Dear Surajit,

Concerning soils samples, I dissolve samples in a 3:1:0.5 mixture of

concentrated HF, HNO3, and HClO4. You might find more details on Major and trace element analyses in the methods section of this paper (Geochemical investigation of a sediment core from the Trajan basin at Portus, the harbor of ancient Rome):

https://www.researchgate.net/publication/260028950_Geochemical_investigation_of_a_sediment_core_from_the_Trajan_basin_at_Portus_the_harbor_of_ancient_Rome

Best regards,

Hugo

Article Geochemical investigation of a sediment core from the Trajan...

 You should try dissolve rocks samples in 1:3 relation of HNO3 and HCL this mixture is usually called "Agua Regia" in souht America, is useful cause is posible dissolve noble metal.