Hello Stefano, 

the method should be the acid digestion with microwave, but it varies depending on the species of veg you have to analyse.

Here you can find two useful publications about sampling preparation and analysis (in Italian).

Hope it can help, 

Andrea

http://www.iss.it/binary/publ/publi/0345.1109237408.pdf

http://www.arpa.emr.it/cms3/documenti/moniter/risultati/20111202/LP2_Az9_AcquaSuoloPianta.pdf

You can use microwave digestion system, i's better way for sample preparation for ICP-MS.  If you do not have this kind of system I can say how you can make it manually with the use of ordinary microwave oven.   

Thank you for your answers!

I know the microwave digestion of samples, but i don't need to quantify total contents of metal but residual contents, caused by herbicides and other agrochemicals.

Regards

Stefano

If so, dry your samples grind them and do an open acid digestion with nitric acid on a heating plate.

But be careful in open acid method you can lose some amount metals such as Hg, As, Cd. 

I aggree. Avoid drying of the acid. Adding a cooler to the glass flask helps, even a crystal helps.