Spontaneous Charge Separation and Sublimation Processes are Ubiquitous in Nature and in Ionization Processes in Mass Spectrometry

Abstract

Ionization processes have been discovered by which small and large as well as volatile and nonvolatile compounds are converted to gas-phase ions when associated with a matrix and exposed to sub-atmospheric pressure. Here, we discuss experiments further defining these simple and unexpected processes. Charge separation is found to be a common process for small molecule chemicals, solids and liquids, passed through an inlet tube from a higher to a lower pressure region, with and without heat applied. This charge separation process produces positively- and negatively-charged particles with widely different efficiencies depending on the compound and its physical state. Circumstantial evidence is presented suggesting that in the new ionization process, charged particles carry analyte into the gas phase, and desolvation of these particles produce the bare ions similar to electrospray ionization, except that solid particles appear likely to be involved. This mechanistic proposition is in agreement with previous theoretical work related to ion emission from ice. Graphical Abstract ᅟ.

Novel ionization processes for use in mass spectrometry: ‘Squeezing’ nonvolatile analyte ions from crystals and droplets

“…The ease of producing charge separation was demonstrated using an inlet tube designed to detect current on the tube and on a downstream detector. Almost any compound passed through the heated inlet tube from atmospheric pressure to vacuum produced charge separation . However, at room temperature, the current measured was significantly less than at higher temperature for most compounds tested.…”

“…Thus, matrices capable of producing ions at ambient temperature are no more likely to contaminate the mass spectrometer ion optics than solvents used with ESI or APCI. Many more matrix compounds assist in analyte ionization using an inlet tube that is heated . Mixing a solution of an analyte with a solution of a MAI matrix, with or without drying before exposure of the sample to the vacuum of a mass spectrometer, has been shown to produce ions of the analyte with charge states similar to ESI .…”

“…In the case of a liquid being introduced into a hot or cooled inlet tube there are a number of charge separation possibilities including shear forces from the flow of air through the tube providing charge separation similar to SSI, frictional/triboelectric charging, and charging by removal of a surface layer possessing a different charge from the bulk solution through bubble formation, as examples. Triboelectric charging occurs with solids, and no doubt accounts for at least some of the charging measured when powders of compounds are passed through an inlet tube and the charges measured on the inlet tube as well as a downstream detector plate . Particle size and roughness would account for differences in measured currents as well as the positions of the compounds studied on a triboelectric series relative to the stainless steel tube, or surface material, through which the particles pass.…”

“…The difficulty in observing discrete mass loss events with a quartz crystal microbalance or by MS measurements might be explained by small particles which do not produce a signal above the noise in the experiment, or more likely by ejection of a continuous stream of charged particles, similar to the charged droplets from ESI . Under these conditions, a continuous weight gain, or loss, is expected and observed . How these gas‐phase particles are produced remains speculation.…”

“…However, the nonvolatility of these MALDI matrix compounds is expected to make the process of removing the matrix from the charged clusters difficult . Less volatile matrices also have the drawback that they will collect on instrument surfaces contaminating the instrument, while matrices that spontaneously sublime in the vacuum will be pumped away similar to volatile solvents …”