A method for purifying nucleic acids or proteins comprising a plurality of piston pumps; and a plurality of nozzles capable of having a plurality of disposable tips which are automatically attachabledetachable. The absorbent structure made of said layers has been compressed to a thickness which is substantially the same over the width of the structure, in such a way that the structure has a higher density in the areas thereof where the layers overlap each other and a lower density in other areas. Preferably, the pod comprises a secondary pod disposed in the primary pod defining a second enclosure for containing the particulate sensitive substrate.