New designs and detection strategies for glow discharge as an alternative spectrochemical source

Abstract

Powerful instrumentation exists for the determination of elemental concentrations in a range of samples, but all such instruments are the product of compromises among desirable properties. Sensitivity, accuracy, precision, portability, expense, sample size, speed, and other aspects of analysis are balanced based on the nature of a problem and the available resources. Opportunities exist for new instruments that strike a different balance from existing methods, particularly if these new techniques offer advantages over existing ones. Glow discharge (GD) is a flexible and potent platform from which to develop new spectrochemical methods, including those for elemental analysis. The methods described here seek to improve upon areas where GD falls short of other methods and to exploit the areas where GD excels, particularly speed and cost.

Glow discharge has predominately been used for the analysis of solids, and it offers high sensitivity and depth resolution at a comparatively low cost in this application. However, it has historically lacked lateral (in the plane of the sample surface) resolution. This document describes a new method, making use of pulsed GD and monochromatic imaging, that adds lateral resolution better than 100 µm.

Traditionally, glow discharges have found little use in the analysis of solutions, largely due to their inefficiency in vaporizing the solvent. Two glow discharges, both operated at atmospheric pressure to improve desolvation, are described and characterized here.

The solution-cathode glow discharge (SCGD) is a simplified and improved version of the electrolyte-cathode glow discharge. A high voltage is applied across a gap between a sample solution and an electrode, forming a plasma that directly samples the solution and produces atomic emission that is used to identify and quantify the solution's elemental constituents. Several studies on the SCGD are described, including characterization and improvement of the source.

The annular glow discharge (AGD) is another glow discharge for the analysis of solutions. Unlike the SCGD, it is operated between two metal electrodes and the solution does not form a fundamental part of the plasma, which allows greater flexibility in solution composition. Both methods offer competitive analytical performance at low cost and with the potential for portability.