The effect of electrospray solvent composition on desorption electrospray ionisation (DESI) efficiency and spatial resolution

Abstract

In desorption electrospray ionisation (DESI) the interaction between the electrospray and the surface is key to two important analytical parameters, the spatial resolution and the sensitivity. We evaluate the effect of the electrospray solvent type, organic solvent fraction with water, analyte solubility and substrate wettability on DESI erosion diameter and material transferral into useful ion signal. To do this five amino acids, glycine, alanine, valine, leucine and phenylalanine are prepared as thin films on three substrates, UV/ozone treated glass, glass and polytetrafluoroethylene (PTFE). Four different solvents, acetonitrile (ACN), methanol (MeOH), ethanol (EtOH) and propan-2-ol (IPA), are used with organic solvent fractions with water varying from 0.1 to 1. These model systems allow the solubility or wettability to be kept constant as other parameters are varied. Additionally, comparison with electrospray ionisation (ESI) allows effects of ionisation efficiency to be determined. It is shown that the DESI efficiency is linearly dependent on the solubility (for these materials at least) and for analytes with solubilities below 1.5 g kg(-1), additional strategies may be required for DESI to be effective. We show that the DESI erosion diameter improves linearly with organic solvent fraction, with an organic solvent fraction of 0.9 instead of 0.5 leading to a 2 fold improvement. Furthermore, this leads to a 35 fold increase in DESI efficiency, defined as the molecular ion yield per unit area. It is shown that these improvements correlate with smaller droplet sizes rather than surface wetting or ionisation.

Deconstructing Desorption Electrospray Ionization: Independent Optimization of Desorption and Ionization by Spray Desorption Collection

“…Increasing the distance between the sprayer and the impact zone results in smaller droplets impacting the surface, which has been shown to increase the efficiency by which ions are transferred from the surface to the mass spectrometer inlet [41]. This transfer efficiency appears to increase over the entire range of sprayer-to-impact zone distances that was investigated in the current study.…”

Imaging of Biological Tissues by Desorption Electrospray Ionization Mass Spectrometry

“…Additionally, the solvent selection for DESI is crucial for sensitivity, as analyte transmission and ionization depends on the extraction of the analyte from the surface into the thin film formed. 13 Many electrospray ionization-compatible solvents and mixtures can be used to assist in the desolvation and ionization process depending on the class of compound of interest during the analysis.…”

“…12 DESI sensitivity is strongly dependent upon the solubility of the analyte in the spray solvent, as desorption relies on the localized microextraction. 13 When used with a software-controlled sample stage, the sample is scanned unidirectionally with lane stepping underneath the DESI ionization probe, and through the time domain, m/z information is correlated with the chemical species' spatial distribution (Figure 1 5. If analyzing stored sections, remove slide from freezer, immediately transfer to vacuum desiccator, and allow ~15-20 min to thaw.…”

Commentary: Challenging convention using ambient ionization and direct analysis mass spectrometric techniques

“…This afforded improved MS selectivity on the millisecond timescale using travelling-wave technology. [7,8] By providing a frank assessment of advantages, disadvantages and current knowledge gaps, Professor Scrivens set the tone for open and honest scientific discussion, with a view to exploring these new techniques. When summarising the disclosed techniques, described by over 30 acronyms and abbreviations, it soon became clear that individual ambient ionization MS techniques often display a degree of commonality in both fundamental (mechanistic considerations) and nomenclature (subtle licence of terminology) aspects, where techniques are somewhat similar, as noted previously.…”