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

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.