INVESTIGATIONS INTO THE APPLICATION OF METHANE ADDITION TO THE NEBULIZER GAS IN INDUCTIVELY COUPLED PLASMA MASS-SPECTROMETRY FOR THE REMOVAL OF POLYATOMIC INTERFERENCES

The addition of methane to the nebulizer gas in inductively coupled plasma mass spectrometry is assessed as a method for the reduction of polyatomic interferences. The effect of nebulizer gas flow rate, power and percentage methane on a range of species in the plasma was studied. The analyte response was not seriously affected by the addition of methane, but the responses of the interfering ions, ArCl+, ArO+, ClO+ and MO+, and also the random background, were reduced when compared with those of an unmodified plasma. Simplex optimization was used to optimize operating parameters for maximum removal of these interferences with methane addition to the plasma. Complete removal of ArCl+ was achieved with methane addition to the nebulizer gas, yielding a detection limit for As of 0.75 ng ml-1 in 10% HCl. The ArO+ and ClO+ were much reduced, the detection limit for Fe being 0.48 and V 2.52 ng ml-1 in 10% HCl. The MO+ was reduced by a factor of 10 under the optimum conditions. Recovery tests for As, Se and V further demonstrated the utility of these optimal conditions in removing interferences when compared with typical operating conditions. Several certified reference materials high in chloride, were successfully analysed for As, Se and V.