A confirmatory method for detection of a banned substance: The validation experience of a routine EU laboratory

The Commission Decision 2002/657/EC is a fundamental reference document for the UE laboratories involved in residue analysis although its implementation has caused some difficulties in the requirements interpretation. In this work a pragmatic validation approach of a quantitative confirmatory method for the detection of 17-alpha-(α-NT) and 17-beta-19-nortestosterone (β-NT) in bovine urine by gas chromatography mass spectrometry is proposed. The 19-nortestosterone is a banned anabolic steroid for which no minimum required performance limit (MRPL) has been laid down, therefore the limit reported in Italian Residue Monitoring Plan (2 μg L−1) has been considered the reference level to evaluate the method performances. The decision limit (CCα) and the detection capability (CCβ) were obtained by the calibration curve procedure. The minimum required performance level (mrpl), which represents the starting concentration of the calibration curves, was preliminary fixed estimating the results dispersion of blank urine samples fortified at 2 μg L−1 for each isomer. The found CCα and CCβ were 1.5 and 1.9 μg L−1 for α-NT and 1.2 and 1.4 μg L−1 for β-NT. The precision (repeatability and within-laboratory reproducibility) and recoveries were suitable for the investigated concentration range (1-3 μg L−1). Finally, the method ruggedness (minor and major changes) has been also demonstrated.