Determination of rare earth elements and Y in ultramafic rocks by ICP-MS after preconcentration using Fe(OH) 3 and Mg(OH) 2 coprecipitation

Abstract

A simple and reliable method to separate rare earth elements (REE) from Mg, Fe, K, Na, Ca and Ba in ultramafic rocks has been developed, thereby concentrating their abundances. The sample (0.3 g) was digested with HF and HNO 3 in a PTFE bomb, placed in a stainless steel container and, after drying, the insoluble residue was dissolved in 6 ml of 10% v/v HNO 3. Following the addition of 50% triethanolamine and 30% m/v NaOH solution, the REE were precipitated along with Mg(OH) 2, such that the majority of Fe, K and Na in the solution could be separated by centrifuging. The precipitate was dissolved in 1 ml HNO 3 and a buffer solution of NH 4Cl/NH 4OH at pH = 9.0 was added to precipitate the REE along with any remaining Fe as Fe(OH) 3, and so achieve separation from Mg, Ca and Ba, which remained in the solution. In this way, REE could be separated from major elements and were concentrated by a factor of about 60. The recovery of REE was more than 95% using this method. Four ultramafic rock reference materials, PCC-1 (USGS), JP-1 (GSJ), DZE-1, DZE-2 (IGGE) and one new proficiency testing sample GeoP7/2 (GAS Serpentinite) were analysed by ICP-MS using indium as an internal standard. The quantitation limits were about 0.02-0.2 ng g -1. Smooth chondrite-normalised REE patterns were obtained with a precision for REE determination of about 2-9%.