EFFECT OF NITRIC AND HYDROCHLORIC ACIDS ON RARE-EARTH ELEMENT (REE) INTENSITIES IN INDUCTIVELY-COUPLED PLASMA EMISSION-SPECTROMETRY

Atypical depressive effects on rare earth element (REE) intensities;, reproducible within experimental error, generated by high concentrations of nitric and hydrochloric acids in an Ar inductively coupled plasma-atomic emission spectrometer (ICP-AES) are reported. It was observed that in the presence of nitric acid, the REEs did not respond as a coherent group, the intensity of each REE line responding differently to increasing acid concentrations. However, in the presence of hydrochloric acid, the response of all REE spectral lines were comparable. These effects were generally similar regardless of the mode of aerosol generated (conventional pneumatic nebulization using concentric and ultrasonic nebulizers (USNs) with aerosol desolvation). However, in the USN the effect was reduced due to removal of nitric acid in desolvation. In the argon ICP, using a conventional pneumatic nebulizer, the excitation temperature (T-exc) decreased with increasing nitric acid concentration from about 7500 K in an aqueous solution to 7000 K in 7 N NHO3. Temperatures did not vary significantly for variations in concentration of hydrochloric acid up to 10 N. Consequently, in the case of HCl, the similar reduction in intensities for all REEs may be due to a physical effect in the aerosol transport system. T-exc, however, for the ultrasonic nebulizer was generally lower by about 500 K. The depressive effects of the REE spectral intensities due to HNO3 were not directly correlative with the thermal (heat of evaporation, heat of fusion, melting and boiling points) and spectroscopic characteristics (ionization and excitation potentials) of the REE spectral lines employed and are not attributable to physical transport effects in the sample introduction system.