REDUCTION OF MATRIX EFFECTS AND MASS DISCRIMINATION IN INDUCTIVELY-COUPLED PLASMA-MASS SPECTROMETRY WITH OPTIMIZED ARGON-NITROGEN PLASMAS

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 an alytical capability of inductively coupled plasma mass spectrometry (I CP-MS) for multi-element analysis. The experiments were all conducted at an optimum sampling position, i.e., by adjusting the aerosol carrie r gas flow rate for maximum sensitivity after each change in power and /or addition of N2. For each set of conditions, the analytical capabil ities of Ar-N2 plasmas in terms of sensitivity, detection limits, mass discrimination and susceptibility to the effect of 0.01 and 0.1 mol d m-3 Na on various analytes (Al, V, Cr, Mn, Fe-59, Fe-57, Co, Ni, Zn, C u, AS, Se-77 , Se-78, Mo, Cd, Sb and Pb) were evaluated. Some improvem ent in sensitivities and detection limits were observed upon addition of N2 to the outer gas in comparison with an Ar plasma at the same pow er. 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 convention al operating conditions (i.e., 1.2 kW, no N2), a mixed-gas plasma with 1 0% N2 at 1.3 kW led to only slightly degraded sensitivities and det ection 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 ra nge, and that of 0.1 mol dm-3 Na was reduced to a uniform level across the mass ran e, allowing the use of a single internal standard.