Quantitative analysis of major and trace elements in NH4HF2-modified silicate rock powders by laser ablation - inductively coupled plasma mass spectrometry

- A new rock sample preparation for LA-ICP-MS bulk analysis by NH4HF2 digestion is developed.
- Rock powder after digestion by NH4HF2 becomes an ultrafine powder (d80 < 8.5 μm).
- Refractory minerals can be dissolved, and the loss of volatile elements is suppressed.
- Forty-five major and trace elements in various geological samples can be determined simultaneously by LA-ICP-MS.

In this paper, we described a NH4HF2 digestion method as sample preparation for the rapid determination of major and trace elements in silicate rocks using laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS). Sample powders digested by NH4HF2 at 230 °C for 3 h form ultrafine powders with a typical grain size d80 < 8.5 μm, and various silicate rocks have a consistent grain morphology and size, allowing us to produce pressed powder pellets that have excellent cohesion and homogeneity suitable for laser ablation micro-analysis without the addition of binder. The influences of the digestion parameters were investigated and optimized, including the evaporation stage of removing residual NH4HF2, sample homogenization, selection of the digestion vessel and calibration strategy of quantitative analysis. The optimized NH4HF2 digestion method was applied to dissolve six silicate rock reference materials (BCR-2, BHVO-2, AGV-2, RGM-2, GSP-2, GSR-1) covering a wide range of rock types. Ten major elements and thirty-five trace elements were simultaneously analyzed by LA-ICP-MS. The analytical results of the six reference materials generally agreed with the recommended values, with discrepancies of less than 10% for most elements. The analytical precision is within 5% for most major elements and within 10% for most trace elements. Compared with previous methods of LA-ICP-MS bulk analysis, our method enables the complete dissolution of refractory minerals, such as zircon, in intermediate-acidic intrusive rocks and limits contamination as well as the loss of volatile elements. Moreover, there are many advantages for the new technique, including reducing matrix effects between reference materials and samples, spiking the internal standard simply and feasibly and sample batch processing. The applicability filed of the new technique in this study was focused on the whole-rock analysis of igneous rock samples, which are from basic rocks to acid rocks (45% < SiO2 < 73%). However, we thought that the NH4HF2 digestion method can be used as a new alternative in LA-ICP-MS for a wider range of geological samples, and will significantly accelerate the application of LA-ICP-MS for the whole-rock analysis.

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