A mechanistic study of electrospray mass spectrometry: charge gradients within electrospray droplets and their influence on ion response.

A model has been developed to account qualitatively for the effects of ion pairing, surface activity, and electrophoretic mobility in electrospray mass spectrometry. The model is tested with various salt and amino acid mixtures. The data suggest that the axial charge gradient arising from electrophoretic separation at droplet genesis may persist within the electrosprayed droplets at least until the first droplet fission, accounting for the field dependence of detected ion clustering of quaternary ammonium salts and for the relatively field-invariant charge distribution of horse heart myoglobin samples.