GROUP SEPARATION BY SOLVENT-EXTRACTION FROM SILICATE ROCK MATRIX OF NB, MO, TA, AND W AT LOW-LEVELS FOR ICP-MS

The extraction efficiency from silicate rock solutions in HF of Nb, Mo, Ta, and W as a function of fluoride activity in HCI solution, 3 mol/l, was investigated. At pF 3.5, N-benzoyl N-phenylhydroxylamine (BPHA) dissolved in CHCl3 proved to be an ideal system for the separation of the four analyte elements from the rock matrix after HF dissolution. All major rock constituents except for titanium are left behind in the aqueous phase. Values for the distribution ratios are 500 +/- 100 for all four elements allowing cross calibration in ICP-MS between enriched isotopes of Mo and W added to the samples before dissolution (isotope dilution) and the neighbouring monoisotopic elements Nb and Ta. Previously such high distribution ratios have been reported for Mo only because Nb, Ta, and W are not normally available as reactive ions due to hydrolysis unless directly released from fluoride complexes by downwards adjustment of fluoride activity of the solution in the presence of BPHA. Analytical data from 20 geochemical reference standards document the practical value of the described analytical procedure. Using ICP-MS and BPHA-pentanol solutions standards, superior detection limits are demonstrated by Nb, Ta, and W values in the sub ppm range obtained for some ultrabasic rock standards for which reference values are not yet available. Using ICP-MS and BPHA-pentanol solutions, superior detection limits are demonstrated by concentration values in the sub ppm range obtained for some ultrabasic rock standards for which reference values are not yet available.