Ochratoxin A in wheat: an intercomparison of procedures.

The Commission of the European Communities' Community Bureau of Reference (BCR) has undertaken a project to improve methodology and to prepare suitable certified reference materials in order to provide a basis for analytical quality control for the determination of ochratoxin A. The first phase of the project, an intercomparison of procedures for the determination of ochratoxin A in wheat at a level of approximately 13 micrograms/kg, is described. The study involved 24 European laboratories which analysed a naturally contaminated wheat and a 'blank' wheat sample (ochratoxin A content < 1 microgram/kg). The participants used a variety of procedures, including chloroform, methanol, toluene and ethyl acetate for extraction, and silica-, reversed phase- and immunoaffinity columns for clean-up. HPLC (one laboratory used TLC) was applied as the determinative step. Several performance characteristics were checked and the ochratoxin A content was determined. Recoveries were found to range from 25 to 100%. The coefficient of variation from all the results calculated on the basis of peak height was 23%. The study showed that the variation of results was influenced more by the clean-up step than by the extraction solvent. Some laboratories suffered significant day-to-day effects while others found difficulties with interfering peaks in the 'blank' material. It is planned for the next study to improve the recovery range, the clean-up step and the reproducibility (within-laboratory, between-days) and to check the influence of co-extractives from the matrix.