INVESTIGATIONS ON MIXED-GAS PLASMAS PRODUCED BY ADDING NITROGEN TO THE PLASMA GAS IN ICP-MS

Additions of nitrogen were made to the argon plasma gas of an inductively coupled plasma (ICP) used as an ion source in mass spectrometry (MS). The studies were carried out at constant nebulizer gas flow rate and uptake rate (i.e. constant nebulization efficiency), with the percentage of nitrogen in the plasma gas (from 0 to 10%) and the power (from 1.0 to 1.4 kW) as the only variables. For each set of operating conditions, the analytical characteristics were assessed for Fe-56+, Fe-57+, Se-76+ and Se-78+. The best results were obtained at 1.2 kW where, although the sensitivity was reduced by a factor of up to 5 upon addition of nitrogen to the plasma, the stability of the plasma was improved, resulting in similar (Se) or improved (Fe) detection limits, by up to a factor of 4. A study of the effect of a matrix containing up to 0.1 M Na was also carried out (at 1.2 kW) while the percentage of nitrogen in the plasma gas was raised from 0 to 10%. The results show that the non-spectroscopic interference of Na can be reduced. In fact, with 5% nitrogen in the plasma gas, the effect of 0.01 M Na was essentially eliminated. Furthermore, the accuracy and precision of the Fe-57+/Fe-56+ and Se-76+/Se-78+ isotopic ratios were found to improve as the amount of nitrogen was increased. For instance, the precision of the mean Fe-57+/Fe-56+ ratio measured in solutions containing from 0 to 0.1 M Na improved by an order of magnitude with 10% nitrogen in the plasma, and that of Se-76+/Se-78+ improved by a factor of 4, which is still quite significant, given the high Ar-76+ background in comparison with the relatively low abundance of Se-76+. Overall, the results suggest that an Ar-N2 mixed-gas plasma may be a better ion source for MS than an all argon ICP.