SPACE-CHARGE EFFECTS AND ION DISTRIBUTION IN PLASMA SOURCE-MASS SPECTROMETRY

Space-charge effects appear to be key in understanding ion-transport processes in plasma-source mass spectrometry. Both theoretical calculations and experimental measurements demonstrate that space charge influences ion transport between the plasma source and mass analyzer. In a theoretical analysis, both drift space and accelerating space in the interface have been considered and preliminary conclusions have been drawn concerning space-charge repulsion and ion-beam divergence in those regions. In the experimental work, spatial patterns of sample deposition on the third stage vacuum orifice plate were recorded while using a microwave plasma source. To measure these patterns, a stainless-steel target was used to cover the orifice and both x-ray fluorescence microanalysis and scanning electron microscopy (SEM) were used to examine the sample material deposited on it. The resulting spatial maps show that analyte deposition is somewhat element-dependent in a way that is correlated with analyte atomic mass. These findings can be explained on the basis of space-charge theory.