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The first action is to identify the expected sizes/molecular weights of the proteins you want to capture. From this you should choose an ultrafiltration devices that has a MWCO close to 1/3 the size of the protein. E.g. for a 100kDa protein choose a 30K MWCO, for a 50kDa protein try using a 10K MWCO. Like with the last example, if you have to pick a MWCO closest to 1/3 the size of the target, choose the lower option. As otherwise the membrane may be too open and let some target protein through. Small molecules will then pass through whilst the protein is retained. However it’s important to note that in picking a 30K membrane, it does not mean that 100% of molecules that are <30kDa will pass through. A good proportion of 25kDa proteins will be capture, less so for 20kDa and so on. For accurate separation with ultrafiltration there must be a 10x size difference between the molecules. In this instance you could be reasonably sure that most molecules <3kDa in size will pass through.