Evolution and evaluation of a jet-assisted glow discharge source for atomic emission spectrometry

• Main Author: Banks, Peter Robert
• Language: English
• Published: 2008
• Online Access: http://hdl.handle.net/2429/2880

Glow discharge atomic emission spectroscopy is a useful analytical method for the direct analysis of conducting solids thereby obviating the need for time-consuming and hazardous dissolution procedures common with other methods. Detection limits for analytical glow discharges, however, are restricted to relatively high analyte concentrations when compared to other methods. One aspect of glow discharge sampling which proves adverse to analytical performance is through significant analyte loss before excitation by the re-deposition of sputtered species back onto the sample surface. Sputtered atoms are typically ejected from the sample surface with a range of energies that extends to 20 eV, however, this ejection energy is quickly thermalized by collisions with support gas species at pressures typically used for analyses. As a consequence, sputtered atoms are readily re-deposited back onto the sample surface, primarily due to diffusion. For a glow discharge using a planar diode electrode geometry, operating at pressures typically used for analytical purposes, up to 95 % of sputtered species re-deposit on the sample surface. Therefore, any method that retards re-deposition would significantly increase the atomization efficiency of glow discharges and increase the sensitivity of the technique. This work addresses the re-deposition problem using a jet assisted source that relies on a directed support gas flow that not only aids sample transport to the excitation region, but impedes re deposition. The original design has gone through a three-stage evolution: each stage correcting certain imbalances found for the previous model which culminates in an emission source capable of sub-ppm level limits of detection and a precision of less than 0.3 % for certain elements. A comprehensive study for the jet flow effects on the sample surface, using Scanning Electron Microscopy and Energy Dispersive X-ray Fluorescence, and the emitting plasma, using atomic emission and absorption spectroscopies, has been conducted. In addition, excitation processes have been studied in the jet-assisted plasma plume as it issues from the anode housing. Results indicate that the dominant atomic excitation process is through electron excitation. The electrons originate from the collision of two argon atoms which reside in metastable states.