Development of isotope dilution-liquid chromatography tandem mass spectrometry for the accurate determination of fluoroquinolones in animal meat products: optimization of chromatographic separation for eliminating matrix effects on isotope ratio measurements.

Isotope dilution-liquid chromatography/tandem mass spectrometry (ID-LC-MS/MS) has been established as a candidate reference method for the accurate determination of three representative fluoroquinolone antibiotics (enrofloxacin, ciprofloxacin, and norfloxacin) in meat products. Enrofloxacin-d₅, ciprofloxacin-¹³C₃¹⁵N, and norfloxacin-d₅ were used as internal standards. After extraction and SPE clean-up, samples were analyzed by using LC-MS/MS in positive ion mode. We observed that the deuterium-labeled internal standards have slightly different LC retention time from their native analogues, which reduces the benefits of using isotope dilution techniques as ion suppression/enhancement effects caused by co-eluting matrix interferences are not completely compensated. In this study, LC conditions were optimized to minimize matrix effects causing different ionization efficiency between the target analytes and their isotope analogues by separating them from significant matrix interferences. The analytical method was validated by measuring samples (chicken breast, bovine muscle, and porcine muscle) gravimetrically fortified in various levels with the target analytes. The method provided accurate analytical results of the target analytes in the range of 5-50 μg/kg with the relative expanded uncertainty of 1-5%.