Simple ultrafiltration apparatus.

All ultrafiltration devices have in common a semipermeable membrane and a means for creating a pressure difference across it. They differ only in the configuration of the membrane, the nature of the force used to create a pressure difference, and the ease with which the environment of the plasma can be controlled so as to simulate physiological conditions. Speed of producing ultrafiltrate provides a poor criterion of performance since most devices can be sealed up to give any desired flow rate. The limiting factor in practice is usually sample size. The apparatus described here permits the ultrafiltration of volumes of plasma down to about 1-5 ml. with the formation of ultrafiltrate equal to half the volume of plasma within 15 min. under controlled conditions of temperature and pH. The principle of the apparatus is as follows. Plasma is separated anaerobically and transferred to an all-glass syringe from which it is forced under a pressure of 4 atmospheres into a cellophane sac. The sac is invaginated in order to provide a high ratio of surface area to capacity and is supported on the outside by a spiral of stainless steel to prevent rupture.