Desorption ionization by charge exchange (DICE) for sample analysis under ambient conditions by mass spectrometry

Abstract

An ambient pressure ionization technique for mass spectrometric analysis of substances present on solid surfaces was developed. A nebulized spray containing molecular ions of a solvent such as toluene can be generated by passing the solvent through a stainless steel capillary held at a high voltage. When the stream of charged droplets produced in this way is directed onto a solid surface, the analytes present on the surface are desorbed and ionized by a charge exchange process. This technique was shown to desorb and generate positively charged molecular ions from compounds that are not readily ionized by some other ambient methods, under positive-ion generation mode. For example, intense signals representing radical cations of 1,4-hydroquinone, limonene, thymol, and several other compounds were observed when the analytes were deposited on a metal surface and exposed to a toluene spray nebulized from the metal capillary maintained at a potential of about +5 kV. In contrast, when the same samples were exposed to a spray of water/methanol/formic acid under customary DESI-like (positive-ion mode) conditions, no peaks representing the analytes were observed.

A Combined Desorption Ionization by Charge Exchange (DICE) and Desorption Electrospray Ionization (DESI) Source for Mass Spectrometry

“…T he recently introduced desorption ionization by charge exchange (DICE) technique has been demonstrated to generate molecular ions from many classes of polar and nonpolar compounds under ambient conditions [1]. Upon further research work, we observed that the DICE technique does not produce gaseous metal cation adducts, even though they are commonly generated by other desorption methods.…”

“…The recently introduced DICE technique, on the other hand, has been demonstrated to generate gaseous molecular ions under ambient conditions from many polar and nonpolar compounds that possess ionization energies lower than that of the selected spray reagent [1]. We envisaged that a combination of the DICE and DESI techniques would enable the ionization of a broader range of analytes within a single desorption ionization experiment.…”

“…Presumably, the desorption process involved in the combined technique is a composite of the mechanisms of droplet "pick up" [15,17], thermal desorption [15], and momentum transfer from the heated nebulizer gas stream [15]. We hypothesized previously that the molecular ions of toluene for the DICE technique could be formed in both the solution phase (by an electrochemical oxidation process) and the gas phase (by an electric discharge process) when the nebulized spray reagent is directed onto a surface [1]. Most likely, the ionization of analytes occurs in both the DICE and the DESI methods in the microdroplets after the analytes have been "picked up" [15,17] by the spray reagent.…”

Cluster SIMS of Organic Materials: Theoretical Insights

“…However, the simulations also show that these organic projectiles made of large molecules require higher kinetic energies to overcome the regime of net deposition, an issue which could be avoided with inert projectiles such as noble gas clusters or organic clusters made of smaller, volatile molecules, such as toluene. 89 For this reason, the molecular clusters described in Reference 66 would not appear to be good candidates for molecular depth profiling and 3D analysis of organic materials. Another remarkable result of these simulations is that because of their large momentum, massive organic clusters under normal incidence also induce more molecular mixing in soft materials than fullerenes.…”

Regulated In Situ Generation of Molecular Ions or Protonated Molecules under Atmospheric-Pressure Helium-Plasma-Ionization Mass Spectrometric Conditions

“…Although the formation of molecular ions by charge transfer has been noted under atmospheric conditions [6,[19][20][21][22], ambient MS methods generate primarily protonated molecules, or mixtures of molecular ions and protonated molecules. The outcome appears to be dependent on several factors (e.g., the strength of the discharge field, the possible presence of solvent and other chemical vapors, the nature of the gases used, and the temperature of the source).…”