Solvent effects on surface activity of aggregate ions in electrospray ionization

Abstract

A principal feature of electrospray ionization (ESI) is the transfer of ions in solution into the gas-phase for analysis by mass spectrometry. The electrospray process is intricate and therefore each stage of the process must be well-characterized in order to optimize the quality of the data obtained. The surface activity of a given ion is a substantial factor in its likelihood of evaporating from droplets formed by the electrospray, and leads to a differential response of one ion over another. Consequently, investigation of the response of a variety of ions in multiple solvents lends insight toward both desolvation processes and the surface activity of the ions studied in the chosen solvent. In the present work, a cationic ionic liquid, butyl methylimidazolium (BMIM), was paired with a counterion and mixed in various solvents. Subsequently, BMIM paired with a different counterion was added to the solution and analyzed by ESI mass spectrometry to determine the relative response ratio between two observable aggregates. The findings assist in the elucidation of differential surface activity of chemically distinct ions in ESI, with respect to changes in solvent. Furthermore, the results obtained suggest acetonitrile is an optimal solvent for the analysis of ions of this type due to a reduction in differential effects, whereas other common ESI solvents prove to enhance the surface activity of specific aggregate ions.