Effect of concomitant elements on the distribution of ions in inductively coupled plasma-mass spectroscopy. Part 1. Elemental ions

The effect of various concomitant elements in ICP-MS was assessed by measuring the distribution of selected singly charged analyte ions (of Al, V, Cr, Mn, Ni, Co, Cu, Zn, As, Mo, Sb, La, Ce, and Pb) as well as doubly charged ions of La and Ce in the plasma. This was accomplished while moving the ICP across and away from the sampling interface. The effect of each different concomitant element was studied individually. 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). A dependence upon both the mass of the matrix element and the mass of the analyte was observed. The suppression seen increased with increasing matrix element mass and decreased with increasing analyte mass. The effect of the mass of the matrix element was the more significant of the two factors. Space-charge effects were found to be significant for matrix elements of much lower mass than has been suggested previously. The age of the sampling interface was also found to have an effect upon the relative observed locations within the plasma of the suppressions and enhancements. Older sampling interfaces resulted in the ‘crossing point’, where an analyte signal in the presence of a matrix element intersects the reference analyte signal, shifting closer to the load coil. This is likely further evidence of space-charge effects. Finally, some evidence was also found for a shift in ion–atom equilibrium through the suppression observed in the presence of 0.02 M Na, an element for which space-charge effects should be negligible.