Retention by Charged Membranes

The most significant claim is for the removal of pyrogens (endotoxins). Table 2.6 17 compares the pyrogen removal efficiency for a positively charged nylon 0.2 ju pore size membrane, Zetapor , and a conventional cellulosic 0.22 ju membrane. Normally, a 10,000 molecular weight cut-off UF membrane is required to remove pyrogens. (These membranes have an equivalent pore-size of 30 A or 0.003 ju). As expected, the conventional MF membrane shows no retention whatsoever, but the positively charged membrane shows better than 97% retention. 

Filter Area 3.9 cm2 Endotoxin Concentration Before Filtration pg/ml Endotoxin Concentration in Filtrate pg/ml  

Procedure E. cott purified endotoxin was added to 0.9 NaCI solution at pH 6.7 and passed through filters contained in a 25mm filter holder. 

It is also claimed that positively charged 1.2 ju pore size MF membranes will retain Pseudomonas diminuta. Data from Pall Corp.18 on their 1.2 ju Nw Posidyne membrane show better than 99.99% retention of Pseudomonas. Their conventional 1.2 ju ULTlPOR N66TM membrane (without the charge) retained less than 50% of the Pseudomonas. In both cases, challenges between 2 x 1010 and 7 x 1012 bacteria per square foot of filter area were used. This means that comparable removal of Pseudomonas can be achieved with more than 10 times the flow rate of a 0.2 ju pore-size membrane. 

Further, if all of the particles retained are considerably smaller than the rated pore size (say less than 25%) and no larger particles are present, the membrane does not clog even with the processing of very high volumes. The small particles do not bridge and cause only a slight change in the pore diameter. 

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