Analysis of REE in Granite and Sandstone by HR ICP-OES - 2021

Rare Earth Deposits & Mining

AnalytikJena reported in 2021 that quantification of rare earth elements (REE) in geological materials by ICP-OES is one of the most challenging analytical routines. Often containing large amounts of alumina and silica, sulfur and refractory metals, etc., the high matrix contents of digested samples require for exceptional plasma robustness, in particular when trace levels of REE ought to be detected and sample dilution has to be avoided.

The vast number of emission lines arising from both matrix and rare earth elements further adds to the complexity, which can be resolved by high spectral
resolution only. This application note describes a method for the analysis of two geological reference materials for granite (GSR-1) and sandstone (GSR-4) for its rare earth constituents including Ce, Dy, Er, Eu, Gd, Ho, La, Lu, Nd, Pr, Sm, and Yb by high-resolution Array ICP-OES on a PlasmaQuant 9100 Elite equipped with HF Kit.

Interference-free analysis in both granite and sandstone was possible for most REE elements thanks to the unique resolving power of the high-resolution optics of the PlasmaQuant 9100 Elite. Prominent spectral interferences from cerium, erbium, iron, and neodymium on sensitive emission lines of dysprosium, erbium, lanthanum, and samarium were successfully corrected by the CSI-Software Tool. Excellent agreement with the certified values and quality RSD values were obtained for granite (GSR-1) and sandstone (GSR-2). Matrix-specific detection limits are given.

The utilization of a Na₂O₂ fusion digestion methodology rather than an acidic digestion allows for a full access to all REE in the investigated samples. The precise analysis (RSD values well below 5%) of the as-prepared, matrix rich digestions was ensured by the high plasma stability as well as the smart torch design of the PlasmaQuant 9100 Elite. Due to the high resolution of the instrument (FWHM Dy 353.170 ≤ 5.5 pm), also severe interferences can be spectrally resolved, resulting in the ability to analyze the majority of the REE without any further mathematical correction algorithms.

In this investigation, the authors concluded that only the elements dysprosium, erbium, lanthanum and samarium required a subsequent mathematical correction by the CSI software tool. The excellent accuracy and precision achieved with the applied methodology demonstrates its suitability for the routine analysis of geological samples for REE.

Furthermore, the analysis of rare earth elements (REE) exhibits several obstacles, such as the ability for complete access to all REE, analysis of matrix-rich samples, and the high potential of severe spectral interferences. The application methodology presented here combines a fusion digestion procedure using a Na₂O₂ flux with the analysis by high-resolution ICP-OES on the PlasmaQuant 9100 Elite.

The fusion digestion allows residue-free digestion of complex geological matrices, hence all analytes, including both light and heavy REE, can be readily analyzed. Since the solutions as prepared carry both the sample and the flux matrix, a highly stable plasma performance and a sample introduction system are required to perform a high-precision and high-accuracy analysis. Here, the PlasmaQuant 9100 Elite features a high-frequency generator for the best plasma stability among ICP-OES instrumentation. Using its V-Shuttle torch, a fully demountable vertically-oriented torch, the analysis of highest matrix content (e.g. 12 g/L fusion flux) is ensured without further dilution that benefits REE detection limits.

Matrix specific detection limits in the ng/L to low µg/L range can be achieved. Severe spectral interferences arising from other REE or elements that often accompany REE, such as iron, complicate their accurate analysis in geological materials. The PlasmaQuant 9100 Elite exhibits two powerful features to overcome such spectral interferences: the high resolution of the optical system and the CSI software tool. The former makes it possible to spectrally resolve even severe interferences without further mathematical correction schemes, and the latter is able to correct for most difficult interferences by applying a mathematical correction to provide an interference-free result, making the PlasmaQuant 9100 Elite the perfect analytical tool for the analysis of REE in any geological sample.