Dear Mohammad, 

This is easily done. If the buffy coat is only to be used for DNA,  simply centrifuge an EDTA whole blood which will leave you with a clear buffy coat sitting on the red cells that can be harvested for DNA extraction (we use simple pasteur pipettes for this). You will be left with ample EDTA plasma for biobanking. Beware though, in inexperienced hands the buffy coat can be drawn up and transferred into the plasma storage, so make sure your SOP states to isolate the buffy coat first!

Also EDTA plasma is not ideal for all analytes as it chelates divalent cations e.g. calcium, magnesium, iron etc so you may need to give this some thought.

BW

Tim 

Hi Mohammad, just to add to Tim's answer, if you are going to freeze buffy coats and plasmas, then Ficoll isolation might a way to go.