be employed such unit having a molecular weight pore size rating of less than about 180 kd molecular weight or about 12 nm the nonionic surfactant or detergents which may be used in the present invention include the nonionic polyoxyethylene detergents for example the polysorbates tweens vinyl polymers pluronics polyoxyethylene polypropylene polymers or co polymers brij sterox aj and tritons most preferred is polyoxyethylene sorbitan monooleate polysorbate 80 tween 80 the buffer employed in the invention is selected from any suitable low ph low conductivity buffer such as phosphate buffers citrate buffers borate buffers acetate buffers and glycine buffers at a ph of about 5 0 the buffer is employed to maintain the ph below 6 and reduce aggregation of the protein thereby allowing more efficient flow through the nanofilter preferably a buffer with a low ionic strength of 50 mm u00b1 u2212 20 is employed preferably a sodium acetate buffer ph 5 0 the method involves transferring the protein of interest into a low ph ph 5 0 6 0 low conductivity buffer 10 20 ms cm containing a non ionic detergent such as tween 80 at a concentration of 0 01 and sucrose at a concentration of between 5 and 10 w v the tangential flow apparatus is in fluid communication with several other vessels a product tank a buffer tank and a feed recirculation tank equipped with an agitator the relationship of these vessels and the fluid flow between.