SOME CHARACTERISTICS OF POLYATOMIC ION SPECTRA IN INDUCTIVELY-COUPLEDPLASMA-MASS SPECTROMETRY

In order to clarify formation mechanisms of polyatomic ion species in shielded inductively coupled plasma mass spectrometry, the effects of the secondary discharge occurring at the tip of the sampling cone on k inetic energies and signal behaviours of polyatomic ion species and on physical properties of the micro plasma in the interface region betwe en the sampling and skimmer cones (interface plasma) were investigated with and without shielding the ICP. Without shielding, where the effe ct of secondary discharge was extensive, all polyatomic ion signals st udied (ArH+, ArO+, NO+, O-2(+) and MO(+); M=Sc, Sr, Zr and La) showed an increase with increasing chamber gas flow rate, and the kinetic ene rgies for these species were in a range of 15-30 eV. However, with the shielded ICP, in which the secondary discharge was eliminated, the si gnals of the ArH+ and ArO+ species decreased while those for NO+, O-2( +) and LaO+ increased with increasing chamber gas Bow rate. The kineti c energies in this instance were 2-5 eV, an order of magnitude lower t han the energies without shielding. The signals for ArH+ and ArO+ spec ies were dependent on the electron number density (n(e)) in the interf ace plasma, in both situations (with and without shielding). This indi cates that the formation of ArH+ and ArO+ may be promoted by the secon dary discharge. Also ab initio calculation was performed for argon pol yatomic ion species (ArX(+), X=H, C, O and N) to obtain spectroscopic data, which were used for the calculation of dissociation equilibrium constants (K-n) and ion number densities ([ArX(+)]). The theoretical c alculation results for [ArX(+)] suggest that the dissociation equilibr ium for ArX(+) species is reached not in the ICP but in the interface plasma with a gas kinetic temperature of 3500 K.