Confirmation of ochratoxins in biological samples by conversion into methyl esters in acidified methanol

There is need to confirm the presence of ochratoxin A (OA) and its metabolites in biological samples following chromatographic analysis. One method is to convert the ochratoxins into different forms. A simplified procedure has been developed for the esterification of OA, GB (dechlorinated GA), O alpha (hydrolyzed GA), and OA-OH (hydroxy-OA) in the presence of methanol and HCl. The percentage conversion of GA into its corresponding methyl ester (OA-Me) was >95% when the sample was incubated for a period of >12 h at 20 degrees C in the presence of 6 N or higher concentrations of HCl and a high relative volume of methanol (95% v/v). Similar results were obtained with OA, OB, and GA-OH when present as pure preparations or in tissue extracts of kidney, liver, bile, and blood. The conversion of O alpha into O alpha-Me was not quantitative. The detection limit of the assay for OA and OA-Me was 1 ng/mL. The procedure can be used for the confirmation of the ochratoxins in biological samples or for the synthesis of their esters.