Investigations into glow discharge plasmas as atomization and excitation sources for analytical atomic spectrometry

Two complementary atomic spectrometric sources utilizing glow discharges for analyte atomization/excitation were investigated to better understand their fundamental operation so that improvements in their design and operation might result. The first source, a glow discharge atomizer operating in argon at low (1-20 torr) pressure, was characterized for the direct analysis of solid samples by cathodic sputtering. Changes in atomic absorbance and emission signals were measured as atomizer pressure, argon flow rate and discharge power were varied to determine an optimal operational regime for analytical measurements. Self-absorption and self-reversal of commonly-used resonance analytical lines was problematic at all flow rates and degraded performance began in emission at flow rates in excess of 0.5 L min;Line profiles and gas kinetic temperatures were then measured for commonly-used analytical transitions for this glow discharge atomizer and for commercial hollow cathode lamps. A strong similarity in gas kinetic temperatures for both of these sources suggested that analytical calibration curves constructed using these lines will exhibit deviation from linearity earlier than when thermal atomization sources are used.;The second source studied was an atmospheric pressure glow discharge formed in a graphite furnace, termed Furnace Atomization Plasma Emission Spectrometry (FAPES). In FAPES, a helium plasma is formed around a 1-mm diameter graphite rod (which is coaxially centred within a graphite tube) to which radio-frequency (rf) power is applied. The graphite furnace is used to thermally desorb analytes from its surface into the plasma where they are excited, enabling simultaneous multielement analysis with small-volume liquid samples. Two-dimensional imaging of this source with a charge-coupled device (CCD) camera revealed the existence of a second region of plasma close to the tube wall. The centre electrode acts as an efficient site for condensation and secondary revaporization of analyte atoms. Altering the dc bias potential of the centre electrode changed the radial distribution of analytes, suggesting that the rf voltages developed are an order of magnitude lower than previously believed. Easily-ionized elements (EIEs) suppressed the radial emission distribution of Cu and Ag probably because of more efficient loss of photons from the plasma through EIE excitation/ionization.