Quantification of ochratoxin A in foods by a stable isotope dilution assay using high-performance liquid chromatography-tandem mass spectrometry

A stable isotope dilution assay (SIDA) was developed for quantification of the mycotoxin ochratoxin A (OTA) by using [²H₅]-OTA as internal standard. The synthesis of labelled OTA was accomplished by acid hydrolysis of unlabelled OTA and subsequent coupling one of the products, ochratoxin α, to [²H₅]-L-phenylalanine. The mycotoxin was quantified in foods by LC-tandem MS after extraction with buffers containing [²H₅]-OTA and clean-up by immuno affinity chromatography or by solid phase extraction on silica. The method showed a sufficient sensitivity with a low detection and quantification limit of 0.5 and 1.4μg/kg, respectively, and good precision in inter-assay studies showing a CV (n=3) of 3.6%. The analysis of certified reference materials resulted in a low bias of 2.1% from the certified values and revealed excellent accuracy of the new method. To prove the suitability of SIDA, OTA was quantified in a number of food samples and resulted mainly in not detectable OTA contents. However, three samples of raisins exceeded the legal limit of 10μg/kg and highlighted the need for further controlling the contamination with the mycotoxin.