Post-column metal complexation of quinolone antibiotics in a quadrupole ion trap

Post-column addition of a metal salt and an auxiliary chelating ligand via a sheath flow offers an alternative strategy for promoting electrospray ionization of analytes via metal cationization. In the present study, a high-performance liquid chromatography (HPLC) mass spectrometer interface that incorporates a modified ionspray source that allows either online metal cationization or protonation is reported. The analytical utility of metal complexation is evaluated relative to protonation as a means for efficient ionization and generation of structurally diagnostic ions during HPLC separation. To improve metal cationization, an auxiliary chelating ligand is added to the sheath flow to coordinate the metal and stabilize the resulting complexes. The methodology was verified using a series of quinolone antibiotics as model analytes. Relative reaction efficiencies were compared for the protonation vs. metal complexation modes, and it was found that metal complexation with an auxiliary chelating ligand gave a three to five times better detection limit than protonation for the quinolones. Detector linearity and optimal reaction conditions are also reported.