Filter material compatibility tends to be overlooked in routine sample filtration. Researchers might often select filter paper or devices based on convenience, and only reconsider when there is a failure or troubleshooting an unexpected result.
There is a wide choice of filter membrane materials available, including glass or natural and synthetic polymers, each of which have unique properties making them compatible with just about any sample.
Understanding these properties and taking a preventative—rather than reactive—approach towards filtration and membrane-sample compatibility, can minimize time spent troubleshooting and maximize filtration efficiency.
Differences between hydrophilic membranes and hydrophobic membranes
A hydrophobic membrane, such as polytetrafluoroethylene (PTFE) will resist any aqueous sample, creating back pressure. Although it is sometimes possible to overcome this back pressure with additional force, there is a risk of membrane rupture and incomplete filtration.
Should there be no alternative, pre-wetting the membrane with an alcohol can reduce this back-pressure effect.
PTFE and other hydrophobic materials are well suited to organic samples and solvents, which result in no resistance or back pressure. However, some organic solvents can absorb into the membrane material, especially when in contact for long periods.
This absorption makes the material swell, reducing pore size and affecting the performance of the filter. Some solvents might also chemically attack the material, releasing extractables into the filtrate. In rare cases, a solvent might partly or fully dissolve the membrane, resulting in breakthrough and potential contamination of the sample.
Aqueous samples are unlikely to damage most membrane materials, particularly hydrophilic. However, pH is a significant factor in determining membrane compatibility.
Strongly acidic or alkaline solvents might not immediately damage a membrane but can have an effect over time. As such, only highly inert membranes such as PTFE are suitable for high and low pH samples.
Depth filtration
In terms of particle retention, filters fall into two categories: surface filters and depth filters. Surface filters, generally referred to as membranes, trap particles exclusively on the top surface. These filters are well suited to samples with low particulate content. However, high particulate content tends to rapidly clog the filter surface.
Pushing a high-particulate sample through a surface filter, such as track-etched polyester, is likely to result in back-pressure build-up and possible breakthrough with enough force. Depth filters on the other hand, are well suited to high-particulate applications, trapping particles within their fiber matrix.
Asymmetric depth filters, made of materials such as polyethersulfone (PES), have an open matrix structure at their top, and a finer matrix structure towards the bottom. This gradient of porosity initially traps large particles and acts as a pre-filter for the denser material underneath, maintaining flow.
For difficult-to-filter samples with high particulates, such as soil samples, non-woven matrix filters provide heavy-duty fine filtration. Non-woven polypropylene (NWPP), for example, provides durability and high loading capacity for particulates.
These non-woven matrices are generally thick pads with layered structures to minimize clogging. Other material options for non-woven matrix filters include glass fiber and cellulose.
Protein binding and sample extractables
In addition to resistance and clogging, membrane-sample compatibility affects the composition of the filtrate. An incompatibility here can lead to sample solutes unexpectedly being retained by the filter (protein binding) or unwanted solutes being released into a sample (extractables) from the filter material or housing.
Certain hydrophilic materials, such as nylon (NYL) and cellulose nitrate (CN), provide a high protein binding capacity. This property makes them unsuitable for protein recovery and analysis, where their use might result in inconsistent or unexpected results.
However, regenerated cellulose (RC) and cellulose acetate (CA) bind virtually no protein, making them well suited for filtering protein-containing solutions. RC also has a broad solvent compatibility. Together with PTFE, RC is a useful general-purpose option to have on-hand.
Extractables are a common symptom of membrane-sample incompatibility and affect sensitive downstream analytical techniques, such as ultra-high-performance liquid chromatography (UHPLC) and high-performance liquid chromatography (HPLC). PTFE, polyvinylidene difluoride (PVDF), and RC are compatible with a wide range of solvents commonly used in HPLC while remaining low in extractables.
We have compiled compatibility tables listing material resistance to common solvents, making them useful for quick reference. Basing filter material selection on compatibility minimizes the likelihood of issues such as slow or ineffective filtration, or sample contamination from extractables, and maximizes filtration efficiency.
For advice on filter material properties and compatibility, contact Cytiva support. Alternatively, use the Whatman filter selector to identify a suitable filter material for your application. Click to get in touch with a Cytiva representative or to request a sample visit.