Investigation of spectrochemical matrix effects in the liquid sampling-atmospheric pressure glow discharge source

Abstract

The liquid sampling-atmospheric pressure glow discharge (LS-APGD) microplasma was evaluated with regards to its spectrochemical robustness in its application as a miniaturized optical emission spectroscopy (OES) source for liquid samples. The susceptibility to perturbations in excitation/ionization conditions was probed across a wide range test species, including transition metals, easily ionized elements (group I), and elements with low second ionization potentials (group II). Spectrochemical metrics included the plasma excitation temperature (T exc), ionization temperatures (T ion), and magnesium (Mg) ionic:atomic (Mg II:Mg I) ratios. The introduction of the 11 different matrix elements into the LS-APGD at concentrations of 500 g mL-1 yielded no significant changes in the optically-determined plasma characteristics, indicating a relative immunity to spectrochemical matrix effects. T exe values for the plasma, using He I as the spectrometric species averaged 2769 ± 79 K across the test matrix, with Mg-based ionization temperature values centered at 6665 ± 151 K. Typical Mg II:Mg I ratios (the so-called robustness parameter) were 0.95 ± 0.3. The lack of appreciable perturbation in excitation/ionization conditions observed here is also manifested in virtually no changes in the probe Mg II and I species' intensities, even at matrix loadings of up 1000 g mL-1 of Ba. These observations indicate that the LS-APGD could serve as an OES source for the analysis of diverse aqueous samples without appreciable spectroscopic matrix effects, though potential physical matrix effects including vaporization effects must be evaluated.

Temporal characterization of fundamental plasma parameters in pulsed liquid electrode plasma (LEP) optical emission spectrometry

“…The 366 active LEP power is comparable to other solution-based microplasmas, for example, the SCGD 367 and the LS-APGD. Typical operating powers for the SCGD and LS-APGD are in the range of 368 60-80 W [35][36][37] and < 50 W [27,38], respectively. 369…”

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.…”

“…24,25,37,40,41 Previous works with the LS-APGD-OES source have yielded fairly low Mg II : Mg I ratios of $1, but perhaps surprisingly showing a good deal of immunity towards additions of easily ionized elements (EIEs) and laser ablation-generated aerosols. 12,13,21 While the above-cited Mg line pair is very convenient to implement, ion : atom transitions of other elements have been employed. Of relevance here, Chan et al 42 and Chan and Hieje 24 have used the Zn II : Zn I (206.2 nm : 213.9 nm) pair for investigating ICP matrix effects, with values for this ratio occurring in the general range of 0.9-1.0.…”

“…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.…”

“…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.…”