Factors Affecting the Ionization Efficiency of Quaternary Ammonium Compounds in Electrospray-Ionspray Mass Spectrometry

A series of quaternary ammonium salts was analysed by the ionspray technique under different conditions in order to study the influence of some important factors on the ionization efficiency. The factors taken into account were droplet charging, sample ion, counter ion, solvent composition, and concentration. Measurement of the spray current leaving the nebulizer has shown that the spray current increases linearly with sample concentration up to at least 50pmol/pL. The measured spray current is approximately 30% of the calculated current corresponding to a given concentration of quaternary ammonium ions in solution leaving the nebulizer at a given flow rate. The results obtained show that tetraalkylammonium salts give the best sensitivity with no influence of sample-ion shape or counter ion. The introduction of other functional groups in the quaternary ammonium ion decreases the sensitivity. The dynamic range for full-scan mass spectra of the compounds studied appears to be linear for concentrations from 0.1 prnoVpL M) up to 10 pmol/pL M).