OXIDE FORMATION IN ELECTROTHERMAL VAPORIZATION INDUCTIVELY-COUPLED PLASMA-MASS SPECTROMETRY

In electrothermal vaporization inductively coupled plasma mass spectro metry (ETV-ICP-MS) using a tungsten furnace, the effects of plasma par ameters and removal of solvent on interfering monoxide ion signals wer e investigated in order to determine rare earth element (REE) impuriti es in high-purity REE oxide samples without spectral interferences. Th e monoxide ion to element ion ratio (MO+/M+) was dependent on the plas ma parameters, showing a decreasing tendency with increasing rf power. To reduce spectral interferences, the plasma parameters were chosen s o as to attain a larger analyte signal and smaller MO+/M+ for a matrix element. The effect of oxide interferences on analyte signal could be further reduced by proper selection of integration time of the ion co unt. Also, a theoretical calculation of MO/M, assuming the Boltzmann e quilibrium of MO in the ICP, was performed to elucidate the mechanism related to oxide formation. In nebulization ICP-MS, experimental MO+/M + values for REEs were in good agreement with theoretical MO/M ones, w hich indicates that oxide ion species in ICP-MS may be derived from un dissociated MO and/or MO+ in the ICP. In ETV-ICP-MS, however, experime ntal MO+/M+ values were two orders of magnitude larger than theoretica l ones, probably owing to the air entering the ICP. Under the optimize d conditions that the oxide formation was minimized, the use of the ET V technique enabled us to determine Tb and Lu impurities at a concentr ation level of 0.01 mug g-1 in high-purity Gd2O3.