Please see this link Introduction to Sample Preparation:
http://www.ivstandards.com/tech/reliability/part04.asp#topLook at the menu on the right and choose Acid digestion of Organic Samples.
I have also attached 2 pdf documents that discuss Wet/Microwave Digestion..
You might find the reference useful:
Source: Nutrition & Food Science, Volume 95, Number 1, 1995 , pp. 28-30(3)
Publisher: Emerald Group Publishing Limited
Abstract:
Provides an overview of the analytical methods used to quantify micronutrients (metals, non-metals and vitamins) present in food. Considers the principles of the methods of analysis employed.
Keywords: Diet; Nutrition
Finally, I am attaching Varian Application Note 33: Rapid measurement of major, minor and trace elements in plant and food material (ICP-OES method). This aplication note references EPA Method 3051A which I have also attaced.