Development of an atmospheric-pressure solution-cathode glow discharge as an ionization source for mass spectrometry

Abstract

Atmospheric-pressure ionization sources have seen increased use in the field of mass spectrometry (MS) in recent years due to their relative simplicity and ability to ionize a variety of analytes found in solid, liquid, or gaseous samples. Currently, however the type of sample that can be analyzed and the ions formed for specific analytes within the sample is still dependent on the type of ionization source used. What is needed is a more versatile ionization source capable of ionizing a range of analytes, from elemental to biomolecular, with the ability to garner both quantitative and qualitative information without the need for additional complementary methods. Recently, interest has turned to sources designed for other analytical techniques, including liquid-electrode glow discharges, as feasible, versatile ionization sources for mass spectrometry. The solution-cathode glow discharge (SCGD) was originally developed as an inexpensive, low-power alternative to inductively coupled plasma (ICP) for optical emission spectroscopy (OES) and was found to have comparable analytical figures of merit. Because of the performance of the SCGD for OES and the fact that other plasma sources have been used for MS for many years, preliminary studies of the SCGD were conducted to determine the capabilities as an atmospheric-pressure ionization source, first for elemental MS and then towards ionization for small molecular species and biopolymers.

Current research focuses on the optimization of the SCGD for small molecule and bimolecular analyses. A parametric evaluation of the source for MS was conducted with a small peptide, angiotensin II, as the model analyte to determine which parameters most affected analyte signal. Similar parametric optimizations were run for small molecules, including pesticides and explosives. Moreover, the SCGD has also been coupled with ultra-high performance liquid chromatography (UHPLC) to further increase the effectiveness SCGD-MS for various biomolecule detection and sequencing. Utilization of UHPLC-SCGD-MS as a coupled method for biomolecular studies, and the controllable fragmentation of the ionization source for primary structure sequencing information were performed. Fundamental studies on the ionization and fragmentation mechanisms within the SCGD source were executed to begin to form an underlying foundation and understanding of the source. These fundamental research endeavors included experiments to increase analyte signal, the effects of addition of reagent molecules to ionization processes, and investigations of internal energy of ions formed within the source. Overall, these studies have advanced knowledge of the SCGD as an ionization source for mass spectrometry and provided a basis for further analysis and investigation of the source for ionization and fragmentation of more complex samples and applications.

The SCGD is a direct current, atmospheric-pressure glow discharge sustained in the open air between a metal anode and a flowing conductive solution, which serves as the cathode. Analytes of interest are introduced into the flowing solution where they are transported into the plasma and ionized before introduction to the mass spectrometer. Our research has found the SCGD is a unique ionization source in that it produces both protonated molecular ions from labile biomolecules and with source parameter changes can yield fragmentation of the same species within the source. The source has also been shown to be capable of producing gaseous ions from elemental species as well as small molecules. As such, a basic understanding of the ionization and fragmentation mechanisms, as well as the source parameters that lead to these conditions, is essential to optimize the system for a range of analytes.