Analysis of rare-earth elements in geological samples by gradient ion chromatography: An alternative to ICP and INAA

Abstract

An application of high-performance ion chromatography (HPIC) for the analysis of rare-earth elements (REE) in geological samples is described. The technique (developed by Heberling and coworkers on synthetic solutions) is shown to be capable of determining 12 of the REE in a wide variety of rock types with an accuracy and precision comparable to that of inductively coupled plasma (ICP) atomic emission spectroscopy. By this HPIC method it is not possible to obtain data for Ho, due to interference by Y, or Lu which is not sufficiently well resolved from Yb. The HPIC instrument used in this study is a Dionex® 4000i Gradient ion chromatograph in which eluant composition and flow rate are microprocessor controlled. Separation of the REE was effected using a Dionex® CS 5 solumn with conditions similar to those outlined by Heberling and coworkers. The eluted REE were reacted with a colour complexing agent and detected photometrically using a UV/visible light detector at a wavelength of 520 nm. Analysis by HPIC requires that the sample be taken into solution and the matrix removed. Sample dissolution and removal of matrix elements, that would otherwise interfere with the chromatogram, were achieved using standard cationexchange procedures routinely applied in the preparation of samples for ICP analysis. Providing that a transition metal-free solution remains after matrix removal, the full range of REE can be determined in ∼20 min. However, if ppb levels of the transition metals are present in the analyte solution, these must be removed during the early part of the HPIC analysis so increasing the run time to ∼42 min. REE data obtained by this HPIC technique for a number of international rock standards compare favourably with the published data on these well-characterised samples.