Mass transport effects in electrohydrodynamic mass spectrometry

Abstract

A homologous series of tetraalkylammonium salts was used to characterize the sampling behavior of electrohydrodynamic (EH) mass spectrometry. Relative sampling efficiencies increased with increasing ion size up to the tetrabutylammonium ion for glycerol solutions, reflecting expected decreases in the strength of ion—solvent interactions with increasing ion size. An unexpectedly low sensitivity for tetrapentylammonium did not result from ion pairing or surface tension effects. The anomaly was more pronounced when LiCl was used as supporting electrolyte in glycerol (increasing viscosity) and was not observed when sampling from ethylene glycol. These effects are consistent with mass transport control of ion concentrations in the sampling region, introducing a diffusion coefficient-based bias. Studies with the tetramethylammonium ion showed systematic decreases in signal intensity with time, characteristic of convection—diffusion control of surface concentrations. A model of convection—diffusion behavior in the EH system was developed. While the model does not explain all aspects of the sampling process, it provides support for the mass transport hypothesis.