The effective method i have found for digestion of samples for heavy metal analysis is di-acid digestion using nitric acid and perchloric acid (3:1 ratio)

Hi Khaled:

 For heavy metals analysis on milk, 1.0  gram of sample can be more than enough.  You you can try wet digestion of your samples adding:

- 5 mL nitric acid 65%

- Digest at 90 degrees for 2 hours, using a hot plate.

after these time, you have to check your dissolution, if its not complete clear, it means your digestion is not fully performed.  Then add: 

+  3.0 to 5.0 mL HNO3 65%, for complete digestion of organic matter.

Do not let it to boil / dry.  Once your sample is clear, add 3 mL of H2O2, heat until effervescense end and coolit down.

Add 5.0 mL double DI water and  filter throught 0.7 micrometers,  transfer quantitatively into a  25,0 mL  volumtric flask.

Analysis of blank off course is requeried.  Use of high quality acids and peroxide also is strongly recommended.

Best 

Mario

For milk and Milk products ; wet digestion is widely practiced. Please see the attached articles for reference. (Milk, Cheese, Yogurt, Chocolate milk sample digestion references.)

Anastasio et al (2006)  described a microwave digestion method. A microwave system was used for acid digestion of all the milk and dairy products.  Samples were dried at 70 °C in a forced stove until dry weight. To a dry sample of 0,3 g, finely crushed, were added 6.0 ml of HNO3 (65%) and 1.0 ml of H2O2 (30%). The solution was filtered, deionised water (50 ml) was added and the sample was then stored in PET bottles.  Pilarczyk (2013)  also suggested the same protocol.

for ICP-AES wet  digestion nitric acid and perchloric acid (4:1),stay over night