Liquid Sampling–Atmospheric Pressure Glow Discharge as a Secondary Excitation Source for Laser Ablation-Generated Aerosols: Parametric Dependence and Robustness to Particle Loading

Abstract

Liquid sampling-atmospheric pressure glow discharge (LS-APGD) microplasma is being developed as a secondary vaporization-excitation source for the optical emission analysis of laser ablation (LA)-generated particle populations. The practicalities of this coupling are evaluated by determining the influence of source parameters on the emission response and the plasma's robustness upon LA introduction of easily ionized elements (EIEs). The influence of discharge current (45-70 mA), LA carrier gas flow rate (0.1-0.8 L min(-1)), and electrode separation distance (0.5-3.5 mm) was studied by measuring Cu emission lines after ablation of a brass sample. Best emission responses were observed for high-discharge currents, low He carrier gas flow rates, and relatively small (<1.5 mm) electrode gaps. Plasma robustness and spectroscopic matrix effects were studied by monitoring Mg(II) : Mg(I) intensity ratios and N2-derived plasma rotational temperatures after the ablation of Sr- and Ca-containing pellets. Plasma robustness investigations showed that the plasma is not appreciably affected by the particle loadings, with the microplasma being slightly more ionizing in the case of Ca introduction. In neither case did the concentration of the concomitant element change the robustness values, implying a high level of robustness. Introduction of the LA particles results in slight increases in the rotational temperatures (∼10% relative), with Ca-containing particles having a greater effect than Sr-containing particles. The observed variation of 9% in the plasma rotational temperature is in the same order of magnitude as the short-term reproducibility determined by the proposed LA-LS-APGD system. The determined rotational temperatures ranged from 1047 to 1212 K upon introducing various amounts of Ca and Sr. The relative immunity to LA particle-induced matrix effects is attributed to the relatively long residence times and high power densities (>10 W mm(-3)) of the LS-APGD microplasma.

Liquid sampling-atmospheric pressure glow discharge excitation of atomic and ionic species

“…Generally, the LS-APGD physical set-up can be easily modied to meet the needs for analyzing different aggregation states (liquid, solid), and for gaining elemental, isotopic, and/or molecular information by coupling the source either to an optical emission or mass spectrometer. [11][12][13][14][15][16][17][18][19][20][21] Here, the LS-APGD microplasma source was sustained between the electrolytic test solution (500 mg g À1 Cu and Zn in 2% HNO 3 (both single element standards CPI International, Santa Rosa, CA, USA)) and a hollow stainless steel counter electrode (316 SS, 1.2 mm i.d., 1.6 mm o.d., IDEX Health and Science, Oak Harbor, WA, USA). The use of such high test solution concentrations, versus previously determined single-mg mL À1 detection limits, 14 allows reasonable emission response levels under non-ideal conditions across the parameter space.…”

“…rotational and excitation temperatures, electron number densities, and plasma robustness), 12 and parametric dependences on particle loading. 21 Photographic images of the LS-APGD-OES set-up are depicted in Fig. 1.…”

“…Up to now, the LS-APGD has proven useful as excitation/ionization source not only for liquid sample analyses with optical emission spectroscopy (OES) [11][12][13][14][15] and mass spectrometry (MS), [16][17][18] but also for the analysis of laser ablation (LA)-generated aerosol particles. 12,[19][20][21] The microplasma has also been employed for the analysis of molecular species when used as ambient desorption/ionization (ADI) source. 22 Overall, the LS-APGD offers a small footprint (components mounted on a 25 cm  30 cm optical breadboard) and is operated with low power (<100 W), low gas (<1 L min À1 ) and solution ow rates (<300 mL min À1 ).…”

“…However, a pre-requisite for the LS-APGD's in-eld application is the fundamental understanding of the source's characteristics for elemental analysis and a full assessment of the LS-APGD's merits, limitations, and usefulness as an excitation/ ionization source. Therefore, recent studies have focused on the investigation of inter-parametric dependencies (discharge current, interelectrode gap and He sheath gas ow rate) on plasma temperature characteristics 12 (gas-rotational, excitation and ionization temperatures) and on plasma robustness upon the introduction of matrix elements in case of both liquid 13,23 and solid (i.e., laser ablation) 21 sampling. While these studies, employing an optical emission spectrometer (OES) for the analysis, provided great insight into the plasma physics of the LS-APGD, more in-depth knowledge regarding the excitation/ ionization conditions is required in order to gain a more complete understanding of the source's mechanisms at an atomic level.…”

“…Considering OES analyses, the inuence of plasma operating conditions (three parameters) on Cu atomic emission response following laser ablation sampling was so far independently investigated by looking at only one parameter at a time. 20,21 However, an inter-parametric approach is considered as crucial for gaining a deeper understanding of the actual driving forces for both atomic and ionic emission. For example, optimizing the LS-APGD for a less pronounced spectral background emission could simplify the spectra, reducing spectral interferences.…”