Isotope dilution GC-MS of benzylpenicillin residues in bovine muscle.

The German provisional prevalidated official method for the determination of residues of benzylpenicillin in muscle (beef) which uses gas chromatography with nitrogen-specific detection (GC-NPD) was successfully adapted to gas chromatography-mass spectrometry (GC-MS) with selected-ion monitoring. The programmed temperature vaporisation injection technique used in the original method was substituted by on-column injection. An isotope dilution procedure for benzylpenicillin using 13C2-labelled benzylpenicillin as internal standard was developed to improve the reliability of the GC-MS results. The spectral overlap of this labelled standard and the analyte caused a non-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 the same time requiring fewer calibration measurements. A comparison of the results obtained by using (i) external standardisation only, (ii) a close analogue of the analyte (phenoxymethylpenicillin) and (iii) 13C2-labelled Pen G as internal standard showed, as was to be expected, that varying recoveries are best corrected by using the isotope-labelled standard.