## 4 Conclusions  
In this study, the HClO\({}_{4}\)-inverse aqua regia method improved the digestion of larger masses of metal sulfide samples containing low concentrations of Os.  
1. Three grams of pyrite were completely dissolved with only 12 mL of mixed acids for Re-Os dating (3 mL of HClO\({}_{4}\) and 9 mL of inverse aqua regia).
2. The oxidation of acids may greatly affect the Os signal but has no influence on the equilibrium of isotope exchange between \({}^{185}\)Re and \({}^{190}\)Os spikes within Re and Os samples.
3. The heating temperature has the greatest influence on the equilibrium of isotope exchange.
4. The Os isotope system was not equilibrated between the spike and sample until the heating temperature reached 190 \({}^{\circ}\)C.  
All authors have read and agree to the published version of the manuscript. Conceptualization, B.G.; Methodology, B.G. and W.L.; writing--original draft preparation, B.G.; Writing--review and editing, W.L., L.Z., and J.G.  
This study was financially funded by the National Natural Science General Project (Grant No. 41603016) and the National Key R&D Program of China (Grant No. 2016 YFC0600106).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample Name** & **Re ug/g** & **\({}^{187}\)Os ng/g** & **Age (Ma)** & **Heating Temperature** \\ \hline JDC & \(16.90\pm 0.21\) & \(24.78\pm 0.17\) & \(139.9\pm 2.4\) & \\ JDC & \(17.01\pm 0.13\) & \(24.71\pm 0.15\) & \(138.6\pm 1.9\) & \\ JDC & \(17.09\pm 0.14\) & \(24.93\pm 0.17\) & \(139.2\pm 2.0\) & \\ \hline JDC & \(16.88\pm 0.12\) & \(24.89\pm 0.21\) & \(140.7\pm 2.1\) & \\ JDC & \(16.94\pm 0.14\) & \(24.89\pm 0.20\) & \(140.2\pm 2.1\) & \\ JDC & \(16.95\pm 0.14\) & \(24.62\pm 0.29\) & \(138.5\pm 2.4\) & \\ \hline JDC & \(16.91\pm 0.20\) & \(19.89\pm 0.16\) & \(112.2\pm 1.9\) & \\ JDC & \(17.73\pm 0.21\) & \(20.44\pm 0.25\) & \(110.0\pm 2.2\) & \\ JDC & \(17.64\pm 0.18\) & \(19.50\pm 0.15\) & \(105.5\pm 1.7\) & \\ \hline CV & \(17.39\pm 0.32\) & \(25.46\pm 0.60\) & \(139.6\pm 3.8\) & \\ \hline \hline \end{tabular} CV: certified values. The mean values and uncertainties (2 s) were calculated using ISOPLOT weighted average.  
\end{table}
Table 6: Analytical results for molybdene RM samples JDC at different temperatures.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**AT** & **ST** & **190 \({}^{\circ}\)C** & **180 \({}^{\circ}\)C** \\ \hline
2 h & 201 \({}^{\circ}\)C & 192 \({}^{\circ}\)C & 181 \({}^{\circ}\)C \\ \hline
4 h & 203 \({}^{\circ}\)C & 191 \({}^{\circ}\)C & 180 \({}^{\circ}\)C \\ \hline
6 h & 202 \({}^{\circ}\)C & 190 \({}^{\circ}\)C & 181 \({}^{\circ}\)C \\ \hline
8 h & 200 \({}^{\circ}\)C & 192 \({}^{\circ}\)C & 182 \({}^{\circ}\)C \\ \hline
10 h & 202 \({}^{\circ}\)C & 191 \({}^{\circ}\)C & 180 \({}^{\circ}\)C \\ \hline
12 h & 201 \({}^{\circ}\)C & 191 \({}^{\circ}\)C & 182 \({}^{\circ}\)C \\ \hline
14 h & 201 \({}^{\circ}\)C & 190 \({}^{\circ}\)C & 183 \({}^{\circ}\)C \\ \hline
16 h & 202 \({}^{\circ}\)C & 191 \({}^{\circ}\)C & 180 \({}^{\circ}\)C \\ \hline
18 h & 200 \({}^{\circ}\)C & 193 \({}^{\circ}\)C & 182 \({}^{\circ}\)C \\ \hline
20 h & 200 \({}^{\circ}\)C & 190 \({}^{\circ}\)C & 181 \({}^{\circ}\)C \\ \hline
22 h & 202 \({}^{\circ}\)C & 192 \({}^{\circ}\)C & 183 \({}^{\circ}\)C \\ \hline
24 h & 200 \({}^{\circ}\)C & 192 \({}^{\circ}\)C & 181 \({}^{\circ}\)C \\ \hline \hline \end{tabular}
\end{table}
Table 5: The stability of different temperature of the oven was monitored during 24 h.  
**Acknowledgments:** This study was financially supported by the National Natural Science General Project (Grant No. 41603016) and the National Key R&D Program of China (Grant No. 2016 YFC0600106).  
**Conflicts of Interest:** The authors declare no conflict of interest.