There are many filter sizes on the market which can be confusing, and this question is often asked

Background

The amount of sample which can be passed through a filter is dependent on a number of factors, the surface area of the membrane only being one of those. The way the membrane is supported and the design of the housing play a big part in the even distribution and filtration ability of the membrane.

Some filter manufacturers will specify the diameter of the housing as the filter size, when in reality, the membrane size might be quite different. Thus, a filter of 30mm or 33mm diamter can sometimes perform less well than a 25mm filter utilising a better design.

Cronus filters are not only certified for HPLC but are guaranteed to have the largest surface area available for filtration for their specified si\e, and are designed to maximise the available throughput. So... which one to use?

HPLC Samples

The majority of samples filtered by syringe filters are sample prior to analysis by an HPLC system. An HPLC vial has a maximum usable volume of 1.8mls typically and should be filled with approx 1ml maximum of sample.

Assuming a moderately clean sample, syringe filters can filter the following approximate volumes of sample:

4mm: 1-2ml

13mm: 1-10ml (could go as high as 50ml)

25mm: 1-50ml (could go as high as 100ml)

However, the higher particulate level in the sample, the lower the volume which can be filtered.

But, for the majority of HPLC samples, a 4mm or 13mm filter is not only good enough, but also less expensive than using a 25mm filter. Instead of trying to save money by moving to a cheaper, less reliable filter, it makes more sense to move to a smaller size filter, save money, and still achieve the performance.

So why do so many people use 25mm or 30mm filters? The simple answer is that they are often just easier to use. If you have plenty of sample then filtering enough for an HPLC sample is really easy, even for higher particulate samples. The available membrane area means that the sample is easy to push through and the filter is less lilley to block up. For really difficult samples, then the addition of a glass fibre pre-filter can help a lot as well, and these are only available in larger filter sizes.