Rapid Method for the Simultaneous Determination of Six Ionophores in Feed by Liquid Chromatography/Mass Spectrometry

A simple and highly sensitive LC/MS method was developed for the simultaneous determination of six ionophores—lasalocid, monensin, laidlomycin, maduramycin, salinomycin, and narasin—in feed. The procedure involved extraction of 1 g of feed with 4 mL of methanol–water (9 + 1, v/v) by shaking on a platform shaker for 45 min. After centrifugation, the extracts were diluted with methanol–water (75 + 25, v/v) and analyzed without any cleanup. The analysis was performed on a Betasil C18 column (150 × 4.6 mm id, 5 μm particle size) connected to an LC/MS system operated in the atmospheric pressure chemical ionization (APCI) mode. We believe this to be the first method that uses the APCI mode for the analysis of ionophores. The mobile phase consisted of 50 mM ammonium acetate as solvent A and acetonitrile–methanol (7 + 3, v/v) as solvent B in a gradient run. Excellent recoveries of 81–120% were found for all compounds at fortification levels of 1–200 μg/g, with RSD ≤15% (except 17% for maduramycin at 2 and 5 μg/g, and 16% for salinomycin at 1 μg/g). At 0.5 μg/g, recoveries of 87–119% were obtained, with RSD ≤20%. However, recovery of lasalocid was 133% and salinomycin 79% in sow and horse feed, respectively. Average RSD values of lasalocid and salinomycin were 22 and 21%, respectively. Finally, proficiency test samples analyzed with the method demonstrated favorable agreement with the certified values.