Development of direct-current, atmospheric-pressure, glow discharges generated in contact with flowing electrolyte solutions for elemental analysis by optical emission spectrometry

Direct-current, atmospheric-pressure, glow discharge (dc-APGD) generated in contact with flowing sample solutions is a new, very promising excitation source for analytical optical emission spectrometry, due to its low maintenance requirements and its analytical performance. Since analyzed solutions act as liquid cathodes, this discharge system is useful for the direct determination of elements dissolved in solutions without having to produce their aerosols by pneumatic nebulization. The review presents different systems and designs of dc-APGD generated with the liquid cathode applied to spectrochemical analysis in the past 20 years. We discuss the effect of experimental conditions on analytical response and performance of the discharge. We include analytical figures of merit obtained with different discharge systems and their application to the analysis of environmental and biological samples containing various trace elements.

Sensitive Determination of Cd in Small-Volume Samples by Miniaturized Liquid Drop Anode Atmospheric Pressure Glow Discharge Optical Emission Spectrometry

A novel liquid drop anode (LDA) direct current atmospheric pressure glow discharge (dc-APGD) system was applied for direct determination of Cd in liquid microsamples (50 μL) by optical emission spectrometry (OES). The microdischarge was generated in open-to-air atmosphere between a solid pin type tungsten cathode and a liquid drop placed on a graphite disk anode. The arrangement of the graphite disk placed on a PTFE chip platform as well as the solid pin type cathode was simple and robust. The limit of detection (LOD) of Cd for the developed LDA-APGD-OES method was 0.20–0.40 μg L^–1, while precision (as the relative standard deviation for the repeated measurements) was within 2–5%. By using the liquid drop of 50 μL, the linearity range of 1–1000 μg L^–1 was achieved. The effect of addition of the low-molecular weight (LMW) organic compounds, easily ionized elements (EIEs), i.e., Ca, K, Mg, and Na, as well as the foreign ions (Al, Cu, Fe, Mn, Zn) to the solution on the <i>in situ</i> atomization and excitation processes occurred during operation of the LDA-APGD system, and the response of Cd was studied. Validation of the proposed method was demonstrated by analysis of <i>Lobster hepatopancreas</i> (TORT-2), pig kidney (ERM-BB186), and groundwater (ERM-CA615) certified reference materials (CRMs) and recoveries of Cd from water samples spiked with 25 μg L^–1 of Cd. Very good agreement between the found and certified values of Cd in the CRMs (the recoveries were within the range of 96.3–99.6%) indicated trueness of the method and its reliability for determination of traces of Cd. In the case of the spiked water samples, the recoveries obtained were in the range from 95.2 to 99.5%