An Improved Procedure for the Determination of Trace Elements in Silicate Rocks Using NH₄HF₂ Digestion

Abstract

Knowledge of the elemental composition of geological samples is of high significance to research in Earth science. For whole‐rock elemental determination, complete sample digestion is essential to obtain bias‐free measurement results. In recent years, NH4HF2 digestion has been proposed for geological sample dissolution; however, this requires 3 h of sample evaporation and 6 h of re‐dissolution of residues. In this study a PTFE beaker, which can withstand 300 °C, was designed to increase the evaporation temperature, reducing the evaporation time to 15 min. Further investigations found that the insoluble fluoride residues formed during the high‐temperature evaporation stage significantly influenced the recoveries of Sc, Y, rare earth elements and Th. Among the tested strategies to suppress the formation of fluorides, the microwave heating method was the most efficient and robust. The optimised NH4HF2 digestion shortened the sample evaporation and re‐dissolution times from 9 h to 21 min, significantly reducing the time consumed for sample evaporation by a factor of ~ 26. A suite of silicate rock reference materials was analysed. The results for most elements were in agreement with reference values with a measurement bias of < 10%.

Evaluation of the digestion capability of ammonium bifluoride for the determination of major and trace elements in Ti-rich minerals by ICP-MS

“…External calibration was employed to calibrate element contents in all samples, and these external standards contained different amounts of the thirty-seven elements. 24 A group of digested international reference materials (AGV-2, BHVO-2, BCR-2, GSP-2 and RGM-2) were used as a quality check.…”

“…Analytical calibration curves for the thirty-seven elements (1, 10, 25 and 50 μg L −1 ) were prepared by gravimetric serial dilution from certified stock multielement solutions (SPEX Certipre Part # CLMS-2, Agilent Calibration Verification Standard-Part # 8500-6942, Part # 8500-6944 and Part # 8500-6948) in 2% (m m −1 ) HNO 3 + 0.05% (m m −1 ) HF. 24 The In internal standard solution was prepared from a single-element standard solution of 1000 μg g −1 (National Center for Analysis and Testing of Steel Materials, China). The 10 μg L −1 In standard was finally constant between the samples and standards.…”

“…18 Lately, NH 4 HF 2 (or NH 4 F) has become popular for complete digestion of various geological rocks and minerals for the determination of the total elements and isotopic compositions, as it exhibits many advantages including efficiency, safety, simple operation and low blank. 18–24 The improvement in digestion temperature (230 °C for NH 4 HF 2 and 250 °C for NH 4 F) increases the rate of fluorination and shortens the digestion time. 18–20 Importantly, NH 4 HF 2 digestion does not use corrosive and toxic HF acid, and provides a low-pressure and favorable alkaline environment to suppress the volatilization loss of Cl, Br and I.…”

Atomic spectrometry update – a review of advances in environmental analysis

“…For whole rock determinations by acid digestion, complete sample digestion is essential for the accurate determination of trace element mass fractions in geological materials, as highlighted by Meisel et al 217 In recent years, NH 4 HF 2 has been proposed as a “green” alternative to HF-based dissolutions but complete digestion can be time-consuming. Zhang et al 219 designed and manufactured a PTFE digestion vessel with a lid that could be operated safely at 300 °C in a procedure to reduce the time for sample evaporation and redissolution in a NH 4 HF 2 digestion. Microwave heating was employed at the redissolution stage to suppress formation of insoluble fluoride residues during the high-temperature evaporation stage.…”

Data Quality in Geochemical Elemental and Isotopic Analysis

“…Though there have been some significant advances in sample preparation methods using microwave digestion methods, infrared radiation digestions, high-pressure asher digestions, automated sample preparation systems (based on artificial intelligence), and the use of alternate reagents such as ammonium fluoride (NH 4 F) and ammonium bifluoride (NH 4 HF 2 ) instead of hazardous HF over the last three decades, the sample preparation remains one of the biggest challenges and limiting factors in the geochemical analysis [36][37][38]. Despite the fact that there have been several important developments in analytical techniques such as ICP-MS and HR-ICP-MS, the growth in the area of sample decompositions and digestion techniques does not commensurate with the innovations witnessed in analytical techniques.…”