## 3 Results and Discussion  
### Influence of HClO\({}_{4}\) Addition on Sample Digestion  
According to previous studies, 1 g of pyrite is usually digested in 20 mL of acids via the inverse aqua regia method [18; 19; 20; 21]. Therefore, a series of experiments were conducted on the basis of this condition. The Os signal was chosen as an indicator to reflect the strength of the dissolving capacity of acids. Considering that the spike is composed of \({}^{190}\)Os and \({}^{185}\)Re, whereas the \({}^{187}\)Os signal is all from the sample, the \({}^{187}\)Os signal was used to reflect the strength of the dissolving capacity of acids. The \({}^{187}\)Os signal was \(3.0\times 10^{4}\) cps from 1 g of pyrite (Figure 1) when the volume of acid was 20 mL using the inverse aqua regia method. The \({}^{187}\)Os signal was \(3.5\times 10^{4}\) cps as the sample weight increased to 2 g. The signal did not increase linearly with increasing sample weight. When 3 g of sample weight was used, the \({}^{187}\)Os signal was only about \(1.2\times 10^{3}\) cps. These results suggest that it was not possible to dissolve more than 1 g of pyrite in 20 mL of acids with the inverse aqua regia method. The \({}^{187}\)Os signal increased significantly with the HClO\({}_{4}\)-inverse aqua regia method indicating that the dissolving capacity of acids was much stronger than that of the inverse aqua regia method. After 1 mL of HClO\({}_{4}\) was added to 19 mL of inverse aqua regia, the \({}^{187}\)Os signal reached \(5.2\times 10^{4}\) cps, which was 1.7 times higher than that of the inverse aqua regia method. The \({}^{187}\)Os signal did not increase as the sample weight increased from 1 g to 3 g. The signal was about 75% of the intensity of the theoretical maximum, and no more than \(1.0\times 10^{4}\) cps of Os signal was obtained when the sample weight reached 3 g. When the volume of HClO\({}_{4}\) was 2 mL, the \({}^{187}\)Os signal from 2 g of pyrite was two-fold higher than that from 1 g of pyrite. However, the signal dropped obviously as the sample weight increased to 3 g. This result suggests that 2 g of pyrite could be completely digested via the HClO\({}_{4}\)-inverse aqua regia method, but the dissolving capacity of acids was too weak to dissolve 3 g of pyrite. When the volume of HClO\({}_{4}\) was 3 mL, we found a positive correlation between the sample weight and Os signal indicating that the dissolving capacity of acids was strong enough to dissolve 3 g of pyrite in total. When the sample weight was at/above 4 g or the volume of HClO\({}_{4}\) was increased to 4 mL, the hazard of the Carius tube exploding obviously increased. Therefore, we recommend that  
Figure 1: Os (**A**) and Re (**B**) signal for different masses of pyrite obtained using the inverse aqua regia and HClO4-inverse aqua regia methods. the maximum desirable sample weight is 3 g for 20 mL of acids, and no more than 3 mL of HClO4 is added to inverse aqua regia. In comparing the inverse aqua regia method with the HClO4-inverse aqua regia method, the addition of HClO4 could improve the dissolving capacity of acids significantly leading to an increased weight of the dissolved sample. However, gas would be formed because of the reaction between the pyrite and acids, and significantly more gas would be formed with an increased volume of acid and sample weight. Increased gas production could lead to higher pressures inside the tube and potential explosions [26]. To reduce this risk, it was necessary to find the minimal volume of digestion medium. When the maximum sample weight was fixed at 3 g and the addition of HClO4 was 3 mL, the \({}^{187}\)Os signal was nearly unchanged when the volume of inverse aqua regia decreased from 17 mL to 9 mL (Figure 2). These data indicate that the dissolving capacity of acids was still strong enough to completely dissolve pyrite. The \({}^{187}\)Os signal decreased to \(1.1\times 10^{5}\) cps when the volume of inverse aqua regia was 7 mL.