Abstract
The mechanism and reduction methods for the formation of interfering ion species in inductively coupled plasma mass spectrometer were described. The interfering ion species were classified into three groups according to the formation mechanism; (1) polyatomic ions due to argon and solvent, (2) refractory monoxides and hydroxides, and (3) doubly charged ions. Polyatomic ions due to argon seem to be formed by the collision-induced reactions of atom with ion in the micro plasma in the interface region between the sampling and skimmer cones (interface plasma), not in the ICP because their dissociation energies are quite small (below 2 eV except for ArH+) and most of polyatomic ions were dissociated into atom and ion in the ICP. Thus, the lowering of the plasma temperature and/or the secondary discharge occurring at the tip of sampling cone led to less frequency of the collision-induced reactions, resulting in the reduction of formation of polyatomic ions. Refractory monoxides and hydroxides whose dissociation energies exceed 4 eV were stable even in the ICP. Monoxide ion to element ion ratio depends on the plasma temperature, the number density of oxygen in the ICP and the time required for the desolvation of analyte particles. The signal for doubly charged ion increased under the condition that the effect of the secondary discharge was large, which was eliminated by reducing the inductively coupling between the ICP and the load coil.
