A method for purifying Lu and Hf for analyses by MC-ICP-MS using TODGA resin

Abstract

A two-stage method is presented to separate Lu and Hf from silicate rock and mineral samples digested by either flux melting or HF–HNO 3 dissolution using TODGA resin from Eichrom Industries. A first-stage chemistry employs a 2-ml cation exchange column that separates the high-field-strength elements (HFSE) and rare-earth elements (REE) from the matrix. Hf–Zr and Yb–Lu are subsequently separated from the HFSE-REE concentrates on a second-stage 0.2-ml TODGA column. This method returns sufficiently pure cuts to overcome all atomic and molecular interferences as well as the detrimental effects of Ti on Hf fractionation associated with MC-ICP-MS. Boric acid serves two roles in this method: (1) it effectively breaks down fluorides formed in the dry-down step after HF–HNO 3 digestions, thereby avoiding use of perchloric acid and/or multiple dry-down and re-dissolution steps, and (2) it is added to the loading solution of the second-stage chemistry to prevent formation of Hf-fluorides that would not be retained by TODGA resin. Using these methods, international rock standards BHVO-1 (processed by both flux melting and HF–HNO 3 dissolution) and BCR-2 (processed by HF–HNO 3 dissolution) yield 176Hf/ 177Hf and 176Lu/ 177Hf ratios that overlap with published values. Permutations of the basic method are presented for samples with significantly elevated LREE contents and for the separation of Lu and Hf from zircon.