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 pot ential, solvent system and the influence of crown ethers on the relative in tensity of the monensin protonated parent ion [M+H](+) in electrospray mass spectrometry (ESI-MS). The analyses were performed on either a triple quad rupole (Quattro-LC) or a quadrupole time-of-flight (Q-tof) mass spectromete r. 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 th e observed signal and the best signal-to-noise was observed at 120 V. The a ddition of 15-crown-5 and 18-crown-6 ethers to the solution led to an incre ase of the protonated ion intensity through successful competition of the c rown ether for the alkali metal cation. The 15-crown-5 ether, though, showe d 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-w ater proved to be the best solvent system for these studies. The best condi tions for the 18-crown-6 ether were then applied to an analysis on a Fourie r-transform ion cyclotron resonance (BioApex II) mass spectrometer which cl early demonstrated the production of the protonated ion.