I intend to use the equipment ICP or AAS...

You can achieve the most quantitative results using an acidic microwave digestion. Depending on the metal concentration in your samples you could use a mixture of 3ml HNO3 (65%), 1 ml HCl (36%) and 1ml HClO4 (70%) for 200 mg of sample, in combination with a 5 min microwave cycle at 500W.

Make sure to respect the order of addition of the acids and do not close the microwave vessels or start the microwave cycle before the reaction is completed at room temperature.

After complete digestion, transfer the acid mixture quantitatively to a 50 ml volumetric flask and you end up with a mixture which can be analysed with ICP or AAS directly.