| Summary: | 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.
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