Field induced ion evaporation from liquid surfaces at atmospheric pressure

TLDR

The study aims to interpret ion mass spectra in terms of ion evaporation or field emission and assess the potential of this process for ion research and analytical applications. A mass spectrometer system was built to sample ions directly from an atmospheric‑pressure spray of highly charged water droplets, using an induced electric field to extract ions through a 25 µm orifice, differential pumping, and a quadrupole mass filter. Approximately 30 univalent anions and cations were observed to evaporate, with Ag⁺, Tl⁺, and Cu⁺ as exceptions, and neutral molecules such as urea and acetanilide were found to remain attached to the ions; these results agree with the authors’ prior theory.

Abstract

A mass spectrometer system which is capable of sampling ions directly from an atmospheric pressure source has been constructed and used to investigate ions which are evaporated from the surface of highly charged water droplets. Solutions of electrolytes are sprayed into the air, and a large charge imparted to the droplets by induction; an electric field applied across the plume of evaporating spray extracts small ions, probably already clustered with solvent molecules, which are drawn through a 25 μm orifice into the vacuum chamber. Differential pumping is applied and the ions are focused into a quadrupole mass filter. Approximately 30 univalent anions and cations of various sizes and structures have been tested, and almost all have been observed to evaporate; significant exceptions are Ag+, Tl+, and Cu+. Furthermore, certain molecules such as urea and acetanilide, when present as neutrals in the sprayed water, are observed to remain attached to the ions when these evaporate. The experimental results are discussed: ion mass spectra are interpreted in terms of ion evaporation (or field emission) and ion–molecule reactions in gas phase, and the potential value of the process for ion research and analytical applications is considered. The main features of the experimental results are consistent with the predictions of the theory developed in a previous paper by the authors.