Development of high-pressure probe electrospray ionization for aqueous solution

Abstract

Probe electrospray ionization (PESI) is a recently developed method that uses a sharp solid needle as electrospray emitter and the sample is loaded to the needle tip by repetitive movement of the needle probe. This method has been previously used for the analysis of sample with high salt concentration and real-world samples without sample pretreatment. Although PESI is also applicable to aqueous solution, the ion signal stability and reproducibility were not satisfactory due to the spontaneous occurrence of corona discharge taking place on the metallic needle tip. In the previous version of PESI, the ionization was performed under an open environment, thus ambient condition such as humidity and the presence of environmental contaminant also affected the performance of PESI. In this paper, we report a modified version of PESI in which the electrospray and the desolvation of charged droplets are performed inside an enclosed chamber which is pressurized to >1 atm with dry air. Under a super atmospheric condition, the dielectric strength of the ambient gas was enhanced and stable electrospray could to be initiated without the occurrence of corona discharge even for liquid sample with high surface tension such as pure water. A comparative study of atmospheric-pressure PESI and high-pressure PESI (HP-PESI) had been performed to demonstrate the improvement of PESI in sensitivity and signal stability for the detection analytes in aqueous solution. HP-PESI offers a higher sensitivity and signal stability over PESI due to the absence of gaseous breakdown, better desolvation, and higher ion sampling rate by the mass spectrometer.