Evaluation of helium-argon mixed gas plasmas for bulk and depth-resolved analyses by radiofrequency glow discharge atomic emission spectroscopy

Abstract

Studies were performed to determine the practical benefits of mixed discharge gases (Ar and He) in the bulk and depth-resolved analysis of solids using a radiofrequency glow discharge atomic emission source. This study examined the characteristics of analyte emission intensity and yield, sample sputter rate and crater shape as a function of added He gas, for both conductive and non-conductive sample matrices. In comparison with pure Ar plasmas, the addition of He does not ultimately improve the limits of detection in the bulk analysis of conductive (metallic) solid samples. However, non-conductive sample matrices such as glasses, which are greatly affected by discharge gas pressure to maintain sputtering conditions (crater shape, sputter rate), may benefit from the addition of He to the plasma gas as a means of enhancing the excitation conditions. The shape of the sputtered craters was minimally affected by the addition of He, indicating that Ar partial pressure is the parameter that most critically affects sputtering characteristics. Overall, the addition of He to the discharge plasma has been found to enhance analyte emission intensity without significantly influencing sputtering characteristics. In terms of depth-resolved analyses, optimized discharge conditions (specifically pressure) derived for the pure Ar case, which are sometimes accompanied by a loss of analytical sensitivity, can be augmented by the addition of He to the discharge to yield improved analytical responses while retaining the desired sputtering characteristics.