Reassessment of HF/HNO3 Decomposition Capability in the High-Pressure Digestion of Felsic Rocks for Multi-Element Determination by ICP-MS

Abstract

Complete dissolution is essential to obtain accurate analytical results for geological samples. Felsic rocks are known to be very difficult to dissolve because of the presence of refractory minerals such as zircon. In this study, we undertook a systematic evaluation of the effect of the HF/HNO3 ratio, digestion time, digestion temperature, digested test portion mass and the presence of insoluble fluorides on analytical results for the felsic rock GSP‐2 using high‐pressure HF and HF/HNO3 digestion. Digestion in mixtures of HF and HNO3 acids is a commonly used method of dissolution for geological samples. However, our results clearly indicate that adding HNO3 inhibited the digestion capabilities of HF for refractory minerals such as zircon. It took 8–12 hr for Zr to be completely recovered in GSP‐2 at 190 °C, whereas it needed about 36 and 72 hr at 160 and 140 °C, respectively. White precipitates were observed in the final solution for test portion mass > 100 mg, irrespective of which of the five different digestion solutions was used (1 ml HF, 2 ml HF, 1 ml HF + 0.5 ml HNO3, 1 ml HF + 1 ml HNO3 and 1.5 ml HF + 1.5 ml HNO3). Environmental scanning electron microscopy showed that these precipitates were mainly composed of AlF3. Instead of further HCl, HNO3 or HClO4 attack, we propose that using ultra‐fine samples and a small sample size is a good way to avoid the formation of insoluble residues (e.g., fluorides). To further investigate the precision and accuracy of the proposed method (using HF alone as the digestion solution during the first acid attack step), a suite of silicate rock reference materials was analysed. Most of the results were found to be in reasonable agreement with the reference values, with a relative error of < 10%.

Simple and cost-effective methods for precise analysis of trace element abundances in geological materials with ICP-MS

“…To achieve rapid and cost-effective digestion, the effect of different acids was also investigated. As seen from Figure 3, 0.5 ml HF in combination with 1ml Lefort aqua regia show the same efficiency with 1ml HF + 1ml Lefort aqua regia and 1 ml HNO3 + 0.5 ml HF for GSP-2 using bomb at 190 °C for 15 h. However, it is suggested that 18 h are needed for complete digestion of GSP-2 at above condition using 1 ml HNO3 + 1 ml HF [12]. In this case, we choose to use the acid of 1ml Lefort aqua regia +0.…”

“…5 Besides, its high Si content can also cause complex isobaric interferences [3], plus causing significant instrumental drift during analysis due to silicate salt deposits in the interiors of the sample/skimmer cones [9]. In this case, the high-pressure digestion (bomb) method remains the method of choice, especially when applied to coarse grained granitic and metamorphic rocks [10][11][12].…”

“…We predict that the byproducts may not be simple fluoride such as AlF3 [12], but likely more than one complex. Our experiments show that the byproducts formed during the high-pressure bomb digestion for 50 mg sample can be readily eliminated by re-dissolution under high pressure after evaporation.…”

“…The degree of drift differs from one mass to the next, and the direction of drift can change frequently [1,28]. Several methods are usually used to reduce the drift interference, single or multiple internal standards, standard additions, isotope dilution, external standard, and external calibration or a combine of several of the above [1,12,16,28,29]. Given the complex nature of instrumental drift as illustrated in Figure 8, it would be difficult to monitor and correct for the drift by using single or even several internal standards [1,30].…”

“…More recently, a mixture of NH4F-HNO3 [3] or HF alone [12] is also used. It is suggested that the addition of HClO4 is more effective to attack refractory minerals, such as those of the spinel group, and helps remove fluorides [7,17].…”