MicroSpin S-200, S-300 and S-400 HR spin columns contain Sephacryl resin of differing pore sizes. They allow DNA purification by the process of gel filtration. Molecules larger than the largest pores in the Sephacryl are excluded from the gel and elute first. Intermediate size molecules penetrate the matrix to varying extents, depending on their size and the resin used. Penetration of the matrix retards progress through the column; very small molecules elute last. The volume required to elute these small molecules is dependent on the volume available both inside and outside the pores i.e. the bed volume.