Efficient and sensitive screening and confirmation of residues of selected polyether ionophore antibiotics in liver and eggs by liquid chromatography-electrospray tandem mass spectrometry

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Abstract
A method using liquid chromatography tandem mass spectrometry (LC-MS-MS) with electrospray (ES) for the determination of traces of narasin, monensin and salinomycin in chicken liver and eggs was developed, validated and used for routine surveillance. The essence of this paper is to demonstrate that one single method can serve very well for two entirely different purposes, i.e., screening and confirmation. Highly reliable confirmation of the identity at low concentrations was demonstrated when residues of narasin were detected and quantified (0.2 to 11 ng g−1) in 50% of the Swedish eggs analysed in 1999. Four daughter ions were detected with ion ratios meeting suggested confirmation criteria for the European Union, even at 0.2 ng g−1. The method was found to be highly cost-effective since both screening and confirmation of 98 liver samples were performed in only two analytical runs (the Swedish national surveillance scheme of 1999, report level 5 ng g−1). The high performance of the method for the different applications was possible due to a combination of the power of ES-LC-MS-MS, a procedure involving screening of pooled samples, and method optimisation of the work-up (automated solid phase extraction), LC and MS parameters. Validation data for narasin (0.5 to 20 ng g−1) in eggs are presented (accuracy 94 to 108%, relative standard deviation 4 to 10%, limit of detection 0.026 ng g−1). The time for an LC-MS-MS run was 4 min, corresponding to 48 s per sample in a pool.