Development of an LC–MS/MS Method for Simultaneous Quantification of Erythromycin A and Seven Metabolites: Initial Residue Profile in Salmon Muscle with Skin

A robust liquid chromatography tandem mass spectrometry (LC–MS/MS) method was developed and validated for determining erythromycin A (ERY A) and its metabolites: erythromycin B (ERY B), erythromycin C (ERY C), erythromycin A N-oxide (ENO), anhydroerythromycin A (AHEA), N-demethylerythromycin A (NDEA), erythromycin A enol ether (EAEE), and pseudoerythromycin A enol ether (PSEE) in salmon muscle with skin. Tissue samples (1.00 g) fortified with ERY A, ERY B, ERY C, ENO, EAEE, and PSEE at 50–200 ng/g and AHEA and NDEA at 500–2000 ng/g, using a surrogate internal standard, were extracted with acetonitrile, defatted hexane, and diluted water, resulting in accuracy from 87% to 108%, precision from 3% to 13%, and estimated LOQ from 0.02 to 1.2 ng/g. Incurred salmon showed ERY A as the major residue at 3 days post-administration and NDEA and AHEA as potential markers exceeding the 500 ng/g level on Day 28.