You can try with aqua regia (HNO3 : HCl = 1:3 volume ration) at room temperature or by applying heat. If the ash is still not dissolved add some distilled water and evaporate the liquid, the once again repeat the procedure of evaporating after adding some distilled water. Be sure that there is not  nitric acid left in the system. Then add some concentrated HClO4, and evaporate. The ash will dissolve. Then once again add distilled water and transfer the liquid into volumetric flask and fill it with distilled water and several drops of nitric acid. f If you have an equipment, most probably the best results will be achieved if you perform a microwave digestion in teflon autoclaves also with aqua regia.

Best regards

Hi there

Depending on the heavy metals you want, there is EPA 3015 and EPA 3051A, just google the methods

I would be worried , however that you ash has already lost many of the volatile heavy metals (Hg, As, Se, etc) 

Hello Gatian,

generally I agree with Sasho's proposal. However, depending on your problem on the one hand you could simplify the method, on the other hand it will be crucial, not to forget to do digestion blinds. Using aqua regis (a.r.) leaches about 95 % of heavy metals from ashes as prepared by you. We used about 0.1 g of ashes. Best crucible material is PFA; normal lab glass might cause problems due to memory effects. Adding  a.r. immediately to the sample can result in material looses due to heavy reaction. So add some distilled water (about 5 mL), add 5 mL HNO3 (65 %; wait) and 15 mL HCl (32 %); leave overnight at room T. Heat up to 95 ºC until near dry . Repeat acid digestion - avoid total dryness (the more repetitions the better the extraction - and the higher the bias by possible acid impurities). Add 20 mL 1N HNO3; stir thoroughly with glass rod and close by cover of 2 sheets of parafilm; leave overnight. Repeat stirring; cover once more (reusing very same cover) and wait until residue is totally settled. Pipette clear supernatant to vessel (possibly PFA) by avoiding residue particles thoroughly. To be on the save side you also could weight your digestion batch (including parafilm cover) after adding 20 mL 1N HNO3  and before pipetting, in order to monitor eventual loss by evaporation. Without loss by evaporation the procedure results in a analysis solution with dilution factor of 200. Do the very same procedure with blinds (possibly in triplicate).

Good luck

D. Zachmann

Thank you very much for your answers!

Agree with Sasho useful remark.