Evaluation of the operating parameters of the liquid sampling-atmospheric pressure glow discharge (LS-APGD) ionization source for elemental mass spectrometry

Abstract

The liquid sampling-atmospheric pressure glow discharge (LS-APGD) has been assessed as an ionization source for elemental analysis with an interdependent, parametric evaluation regarding sheath/cooling gas flow rate, discharge current, liquid flow rate, and the distance between the plasma and the sampling cone of the mass spectrometer. In order to better understand plasma processes (and different from previous reports), no form of collision/reaction processing was performed to remove molecular interferents. The evaluation was performed employing five test elements: cesium, silver, lead, lanthanum and nickel (10(-4) mol L(-1) in 1 mol L(-1) HNO3). The intensity of the atomic ions, levels of spectral background, the signal-to-background ratios, and the atomic-to-oxide/hydroxide adduct ratios were monitored in order to obtain fundamental understanding with regards to not only how each parameter effects the performance of this LS-APGD source, but also the inter-parametric effects. The results indicate that the discharge current and the liquid sampling flow rates are the key aspects that control the spectral composition. A compromise set of operating conditions was determined: sheath gas flow rate = 0.9 L min(-1), discharge current = 10 mA, solution flow rate = 10 μL min(-1), and sampling distance = 1 cm. Limits of detection (LODs) were calculated using the SBR-RSDB (signal-to-background ratio/relative standard deviation of the background) approach under the optimized condition. The LODs for the test elementals ranged from 15 to 400 ng mL(-1) for 10 μL injections, with absolute mass values from 0.2 to 4 ng.

Preliminary Figures of Merit for Isotope Ratio Measurements: The Liquid Sampling-Atmospheric Pressure Glow Discharge Microplasma Ionization Source Coupled to an Orbitrap Mass Analyzer

“…What changes appreciably as the current increases, though, is the ability to vaporize the eluate and desolvate (decluster) hydrated analyte ions. This set of trends is entirely consistent with the more detailed parametric evaluation of the microplasma on the Paul-trap instrument [18]. To this end, as seen in the previous sections, the 235 U/…”

“…Previous LS-APGD efforts on this Orbitrap instrument and on the Thermo Scientific LCQ Advantage Max at Clemson University resulted in similar dependencies relative to the plasma operation conditions of solution feed rate, discharge current, and ion sampling distance [18]. Those works dealt specifically with the responses of atomic ions.…”

“…As suggested above, under the supposition that the oxides are formed in the mass analyzer, the initial plasma parameters were the same as those found for atomic ion analysis; more atomic ions lead to greater amounts of oxides. Ultimately, the LS-APGD was run at the following conditions throughout this effort: electrolyte (sample) flow = 30 μL min , and a discharge current = 30 mA, equivalent to the previous works [17,18]. The parameters that were varied and optimized on the Orbitrap were the Fourier transform (FT) digitization window, the ion accumulation time prior to injection into the Orbitrap (termed injection time in the system software), the number of averaged micro scans that are averaged, and the number of scans making up the spectral data.…”

“…The basic components of the LS-APGD ion source have been described previously and are shown in Figure 1 [16][17][18]. The ion source consists of a anode that is made of a metal electrode (SS, weldable feedthrough, MDC Vacuum Products,LLC, Hayward California) that has a potential applied to it from a Glassman (High Bridge, NJ) Model EH power supply (0-100 mA, 0-1 kV) and a cathode.…”

“…The normal operating parameters of the LS-APGD are gas flow rates of ≤1 L min -1 , liquid flow rates of 5-40 μL min -1 , and power consumption of only 50 W. Further, the LS-APGD is run in total consumption mode when operated correctly, which reduces waste streams to zero. Prior work out of the Marcus group using the LS-APGD for mass spectrometry shows promise as a miniature ion source [16][17][18]. While other elemental/atomic ion sources are not suitable to analyze molecular species due to excessive thermal dissociation, the LS-APGD source has also been shown to be a viable ion source for organics by simple changing the electrolytic solution makeup [19].…”