Determination of rare earth elements and thorium in sea water by inductively coupled plasma atomic emission spectrometry

The concentration of rare earth elements in sea water have been obtained by neutron activation analysis and isotope dilution mass spectrometry. Rare earth elements and Th can be determined with higher sensitivities by inductively coupled plasma atomic emission spectrometry (ICP-AES). However, direct determination of these elements of ppt level in sea water is difficult because of inadequate sensitivity. So, determination of these elements in sea water was made by ICP-AES combined with preconcentration by co-precipitation with ferric hydroxide. A ICP-AES spectrophotometer SPS 1100 of Seiko Instruments and Electronics LTD was used. Spectral band width was 0.005 nm. RF power was 1.3 kW. Argon flow rates of plasma gas, aux. gas and carrier gas were 15 l/min, 0.3 l/min and 0.4 l/min respectively. The recovery of these elements by co-precipitation was more than 95 % at pH 6.5, and the separation of these elements from macrocomponents in sea water was also excellent. Almost all the amount of iron remaining in sample solution was separated with MIBK in 6 M HCl. Sample solution of 10 ml was obtained from 100 ml of sea water. Ca, Fe, and U in the sample solution increased the background at each wavelength for rare earth elements and Th. These matrix elements in the sample solution were added to blank and standard solutions. The background correction was also made in all runs. The overlap of spectral lines was not observed except for Tb, Gd, and Pr cases. The concentration of rare earth elements and Th in the range of 0.1 ng/l ∼ 56 ng/l in sea water could be determined. (author)