Matrix-independent separation of rare-earth elements and yttrium from geological materials using constant calcium content-oxalate precipitation and cation exchange for determination by high-resolution inductively coupled plasma atomic emission spectrometry (ICP-AES)

Abstract

Inductively coupled plasma atomic emission spectrometry (ICP-AES) is a precise and accurate analytical technique for determination of rare-earth elements (REE) in geologic materials provided the REE are separated from matrix elements. The separation method reported here uses cation exchange following an oxalate precipitation separation. Oxalate precipitation separates REE and Y with Ca from other matrix elements. Calcium oxalate is an effective carrier for REE, but is much more effective for light and middle REE (LREE and MREE) than for heavy REE (HREE). Complete REE recovery (especially of HREE) requires a sufficiently high Ca content in the sample. The method used here is to bring each sample (1 g) up to a Ca content of 107 mg by addition of calcium carbonate prior to sample digestion. Subsequent ion exchange with a mixed resin gives complete recovery of REE and Y (and in some cases Sc) without modification for sample type because the salt load is constant no matter what the starting material. Analysis of nine REE, Y and Sc is carried out using a very high-resolution sequential scanning ICP-AES system. High resolution minimizes RE inter-element interferences, and use of mixed-element standards (mixed in geologic proportions) minimizes corrections, usually to a negligible level. Analyses of U.S.G.S. standard reference materials BHVO- 1, DNC- 1 and TLM- 1 compare well with published values for REE and Y. Recovery of Sc is apparently not always complete.