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Centrifugal filters for SW 25.1 and 50.1 swinging-bucket ultracentrifuge rotors have been tested up to the maximum speeds allowed, 90,000 and 300,000 g , respectively. The filters are 1 and 0.5 in. in diameter and accept standard 25-mm polycarbonate filter membranes. The filter membranes are both cup-shaped to prevent loss of particulates to the support materials of the filters. The filter for the SW 25.1 rotor can take 0.75 ml and that of the 50.1 head 0.5 ml. The fluid retained after centrifuging consists of the fluid on the filter membrane and in its pores and that retained by the material filtered. The calculated volume of the pores of the 0.2-μm filter was 0.46 μl. Total liquid retentions of about 0.8 μl have been achieved with both filters using a particulate concentration of 0.5 mg/ml.

A filter assay was developed for the specific binding of labeled ligands to the acetylcholine receptor in an unpurified suspension of membrane fragments from Torpedo californica . Binding to the filter membrane and the standard deviation of replicates were studied, and it was possible to reduce relative standard deviation to about 3% by accounting for the variable weight of the particles dispensed, stopping the loss of particles to the filter apparatus, regulating the transmembrane pressure during filtration, controlling the time the filter remains subject to suction after all the liquid has passed through the filter, and correcting for variable counting efficiency. A method for dispensing equal aliquots of suspended particulates is also described.

An improved ultracentrifugal filter for the Beckman SW 50.1 rotor has been tested to 300,000g. The filter is 0.5 in. in diameter, accepts standard 25-mm polycarbonate filter membranes and filters 0.5 ml of solution.