Apparatus for molecular weight separation

A microdialysis unit for rapidly purifying a biomolecular sample of small volume by removing substantially all low molecular weight molecules and retaining substantially all intermediate molecular weight biomolecules, the microdialysis unit comprising:

• (a) a first stage sample separator having a sample separator having a sample inlet, a separation membrane and a separated sample outlet that is connected to
• (b) a capillary assembly having at least one dialysis tube having opposite ends of a sample inlet and sample outlet attached to a non-porous inlet and outlet;
• (c) a shell surrounding said capillary assembly and forming an annular space therebetween, said non-porous inlet and outlet extending through said shell and extending through said shell and extending beyond said shell, said shell further having a buffer inlet and a buffer outlet;
• and
• (d) a buffer that flows through said buffer inlet into said annular space and out said buffer outlet, said buffer inlet and buffer outlet arranged so that said buffer flows in a counterflow relationship to said biomolecular sample, wherein
• (e) said biomolecular sample is flowed through said first stage sample separator and into said capillary assembly while said buffer is flowed through said shell so that said low molecular weight molecules pass from said biomolecular sample through said dialysis tube and into said buffer, while said biomolecules are retained and passed to said sample outlet.