Fundamental studies of mixed gas plasmas in ICP-MS
Inductively coupled plasma - mass spectrometry is widely accepted as the leading technique for trace element analysis. It suffers however, from a range of interferences both spectral and non - spectral which limit the range of analytes and substrates which may be successfully analyzed. This study has investigated the use of mixed gas plasmas as a means of reducing or removing these interference effects. In addition the use of mixed gas plasmas for sensitivity enhancement has been assessed using both wet and dry sample introduction systems. Simplex optimization procedures have been used throughout. The addition of nitrogen to all three argon gas flows of the ICP was evaluated, and the instrumental operating conditions were optimized to allow for the maximum removal of the ArCl+ interference on As and Se. The addition of nitrogen to the nebulizer gas was particularly effective in the removal of the ArCl+. The use of nitrogen addition also facilitated the determination of arsenic in samples where it was previously found to be impossible. The addition of methane was also applied to all three gas flows of the ICP, however, in this case only the addition to the nebulizer gas was successful in removing interferences. These included ArO+, ClO+ and CeO+ in addition to ArCl+ . Detection limits, recoveries and CRM analysis were all improved by the use of methane addition when compared to nitrogen addition or standard argon plasmas. Hydrogen addition was applied solely to the nebulizer gas and found to significantly increase the magnitude of the interferences from argon based polyatomic ions, whilst reducing those from metal oxides. The addition of hydrogen was useful in reducing MO+ interferences, but the removal of other interferences proved unsuccessful. The addition of ethene to the nebulizer gas has been used to remove many of the polyatomic interferences encountered in ICP-MS, including those based on Na, S and P. Ethene addition was as effective as, and usually better than, the addition of both methane and nitrogen. Detection limits, recoveries, calibrations and CRM analysis were all improved by its use. The use of ethene addition has lead to improvement in the determination of As, Cu, Fe, Gd, Ni, Se, and V in the presence of the relevant interference precursors. Most importantly it has been shown that a whole range of interferences can be reduced simultaneously with the addition of ethene. The plasma operating parameters and ion optics settings of a commercial ICP-MS instrument were optimized to yield maximum SBRs for elements across the mass range. It was shown that simple manual tuning of the instrument did not effect the SBRs, when compared to the simplex optimized conditions. Finally it was shown that the addition of molecular gases to the ICP, did not result in any significant improvements in sensitivity, when compared to the all argon system. Finally it has been shown that the addition of a relatively small amount of hydrogen to the nebulizer gas can lead to an enhancement in the signal obtained using laser ablation sampling. This enhancement was greatest for low mass elements and had no effect on the heavy mass element signal.
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