Alkaline Digestion for Cr(VI)

I'm trying to digest a sand sample in order to test the adsorption of hexavalent chromium. I was previously using microwave digestion, using acid digestion method (1:3 HNO3:HCl). I have afterwards read several journals that state that acid digestion is for total chromium. Somehow they suggest alkaline digestion for digesting the sand sample for hexavalent chromium. However, this method uses a hot plate. Is there any way to achieve alkaline digestion using microwave digestion?