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

Abstract

The liquid sampling-atmospheric pressure glow discharge (LS-APGD) was characterized with respect to the effects of interrelated operational source parameters on the excitation of atomic (I) and ionic (II) states for expanding the fundamental understanding of this microplasma's characteristics as an excitation source for optical emission spectroscopy (OES) analyses. Parameters that were investigated for identifying the key driving forces for atomic and ionic excitation conditions were discharge current, interelectrode gap, and He sheath and counter gas flows. The addition of the He counter gas flow allowed assessment of the additional parameter relevant when aerosol samples are introduced following laser ablation sampling of solid matrices. The introduction of the analytes (500 μg g⁻¹ copper and zinc in 2% HNO₃) in liquid form through the solution capillary permitted the investigation of source parameter effects, without introducing additional influences from solid sampling such as heterogeneous particle populations. Individual driving forces for excitation/ionization conditions and inter-parametric dependencies were assessed by changing the operating conditions according to a design of experiment (DOE) plan and monitoring Zn and Cu atomic and ionic emission lines (Zn I 213.9 nm, Cu I 324.7 nm, Zn I 481.1 nm, and Zn II 202.5 nm). Pareto plots of standardized effects were used for evaluating levels of significance as well as magnitudes of both individual and inter-active parametric effects on emission responses, background emissions and signal-to-background ratios as well as the LS-APGD's tolerance against changes in excitation conditions (i.e. robustness). The results indicate that parameter settings leading to high plasma power density are the key driving forces for enhanced analyte emission, with the inter-electrode distances showing the most pronounced influences for the investigated parameter space.