Abstract
The spatial distribution of background species and analyte in a He FAPES source have been determined using a charge-coupled device (CCD) imaging system. Imaging of the He 667.82 nm support gas line reveals the presence of two distinct plasmas: an intense luminous zone located around the centre electrode and a diffuse plasma near the tube walls. In a graphite tube at room temperature, the plasma system exhibits a significant dependence on the d.c. bias of the centre electrode for a constant forward r.f. power of 50 W. Polyatomic background species exhibit similar overall distributions and response to changes in the centre electrode d.c. bias. The ingress of atmospheric nitrogen into the plasma can be indirectly detected at elevated tube wall temperatures. Images obtained for Ag atomized into 30 W and 50 W plasmas point out the role of the centre electrode as a primary condensation and secondary re-volatilization site. Condensation and re-evaporation is alleviated when Ag is atomized into a 70 W plasma as the result of plasma-induced heating of the centre electrode.
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