Development of an integrated, single electrode liquid sampling – atmospheric pressure glow discharge microplasma ionization source

Abstract

The liquid sampling – atmospheric pressure glow discharge (LS-APGD) has been demonstrated as a versatile source for optical emission spectroscopy (OES) and mass spectrometry (MS). The initial LS-APGD design consisted of a solution cathode and a stainless-steel counter electrode. While shown to perform well across various MS applications, efforts to improve upon the initial design of this system have been ongoing. In this study, an integrated, single electrode design is presented in which the plasma is generated between the solution electrode and the sampling orifice of the mass spectrometer. The driving force, in this case, is the reduction of system components and moving parts, thus simplifying the ionization source. This preliminary study includes an optimization of the operating parameters with a focus on maintaining a stable plasma and improving analytical performance. The multi-parametric evaluation completed here included discharge current, solution flow rate, and the electrode/sampling orifice gap. Most significantly, the manner in which the plasma is powered has changed from the solution cathode being held at ground potential to being powered negatively versus the grounded sampling orifice. Comparisons between the formation of solvent related species in the original source design and this version were performed. With optimal conditions determined, initial LODs were established for Rb, Ag, Tl, and U, with values ranging from 0.02–0.17 ng mL−1 for 20 μL injections, demonstrating increased sensitivity (72-1900×) relative to the previous two-electrode design on the same MS system which employed 50 μL injections.