Desempenho analítico da espectrometria de emissão óptica com fonte de plasma indutivamente acoplado na determinação de elementos terras raras em amostras geológicas

Abstract

The determination of rare earth elements (REEs) is extremely important to geological and environmental studies, these elements are indicators of environmental processes and rocks formation, showing a distinctgeochemical signature governed by their similar chemical properties, especially charge and ionic radius. Several analytical techniques can be applied for determination of REEs, however, some of them are expensive or require laborious sample preparation procedures for routine analysis. Inductively Coupled Plasma Optical Emission Spectrometry (ICP OES) is a multielement technique, flexible, with high sample throughput, good reproducibility and relatively low detection limits. Traditionally, these elements are determined after separation and preconcentration procedures, and the analysis becomes costly and time consuming. This work aimed to evaluate the determination of REEs in geological samples by ICP OES, without the separation and preconcentration procedures. For that, the evaluation of operating conditions suitable for this type of matrix and investigation of interferences were considered. It is important to point out that it is about the determination of trace elements in a very complex matrix, rich in Si, Al, Fe, Ti, Mn, Mg, Ca, Na and K, where several of these constituents can cause critical interferences. The operating conditions of ICP OES were optimized aiming for the best condition for REEs determination (La, Nd, Eu, Gd, Dy, Er and Yb) based on robustness (Mg II / Mg I) and sensitivity (signal to background ratio SBR value). Doehlert design and univariate experiments were applied for the optimization ofthe instrumental parameters. Sets of operating conditions pre-selected were compared based on the values of Mg II / Mg I, SBR, limit of detection (LOD), background equivalent concentration (BEC) and relative standarddeviation of the blank solution (RSD%,blk). Analytical data indicated that the application of lower RF power applied could improve the analytical signal with lower background radiation and also lower RSD%,blk. The optimization of the operating conditions was not enough to overcome some of the interference effects, which refer to spectral and non-spectral interferences. Among the major constituents of geological samples, the main interferents were Al, Fe, Ti, Mn and Ca. There was a different behavior of the REEs in the presence of HNO3 and HCl, and theHNO3 effect on the emission signal of the REEs was more critical. In fact, the interfering effects were more severe in high acid concentration (> 10 %). The information obtained led to the proposal of a calibration strategy with the application of the interelement correction (IEC). A multielement analytical curve was prepared containing analytes and interferingelements (Al, Fe, Ti, Mn and Ca) in the same acid conditions used for the samples digested. The analytical performance of the proposed procedure was evaluated by the linearity of the calibration curve, limits of detection and quantification, precision and accuracy. Two analytical lines were selected for each REEs studied and eight certified reference materials were applied, considering different geological materials including rocks, soils and sediments. The procedure showed good results, linearity, precision and accuracy acceptable. A set of sedimentsamples was used to demonstrate the application of the developed procedure for the determination of REEs in geological samples.