Development and Validation of a Quantitative Gas Chromatography – Mass Spectrometry Confirmatory Method for Phenylbutazone in Equine Plasma

The non-steroidal anti-inflammatory drug phenylbutazone is known to have a number of adverse side effects, however, its use in the equine remains widespread. To protect the human consumer, an action level of 50 ng mL−1 in plasma has been agreed by the European Union and regulatory compliance requires a methodology validated to the EU guidelines SANCO/1805/200 revision 6. Accordingly, a GC-MS method has been developed and validated to serve this purpose. The method counters the adverse effects of strong protein binding and the instability of phenylbutazone at low pH. Selected ion monitoring (SIM) allows the acquisition of three diagnostic ions and the use of a deuterated internal standard secures good quantitative performance.