Sampling preparation

It depends on the sample matrix that you want to digest. I nearly always use HNO3 for oleochemicals. Si plus HNO3 is probably not going to work.

We need more information. It depends on the sample matrix that you want to digest and which system you have, if is open system like hot block or if is closed system like microwave assisted digestion (and in this case which model of rotor/vessel you have available considering the maximum pressure you can play)..... So, the recipe will depend on the sample matrix and which system options you have available in the laboratory to carry out the method with the best cost benefit.