REDUCTION OF THE EFFECTS OF CONCOMITANT ELEMENTS IN INDUCTIVELY COUPLED PLASMA MASS-SPECTROMETRY BY ADDING NITROGEN TO THE PLASMA GAS

Mixed-gas plasmas were produced in inductively coupled plasma mass spectrometry (ICP-MS) by adding 2-10% N2 to the plasma (outer) gas and keeping both the r.f. power and the nebulization efficiency (i.e., nebulizer flow rate and sample delivery rate) constant. The resulting ion sources were characterized (and compared with a conventional Ar plasma) in terms of sensitivity, mass discrimination and susceptibility to the effect of up to 0.1 mol dm-3 K, for several analytes (Al, V, Cr, Fe, Co, Ni, Cu, Zn, Se, Cd and Pb). Sensitivity decreased upon addition of N2 (consistent with a shift of the initial radiation zone away from the sampler) along with the signal-to-noise (S/N) and signal-to-background (S/B) ratios, except for those analytes suffering from interferences from Ar polyatomic species (Cr, Fe and Se) where N2 then improved S/N and S/B. The decrease in sensitivity was smaller for Cd, Zn and Se, as N2 probably improved the ionization of these elements (which are not 100% ionized in an Ar plasma). Mass discrimination was reduced in the m/z range 51-78. Finally, the effect of 0.01 mol dm-3 K was eliminated with 10% N2 in the m/z range 51-114. The amount of N2 could also be adjusted to eliminate the effect of 0.1 mol dm-3 K on a given analyte (but not for the whole mass range).