Electrospray ionization and collision-induced dissociation of antibiotic polyether ionophores

A series of polyether ionophores (lasalocid, monensin-A, narasin-A and salinomycin) was investigated by electrospray ionization (ESI) tandem mass spectrometry (MS/MS). A fast short-column liquid chromatographic (LC) method, using a ternary gradient sequence, was developed to separate the ionophores in less than 4 minutes. Several mobile phase parameters such as pH and solvent additives were investigated, and their impact on both chromatographic and mass spectrometric behavior was studied. The ionophores readily formed stable complexes with various metal cations present as impurities in the solvents. Therefore, a method was developed to convert the ionophores into only sodium adduct species prior to ESI analysis. This procedure maximized both sensitivity and specificity in the subsequent MS/MS step. Low energy collision-induced dissociation (CID) of the sodium adduct ions resulted in a number of structure-diagnostic product ions for all ionophores. The proposed interpretation of these fragment ions is presented in this work. Additionally, the fragmentation of a series of monovalent carboxylic alkali-metal ion complexes of lasalocid was investigated under CID conditions. A comparison of the CID spectra revealed a decreasing degree of fragmentation in the order [M + H]+ approximately [M + NH4]+ > [M + Li]+ > [M-H + 2Li]+ > [M + Na]+ >> [M + Cs]+. To demonstrate the analytical potential of the LC/MS/MS method, its application to the determination of salinomycin in catfood samples was investigated. Salinomycin was recovered from catfood by a microwave solvent extraction procedure, and subsequently analyzed using LC/MS/MS of the sodium adduct ions.