Dissolution techniques for determination of rare earth elements in phosphate products: acid digestion or alkaline fusion?

Abstract

Several studies have been carried out to compare and/or improve instrumental analytical techniques capable of quantifying trace elements, rare earths included. Sample dissolution is a key step in this process and the choice of the most appropriate technique depends on many factors, e.g., digestion power, laboratory workability, available instrumentation, and timing. Alkaline fusion and acid digestion are often the preferred techniques for decomposition of geological samples due to their precision and accuracy. The aim of this study was to evaluate dissolution techniques for determination of REE in phosphate products by ICP-MS. For this, twenty samples of phosphate products were selected from the fertilizer quality control program of the Brazilian Ministry of Agriculture, Livestock, and Food Supply. Samples of these products were digested by either microwave digestion or alkaline fusion. Both methods yielded accurate results, with alkaline fusion being slightly more efficient than microwave digestion in the extraction of light REE. However, this difference between methods was not observed for heavy REE and it did not affect the REE signatures. The information provided in this paper can assist in the choice of dissolution techniques for REE quantification, especially in phosphate fertilizers. In addition, the equations provided in this paper may also be used to interconvert the results from both methods.