the extraction procedure (recoveries of oxalic acid in the additions method between 92.85% ± 3.70% and 103.65% ± 7.70%) and precision for the over-all procedure (relative standard deviation between 5.90% and 8.20%), and low time consuming (30 min for the HPLC analysis) were observed. It makes this procedure suitable for monitoring residual oxalates in cork stoppers either by the industry itself or by government quality control. Quantification of oxalic acid in 60 randomly obtained samples from the market gave the value of 173.8 ± 15.4 µg/g (X ± SE).

Key words: cork stoppers, oxalates, HPLC/electrochemical detection

Submitted on February 14, 1994