On the stability of complex ions in ionic liquid Ion sources

Abstract

Ionic liquids are molten salts at room temperature that consist of positive and negative ions. These liquids can be used in electrosprays to produce ion beams. Ionic liquid ion source (ILIS) beams typically consist of single ions and solvated ions. It has been observed that solvated ions are not always stable and can break up into lighter ions midflight. Past experiments show that the stability of solvated ions depends on the molecular composition of the ionic liquid. Based on these results, it has been hypothesized that the stability of solvated ions increases with increasing molecular complexity of the ions. The focus of this work is to test this hypothesis by characterizing ionic liquids of different molecular complexities under controlled conditions. A time of flight mass spectrometer and a retarding potential analyzer were developed specifically for this purpose. The ion beam composition and energy distribution were measured at various temperatures and source voltages for each ionic liquid. With some exceptions, the observed trend was in agreement with the results of past experiments and with the hypothesis. The exceptions to the expected trend may have resulted from the limitations of the detectors. The data from this work can be used to test the hypothesis with moderate confidence. Future study requires improvements to the detectors, namely the retarding potential analyzer, so that the hypothesis can be evaluated more conclusively.