ANALYTICAL PERFORMANCE OF THE MICROWAVE PLASMA TORCH IN THE DETERMINATION OF RARE-EARTH ELEMENTS WITH OPTICAL-EMISSION SPECTROMETRY

A recently developed microwave discharge device called the microwave plasma torch (MPT) is used in this work to evaluate its analytical performance in the determination of rare earth elements with optical emission spectrometry. Optimization of the instrument was performed by using individual elements with emphasis on the plasma observation zone, carrier and support gas flow rates. The detection limits for 15 rare earth elements obtained by MPT atomic emission spectrometry at 70 W are in the order of ng/ml. The precision (R.S.D.%) calculated with eleven continuous measurements at a concentration of about two orders of magnitude higher than the detection limits is in the range of 2.0 (Y) to 4.9 (Tb). Some comparisons were made between inductively coupled plasma and MPT methods, and the results show that in some cases, most of the sensitivity lines observed in these two sources for rare-earth elements are not identical. Compared with the ICP emission spectrum, the MPT source gives relatively simple spectra and much lower background emission. A preliminary examination of matrix effects with varying matrix concentrations shows that some easily ionized elements give significant enhancement on analyte signal and that aluminum suppresses the signals considerably. For some other elements (Co, Ni and Zn) no remarkable matrix effects are observed at a concentration level of 10 mu g/ml.