Selected reaction monitoring (SRM) of quinolone drugs showed1 different sensitivities in aqueous solution vs. biological extract. The authors suggested formation of two singly protonated ions with different behavior, one undergoing loss of H2O and the other loss of CO2, so that SRM transitions might depend on the ratios of these forms generated by the electrospray. These surprising results prompted us to reexamine several quinolone drugs and some simpler compounds to further elucidate the mechanisms. We find that the relative contributions of loss of H2O vs. loss of CO2 in MS/MS experiments depend not only on molecular structure and collision energy but, in certain cases, on the cone voltage as well. We further find that many fragment ions formed by loss of H2O can attach a water molecule in the collision cell, whereas ions formed by loss of CO2 do not. Since re-attachment of H2O can happen in the cone region as well as in the collision cell, this effect leads to the dependence of MS/MS spectra on the cone voltage used in creating the precursor ion, which explains the formerly observed effect on SRM ratios. Our results support the earlier conclusion that varying amounts of two ions of the same m/z are responsible for problems in the analysis of these drugs, but the origin is in dehydration/rehydration reactions. Thus, SRM's for certain complex compounds may be comparable only when monitored under identical conditions, including the voltage used in the production of the protonated ions in the ESI source.
Citation: Rapid Communications in Mass Spectrometry
Pub Type: Journals
mass spectrometry, norfloxacin, difloxacin, quinolone drugs, decarboxylation, dehydration