1、About 50 mg of sample powder was weighed into a Teflon bomb,
2、Moistened with a few drops of ultrapure water, followed by addition of 1 ml HNO3 and 1 ml HF.
3、The sealed bomb was heated at 190°C in oven for >48 h.
4、After cooling, the bomb was opened and evaporated at 115 °C to incipient dryness. This was followed by adding 1ml HNO3 and evaporating to the second round of incipient dryness.
5、The resultant salt was re-dissolved by adding 3 ml of 30% HNO3 and resealed and heated in the bomb at 190°C for >12 h.
6、Finally, the final solution was transferred to a polyethylene bottle and diluted to ~100 g (corresponding to a dilution factor of ~2000) with mixture of 2% HNO3 for ICP-MS analysis.