Improved compatibility of liquid chromatography with electrospray tandem mass spectrometry for tracing occurrence of barbital homologous residues in animal tissues

This work describes a liquid chromatography-tandem mass spectrometry (LC-MS/MS) procedure for multiplex screening, ultratrace quantification and reliable confirmation of barbital series residues in animal-derived food matrices. The method is developed based on a distinct dependency of the electrospray ionization (ESI) response of nine structural homologues on LC eluent properties and gas-phase ion chemistry during the ESI process. The “wrong-way-round” negative ionization aspect has been explored to optimize the compatibility of the hyphenated LC-MS/MS technique, which facilitates detection limits at 30-100-fold lower than 0.01 ppm without derivatization or post-column basification step. A mobile phase using methanol modified with 0.01% acetic acid is adopted to achieve an approximately 2-9-fold increase in signal-to-noise ratio over the results under suboptimal conditions. There is no significant differential matrix effects or deuterium isotope effects on chromatographic retention and ESI responsiveness at all levels across the different analyte-matrix pairs. Mean recoveries ranged from 79.6% (barbital) to 108% (secobarbital) at fortified levels of 0.5-20 ng/g within relative standard deviations less than 11%. Between-run repeatability and within-laboratory reproducibility were 3-11% and 5-13%, respectively. An ion ratio criterion for valid detection limit data for simultaneous screening of homologous multiresidues in complex sample matrices is proposed. The satisfactory applicability of the newly described procedure to 43 real samples including pork, poultry meat, swine liver, fish tissue and shrimp muscle demonstrated the LC-MS/MS technique with facile sample handling can serve as an attractive alternative analytical method accepted for regulatory purpose.