Isotope dilution GC-MS of benzylpenicillin residues in bovine muscle

The German provisional prevalidated official method for the determination o f residues of benzylpenicillin in muscle (beef) which uses gas chromatograp hy with nitrogen-specific detection (GC-NPD) was successfully adapted to ga s chromatography-mass spectrometry (GC-MS) with selected-ion monitoring. Th e programmed temperature vaporisation injection technique used in the origi nal method was substituted by on-column injection. An isotope dilution proc edure for benzylpenicillin using C-13(2)-labelled benzylpenicillin as inter nal standard was developed to improve the reliability of the CC-MS results. The spectral overlap of this labelled standard and the analyte caused a no n-linear relationship in calibration for which linear bracketing and single -point calibration were compared. Single-point calibration proved to be the superior method, showing less deviation from the 'true' value, while at th e same time requiring fewer calibration measurements. A comparison of the r esults obtained by using (i) external standardisation only, (ii) a close an alogue of the analyte (phenoxymethylpenicillin) and (iii) C-13(2)-labelled Pen G as internal standard showed, as was to be expected, that varying reco veries are best corrected by using the isotope-labelled standard.