A study of the effect of pH, solvent system, cone potential and the addition of crown ethers on the formation of the monensin protonated parent ion in electrospray mass spectrometry

A systematic investigation has been performed on the effect of pH, cone potential, solvent system and the influence of crown ethers on the relative intensity of the monensin protonated parent ion [M+H](+) in electrospray mass spectrometry (ESI-MS). The analyses were performed on either a triple quadrupole (Quattro-LC) or a quadrupole time-of-flight (Q-tof) mass spectrometer. The pH variation showed that it is possible to detect the protonated ion at low pH. An increase in the cone potential resulted in an increase of the observed signal and the best signal-to-noise was observed at 120 V. The addition of 15-crown-5 and 18-crown-6 ethers to the solution led to an increase of the protonated ion intensity through successful competition of the crown ether for the alkali metal cation. The 15-crown-5 ether, though, showed a distinct disadvantage over the 18-crown-6 ether due to retention of its sodium adduct in the source and detector of the instrument. Acetonitrile-water proved to be the best solvent system for these studies. The best conditions for the 18-crown-6 ether were then applied to an analysis on a Fourier-transform ion cyclotron resonance (BioApex II) mass spectrometer which clearly demonstrated the production of the protonated ion.