Background and objective In Japan, the residue definition for fosetyl-Al was “sum of fosetyl-Al and phosphonic acid expressed as fosetyl-Al” and its current provisional MRL in cereals was under review. For establishment and enforcement of fosetyl-Al MRL in cereals, a new analytical method for fosetyl-Al and phosphonic acid in cereals should be developed and validated. Methods The new method involved water extraction, clean-up using tandem cation- and anion-exchange mini-columns, and determination by LC-MS/MS. It was validated in single laboratory and multiple laboratories. Using the method, 41 samples of rice, wheat and barley were analyzed. Results In the multiple-laboratory validation: repeatability and reproducibility for three concentrations of fosetyl-Al and phosphonic acid were in ranges of 4.8–20% and 5.9–34%; calculated sum of fosetyl-Al and phosphonic acid, expressed as fosetyl-Al, showed good recoveries; linearity was observed for fosetyl-Al and phosphonic acid in ranges of 0.005–0.4 and 0.025–2.0 mg/kg; and specificity was sufficient. The method was verified for rice matrices. In 41 samples, phosphonic acid was detected up to 0.2 mg/kg while fosetyl was not. Conclusion The method was successfully validated with high precision, accuracy, linearity, and specificity and capable of analyzing fosetyl-Al and phosphonic acid with the practical LOQ of 0.01 and 0.05 mg/kg. The LOQs and concentrations of phosphonic acid in samples indicate that a potential MRL would be 0.5 mg/kg for fosetyl-Al in cereals. Highlights The validated method was simpler than many methods and did not require derivatization or matrix matched or isotopically labeled internal standards.

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