Rapid quantification of ionophores in feeds by liquid chromatography-tandem mass spectrometry

Ionophores are widely used in veterinary medicine as coccidiostats and for improving nutrient utilization in livestock production. Because of widespread use, ionophores sometimes cause poisoning in livestock. Quantifying concentration of these compounds in feeds for diagnostic purposes is needed. A method with a single step of solvent extraction was developed for rapid simultaneous quantification of monensin, lasalocid, salinomycin, and narasin in feeds by liquid chromatography-tandem mass spectrometry (LC-MS/MS). The ionophores were extracted using methanol:water (90:10). With the high specificity and high sensitivity of tandem mass spectrometry, the extract was introduced for measurement without further processing. The effect of particle size of feeds on extraction efficiency was also investigated. It was found that feeds passing through a 1-mm filter or sieve show better quantitative extraction. Nigericin was used as internal standard for the measurement. The method was validated by fortification of the selected ionophore compounds in horse feed at different concentrations. The typical recovery rate was 69-122%. Meanwhile, various interlaboratory proficiency test samples of different matrices were also quantified as part of the procedure for method validation. A good agreement was found between results and the suggested values. The method is very sensitive, with detection limits between 0.018 µg/g and 0.056 µg/g for the compounds tested. Results showed that the lower limit of quantification was 0.2 µg/g for the ionophore compounds, which is much lower than the contents of the ionophores in medicated feeds, which is generally approximately 10-100 µg/g feed.