Effect of concomitant elements on the distribution of ions in inductively coupled plasma mass spectrometry - part 2: polyatomic ions

The effect of various concomitant elements in inductively coupled plasma mass spectrometry (ICP-MS) was assessed by measuring the distribution of selected polyatomic singly-charged ions, i.e. oxides of selected analytes (LaO+ and CeO+) as well as some background ions (Ar-2(+), ArO+ and CO2+), in the plasma. This was accomplished while moving the ICP across and away from the sampling interface with or without a single concomitant element. This study included concomitant elements that were similar in ionization potential, but different in mass (Na, K, Cs and Cl, I), as well as similar in mass but different in ionization potential (K and Cl, Cs and I). Space-charge effects were dominant in many cases. However, the axial profiles of the LaO+/(La+ + LaO+) ratio, which should be fairly independent of such effects, were only affected by easily ionized elements (E.I.E.s), i.e. alkalis. A shift of the whole axial profile to lower height A.L.C. was observed in the presence of 0.02 M K or Cs, which is strong evidence for earlier desolvation. However, no shift was observed with 0.02 M Na, where the profile was only suppressed at low heights A.L.C., where the electron density would be most affected. This is consistent with a shift in ionization equilibrium. Further evidence for the latter was observed in the Ar-2(+) axial profiles, which were shifted to greater height A.L.C. in the presence of E.I.E.s. Finally, CO2+ was found to behave like the analyte oxides and the analytes, in contrast to ArO+. This implies that CO2+ originates from dissolved carbon dioxide in the sample. (C) 2001 Elsevier Science B.V. All rights reserved.