Assessment of Dissolution of Silicate Rock Reference Materials with Ammonium Bifluoride and Nitric Acid in a Microwave Oven

Abstract

The complete dissolution of representative test portions of powdered rock samples for the determination of the mass fractions of trace elements by ICP-MS relies either on aggressive and time-consuming acid digestions or fusion/sintering with appropriate fluxes. Here, we evaluate a microwave oven dissolution method that employs a solution of NH 4 HF 2 and HNO 3 . The entire procedure occurs in a closed vessel system and takes up to 4 h. In hundreds of digestions, the precipitation of fluorides was never observed. Replicate decomposition of 100 mg of twenty-one international reference materials (RMs) of igneous rocks, and also one of a shale presented mostly satisfactory recoveries of forty-one trace elements. Important exceptions were Zr and Hf in G-2 and GSP-2 (mean recoveries of ca. 70%), although for four other felsic rock RMs, the digestion was complete. For ultramafic rock RMs, we present Cr results that indicate quantitative dissolution of Cr-bearing phases. We discuss the findings and conclude that advances in sample preparation of geological materials for instrumental analysis would benefit from a better understanding of how specific characteristics, such as composition and crystallinity of certain minerals, may affect their reactivity.

Rapid decomposition of geological samples by ammonium bifluoride (NH4HF2) for combined Hf‐Nd‐Sr isotope analyses

“…Both ammonium fluoride (NH 4 F) and ammonium acid fluorides (NH 4 F· x HF with x = 1, 1.5 or 2) were found to be effective in breaking down solid inorganic matrices and refractory mineral separates 18–21 . Ammonium bifluoride (ABF, NH 4 F·HF or NH 4 HF 2 ) dissolution at elevated temperature has recently been introduced and systematically investigated in decomposition of geological samples 22–24 and demonstrated to be successful in breaking down granodiorites with excellent recoveries of Zr and Hf 22 …”

Hydrofluoric Acid‐Free Digestion of Geological Samples for the Quantification of Rare Earth Elements by Inductively Coupled Plasma‐Optical Emission Spectrometry

“…As a substitute for HF in the digestions of silicate rock samples, some authors proposed the use of NH 4 F (Hu et al , ) and NH 4 HF 2 (Zhang et al , Magaldi et al ) for the digestion of igneous rocks, shales (Hu et al , , Zhang et al , Magaldi et al ) and soils (Hu et al ). After the digestion, ICP‐MS was used for the quantification of various elements, including REEs.…”

“…After the digestion, ICP‐MS was used for the quantification of various elements, including REEs. Although good recoveries were found for most of the elements (Hu et al , , Zhang et al , Magaldi et al ), and fluorides precipitation was not observed in most of the works (Zhang et al , Hu et al , Magaldi et al ), the methods still need of a safe handling of ammonium fluoride salts when dissolved in water.…”

“…However, these methods may present certain disadvantages or may not be employed for all types of samples. The repeated additions of HNO 3 (Eggins et al 1997, Yokoyama et al 1999, Yu et al 2001 or HClO 4 (Takei et al 2001, Hu et al 2013, Magaldi et al 2018) and HF evaporation steps may be time-consuming (Pinto et al 2012), besides the disadvantage of exposure to hazardous HF vapours. Furthermore, this method cannot be employed for samples containing (Mg + Ca)/Al ratios < 1, because of the trace element co-precipitation with AlF 3 (Takei et al 2001).…”

“…Furthermore, this method cannot be employed for samples containing (Mg + Ca)/Al ratios < 1, because of the trace element co‐precipitation with AlF 3 (Takei et al ). Moreover, adding Mg to reach (Mg + Ca)/Al ratios ˃ 1 in order to avoid AlF 3 precipitation is laborious in routine analyses of geological samples (Hu et al , Chen et al , Magaldi et al ). The recommended use of small quantities of samples to prevent AlF 3 precipitation during the digestion with HF does not apply for all samples.…”