Effects of Bias Voltage and Easily-ionized Elements on the Spatial Distribution of Analytes in Furnace Atomization Plasma Emission Spectrometry

Abstract

The effect of bias voltage and the presence of easily-ionized elements (EIEs) on the spatial distribution of excited-state atoms and ions of Cu, Ag, Cs and Ca in furnace atomization plasma emission spectrometry is presented. The dc bias of the centre electrode significantly affects the spatial distribution of He I, Cu I, Ag I, Cs I, and Ca II emission in the absence of EIEs. A reasonably uniform distribution of excited-state analyte atoms over the central cross-section of the tube occurs when the centre electrode is self-biased during the course of an atomization transient. A depleted area of Cs I emission around the centre electrode coupled with enhanced Ca II emission in the same region reveals that ionization of analytes is most pronounced in this region. With positive dc bias, concentric rings of enhanced emission occur between the centre electrode and the tube wall for analyte atoms and the He I plasma gas, although the overall breadth of analyte emission distribution is decreased. With NaCl, NaNO ₃ and CsCl serving as EIEs, analyte emission from Ag I, Cu I and Ca II in the region between the centre electrode and the tube wall is strongly suppressed with self-bias. The degree of the suppression depends on the extent of vapour cloud overlap between analyte and EIE. In general, equimolar amounts of NaNO ₃ and CsCl suppress analyte emission similarly and both produce a greater suppression than NaCl. Equal amounts of Fe, added as an interfering matrix, produces a suppression of analyte emission similar to that of EIEs, suggesting that the primary cause of suppression is the loss of energy from the plasma (as photons) due to excitation and ionization of matrix vapour. Control of the dc bias enhances the radial distribution of excited analyte atoms in the presence of EIEs and Fe, but only at low (≤2 µg) interferent loadings.