Minimization of spectral interferences in inductively coupled plasma mass spectrometry by simplex optimization and nitrogen addition to the aerosol carrier gas for multi-element environmental analysis
Low-level trace element determinations by inductively coupled plasma mass spectrometry in environmental samples, e.g., groundwater, can be seriously hampered by spectral interferences originating from matrix-components and/or the argon plasma. Two approaches to reduce the level of interference were investigated: variable step size simplex optimization with a number of response factors (analyte signals, precision, oxide and other polyatomic interferences) was applied to reduce simultaneously a number of molecular ion interferences; and nitrogen was added to the aerosol carrier gas and the nitrogen flow rate was used as an additional variable in the simplex optimization. Simplex optimization alone resulted in a more consistent choice for the settings of the instrumental variables and in reduction of the levels of polyatomic interferences. By the addition of nitrogen spectral interferences were further reduced by a factor of 1.5 to 3. For each set of optimum instrumental settings the spectral interferences were quantified as apparent analyte concentrations. Final levels of interference were <3 µg l–1 for Cr, Cu, Ni, Se and Zn, and <10 µg l–1 for As and V in 0.02 mol l–1 Na, 0.02 mol l–1 Cl, 0.01 mol l–1 Ca, 0.01 mol l–1 Mg or 0.004 mol l–1 SO4 solutions. Matrix-induced sensitivity changes were also reduced by the addition of nitrogen.