Mixed Ar-N-2 plasmas: Simple antidotes to matrix effects in ICP-MS

The addition of 2-10% nitrogen to the outer gas of the Ar plasma at 1.2-1.4 kW was investigated in an attempt to find robust conditions in inductively coupled plasma mass spectrometry (ICP-MS). The experiments were conducted at fixed nebulization efficiency (i.e. constant nebulizer flow rate, sample delivery rate, and sampling depth in the plasma, all optimized in terms of sensitivity with an Ar plasma at 1.2 kW). In each set of conditions, the a nalytical capabilities of Ar-N-2 plasmas were evaluated for analytes across the mass range. In comparison to an Ar plasma at the same power, 10% N-2 p rovided the most robust conditions. Not only were the 20% suppression induc ed by 0.01 M Na and 82% suppression induced by 0.1 M Na transformed into sl ight enhancements, but the analyte signals measured in these two matrices w ere very similar. This relative freedom from Na matrix effects was achieved while sensitivity was only reduced by a factor of about 2 for most element s. In fact, the sensitivity for Zn, Se and Sb remained similar while that f or As was enhanced by a factor of 2, compared to those observed in an Ar pl asma. Although detection limits are expected to be more substantially degra ded than sensitivity for the majority of elements, some improvements should result (for Fe and Se) because of a decrease in spectroscopic interference s. Nonetheless, this is a small price to pay for as much freedom from the n on-spectroscopic interference caused by Na. Furthermore, these robust condi tions also reduced mass discrimination compared to that observed in an Ar p lasma.