Accurate determination of the elemental composition and content of geological samples is the basis for conducting geoscience research. In the analysis of geological samples, the complete digestion of the sample is an essential prerequisite for obtaining high-quality measurement results. At present, conventional trace element analysis uses high-temperature and high-pressure steel jacket digestion technology to ensure the full digestion of the sample, however, it is often difficult to completely digest some rocks rich in refractory minerals such as spinel, zircon, chromite and rutile. Moreover, even with a long digestion time (4-5 days), the desired digestion effect cannot be obtained.
In response to the above problems, Yanhong Liu, senior engineer of the Laboratory of Rock and Mineral Sampling and Analysis of the Institute of Geology and Geophysics, Chinese Academy of Sciences, and her collaborators systematically studied the influence of particle size changes on the trace element analysis results of typical difficult-to-digest geological samples (peridotite and granitic rocks) instead of focusing on the traditional idea of changing the parameters of the digestion experiment (such as temperature, pressure or time), but focusing on the particle size of the sample powder (Fig. 1).
Fig.1 Ultrafine powder is used for trace element analysis in geological samples
The results show that the initial 200 mesh (<74 μm) peridotite sample powder can be ground to 800 mesh (< 20 μm) in 6 minutes when ultrafine grinding with tungsten carbide with a diameter of 3 mm can be used. Using this 800 mesh ultrafine powder for steel jacket digestion, the full digestion of peridotite can be achieved in 2 hours, and no insoluble fluoride precipitation is observed. In addition, the results of the digestion of elements such as Cr, Zr, and Hf from ultrafine powders were improved by 13%–48% due to the full digestion of refractory minerals such as spinel, chromite, and zircon compared to the digestion results of 2-hour 200 mesh powder (Figures 1 and 2).
This new technology improves the quality of analytical elemental data while increasing digestion efficiency by a factor of 60 (2 hours versus 120 hours). Although the method is based on trace element analysis, it has the potential to be used in other geological analyses (e.g., isotope analysis) or in other solid sample decomposition techniques (e.g., melting, sintering, combustion, and ashing).
Fig.2. Relative deviation of the digestion results of initial powder and ultrafine powder of granite samples
研究成果发表于国际分析化学期刊Analytical Chemistry（刘艳红，郭顺，李文君，薛丁帅，李潮峰，万博. Rapid and Complete Digestion of Refractory Geological Samples Using Ultrafine Powder for Accurate Analyses of Trace Elements[J]. Analytical Chemistry, 2024. DOI：10.1021/acs.analchem.3c05888）。 研究受先导B专项（XDB0710000）和国家自然科学基金（42073022和41703021）共同资助。
Editor: Fu Shixu (East China Normal University)
Proofreading: Wan Peng