An evaluation of chromatographic modes for the determination of rare earth elements in geological materials by HPLC-ICP-MS.

Abstract

Scandium, Y and 15 lanthanides, commonly classified as rare earth elements (REE) are indispensable to modern technology and constitute integral components for the manufacture of advanced technological materials. Their determination in geological materials by ICP-MS are susceptible to numerous spectroscopic and matrix interferences that inhibit accurate quantification. The chromatographic separation of individual REE from interferents prior to detection by ICP-MS has been established as an effective approach for overcoming these interferences. However, application of this approach in literature does not address both mutual REE and matrix induced spectroscopic interferences. Furthermore it requires extensive sample pre-treatment, introducing errors in analysis. This study investigates established chromatographic modes of REE separation (ion pair and ion exchange chromatography) on their capability to separate individual REE and sample matrix components without sample pre-treatment. It further aims to evaluate the potential of these chromatographic methods to be integrated to a hyphenated HPLC-ICP-MS technique for elimination of interferences which affect ICP-MS REE quantification. Ion pair and ion exchange methods were optimised using HPLC with post-column derivatisation and UV-Vis detection (as a preliminary detection method). These methods were compared on the basis of their REE separation efficiency and capability to address ICP-MS spectroscopic interferences that affect REE determination. The potential integration of the preferred chromatographic method (ion pair chromatography) prior to ICP-MS detection was evaluated by analysis of CRMs before and after separation. The influence of mobile phase composition on analytical performance of ICP-MS for REE determination was assessed. This study has shown that ion pair chromatography has the potential to achieve complete separation of individual REE without interference from sample matrix components and avoiding sample pre-treatment procedures. However, the surfactant properties of the ion pair reagent and organic and sodium content of its mobile phase composition can impair ICP-MS sensitivity. This study also provides insight to challenges that will be encountered during application of ion pair chromatography with ICP-MS detection (HPLC-ICP-MS) and presents appropriate suggestions and improvements. These will enhance the potential application of ion pair chromatography with HPLC-ICP-MS such that a more effective means for accurate and precise determination of REE in geological materials can be achieved.