Self-assembled filter apparatus

For small volumes, 13 or 25 mm diameter, filtration membranes may be fitted into plastic or stainless steel holders (e.g. the Swinnex filter holder made by the Millipore Corp. Appendix 3) which, after autoclaving, are fitted onto a syringe containing the liquid to be sterilised. Care must be taken in the assembling of the membrane in the holder as incorrect assembly leads to the escape of the membrane from its retaining gaskets with subsequent failure of the filtration process. The plastic holders have a limited life time as they distort on autoclaving. 

For volumes up to about 300 ml stainless steel filter holders are available (see below). These take a 47 or 50 mm diameter membrane filter and incorporate an upper chamber which holds either 100 ml 

If it is necessary to refill the dispensing pressure vessel during filtration it is important that the pressure above the membrane filter does not fall or contamination may result. To avoid such a pressure fall valves may be fitted. Foot operated systems are available from Schleicher and Schuell which allow the flow of liquid to be interrupted at regular intervals this is very useful when filling 100 ml bottles from an otherwise continuous flow of 10 1 of sterile medium. However, we have found the system cumbersome to use and we 

Although the Millipore Corporation offer a very wide range of filter holders and accessories, other companies provide valuable competition e.g. Andaman Co., Ltd. (Appendix 3) are agents for Schleicher Schuell who manufacture a comparable range of filters and holders etc., and V.A. Howe supply Sartorius membranes and filter holders. 

Although superficially more expensive, the use of a disposable filter apparatus is far more reliable than the self assembled varieties, which are not recommended for the occasional user. 

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