Millipore produces a diverse collection of syringe-driven filter units – the Millex™ range. The devices are plastic disposable filters fitted with female Luer-Lok™ inlets and Luer-slip outlets. The filters are available in four different sizes (4mm, 13mm, 25mm and 33mm) and a range of different membrane types are available (PVDF, PES, cellulose, glass fibre, PTFE and Nylon). Of course, many pore sizes are available from pre-filtration glass fibre and 5μm filters, through intermediate 0.8μm and 0.45μm filters and right down to 0.2μm and 0.1μm filters.
The membranes provide a range of applications. Prefiltration can be achieved using borosilicate glass filters. Cellulose filters are provided for generic clarification purposes and small particle removal. For applications where protein recovery is important, very low protein binding PVDF membranes are available in all filter sizes. Other low-protein binding filters are composed from PES to allow faster flow rates than PVDF. Lastly, Nylon and PTFE membranes are provided for the filtration of either aqueous or organic solvents. The PTFE membranes are available in either hydrophilic or hydrophobic form, the latter useful for filtration of hydrophobic substances, the sterilisation of gasses and the exclusion of aqueous substances (i.e. attachment to pipettors).
Several specialised variants of the standard filter units are available. The HPLC certified filters have been specially designed and tested to remove any extractable materials with may interfere with UV absorbance measurements of the filtrate (standards and test methods are available from Millipore). There is also the Millex range designed for low levels of extractables that may interfere with an/ion exchange chromatography. Again, these are manufactured and tested to standards (available from Millipore). The third category of specialist Millex filters is the Medical Millex range which is certified to meet USP Class VI Biological tests for plastics, meaning that no toxic products remain in, or will be leeched from, the plastic filters or filter housings.
The retention volumes of these filters is fairly small in comparison to re-usable filter housings. The 33mm, 25mm, 13mm and 4mm filter units retain 200μL, 100μL, 25μL and 10μL respectively, although they require an air-purge to attain this volume. The retention volume becomes much larger when an air-purge is not possible.
The pressure handling of these filter units is quite good, and although it is possible to generate enough pressure with some smaller syringes to damage the filters, it is unlikely to happen when used with care and common sense. The 33mm, 25mm, 13mm and 4mm filters with withstand 150psi (10bar), 75 psi (5bar), 100 psi (7bar), and 200psi (14bar) respectively.
The filters are available in bulk packs of 50, 100, 250 or 1000. The boxes open from the front to form a convenient ‘hopper’ type dispenser. The filter units are available sterile and individually packaged, or unsterilised. Housings are generally of HDPE, PVC, Eastar™ or polypropylene. Only the polypropylene filters may be sterilised by autoclave; the remainder are pre-sterilised by ethylene oxide or gamma irradiation.
The sterile units are individually sealed in paper-backed flexible plastic containers that quickly and easily peel open from the back, revealing the female Luer-Lok™. This allows the user to hold the filter through it’s packaging and attach it to the syringe whilst maintaining sterility of the outlet. The paper backing on each filter contains the information about the enclosed membrane type and filter size. The 33mm (and Medical Millex) filter units are also colour coded according to their membrane type. The manual contained with these units provides useful information on solvent compatibility
Although there is a wide range of sizes, applications, membranes and housings for this filter range, the combinations are not completely comprehensive. The 25mm filter size contains the most comprehensive range and most options are available in these units.
Scott Coutts
Department of Microbiology
MonashUniversity
Australia