Reduction of matrix effects and mass discrimination in inductively coupled plasma mass spectrometry with optimized argon–nitrogen plasmas

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Abstract
The effect of power (1.2–1.4 kW) and the addition of N2(2–10%) to the outer gas of an Ar plasma was studied in an attempt to improve the analytical capability of inductively coupled plasma mass spectrometry (ICP-MS) for multi-element analysis. The experiments were all conducted at an optimum sampling position, i.e., by adjusting the aerosol carrier gas flow rate for maximum sensitivity after each change in power and/or addition of N2. For each set of conditions, the analytical capabilities of Ar–N2 plasmas in terms of sensitivity, detection limits, mass discrimination and susceptibility to the effect of 0.01 and 0.1 mol dm–3 Na on various analytes (Al, V, Cr, Mn, 56Fe, 57Fe, Co, Ni, Zn, Cu, As, 77Se, 78Se, Mo, Cd, Sb and Pb) were evaluated. Some improvement in sensitivities and detection limits were observed upon addition of N2 to the outer gas in comparison with an Ar plasma at the same power. Both mass discrimination and the effect of either 0.01 or 0.1 mol dm–3 Na were reduced with the addition of N2. Compared with conventional operating conditions (i.e., 1.2 kW, no N2), a mixed-gas plasma with 10% N2 at 1.3 kW led to only slightly degraded sensitivities and detection limits (in fact, improvements were observed for a few elements); but the effect of 0.01 mol dm–3 Na was eliminated across the mass range, and that of 0.1 mol dm–3 Na was reduced to a uniform level across the mass range, allowing the use of a single internal standard.