Abstract
Vertical spatial profiles of a six-electrode, direct-current plasma emission source were used to study the influence of plasma length on the interference of Na on Ca and Zn and P on Ca. For the shortest plasma (11.5 mm), depression of the Ca atom line is observed in the region up to 10 mm above the tip of the sample bullet when Na is present. A cross-over to enhancement occurs for higher regions. This is directly opposite to the observations in the ICP for the Na interference on Ca. Increasing the plasma length causes downward shifts in the cross-over point which are not a simple effect of the lowering of the sample bullet. When P is present, the Ca atom emission is initially depressed in the region directly above the sample bullet. At higher observation heights, little effect is observed. Similar behavior is observed for the Ca ion line in the presence of P, suggesting the possibility of a classical vaporization-type interference mechanism. The interference effects studied here can be virtually eliminated by careful selection of plasma length and observation height.
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