When conducting routine sample filtration, the compatibility of filter materials is often neglected. Researchers frequently choose filter paper or devices for their convenience, only revisiting their decision after encountering a failure or needing to troubleshoot an unexpected outcome.
There is a broad selection of filter membrane materials accessible, including glass and both natural and synthetic polymers, each possessing distinct characteristics that can make them suitable for nearly any sample type.
By understanding these characteristics and adopting a proactive approach to filtration and membrane-sample compatibility, researchers can significantly reduce troubleshooting time and enhance filtration efficiency.
Differences Between Hydrophilic and Hydrophobic Membranes
Hydrophobic membranes, such as polytetrafluoroethylene (PTFE), resist aqueous samples, which leads to back pressure. Although it is sometimes possible to counter this back pressure with increased force, this method carries the risk of membrane rupture and incomplete filtration.
If no alternatives exist, pre-wetting the membrane with an alcohol may help alleviate back-pressure issues.
PTFE and similar hydrophobic materials are ideal for organic samples and solvents, as they present no resistance or back pressure. However, some organic solvents can permeate the membrane material, especially with extended contact.
This absorption causes swelling in the material, thereby reducing pore size and impacting filter performance. Additionally, some solvents may chemically attack the membrane, releasing extractables into the filtrate. In rare instances, a solvent can partially or completely dissolve the membrane, leading to breakthrough and potential contamination of the sample.
In contrast, aqueous samples are typically safe for most membrane materials, especially hydrophilic options. However, pH is a crucial factor in membrane compatibility.
Strongly acidic or alkaline solvents may not damage a membrane immediately but can have long-term effects. Therefore, only highly inert membranes like PTFE are recommended for extreme pH samples.
Depth Filtration
Filters are generally categorized into two types based on particle retention: surface filters and depth filters. Surface filters, commonly referred to as membranes, capture particles only on their surface. They are best for samples with low particulate content. In contrast, samples with high particulate loads can quickly clog these filters.
Attempting to push a high-particulate sample through a surface filter like track-etched polyester can lead to back-pressure build-up and potential breakthrough if sufficient force is applied. Depth filters, however, are designed for high-particulate applications and capture particles within their fiber matrix.
Asymmetric depth filters, such as those made from polyethersulfone (PES), feature a coarser structure at the top and a denser matrix at the bottom. This porosity gradient initially captures larger particles, acting as a pre-filter for the finer material below and maintaining fluid flow.
For challenging samples with high particulate content, like soil samples, non-woven matrix filters are ideal for robust fine filtration. Non-woven polypropylene (NWPP), for example, offers durability and a high capacity for particulates.
These non-woven matrices usually consist of thick pads with layered designs to reduce clogging. Other options for non-woven matrix filters include glass fiber and cellulose.
Protein Binding and Sample Extractables
Beyond clogging and resistance, membrane-sample compatibility influences the composition of the filtrate. Incompatibility can result in the unexpected retention of sample solutes by the filter (protein binding) or undesirable solutes being leached into the sample (extractables) from the filter material or its housing.
Some hydrophilic materials, like nylon (NYL) and cellulose nitrate (CN), exhibit a high capacity for binding proteins. This trait makes them unsuitable for applications involving protein recovery and analysis, where their use can yield inconsistent or unexpected outcomes.
Conversely, regenerated cellulose (RC) and cellulose acetate (CA) have minimal protein-binding properties, making them appropriate for filtering protein-rich solutions. RC also demonstrates broad solvent compatibility. Together with PTFE, RC is a valuable general-purpose option to keep available.
Extractables commonly indicate membrane-sample incompatibility and can interfere with 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 often used in HPLC, while maintaining low levels of extractables.
We have developed compatibility tables that outline material resistance to common solvents to serve as quick reference tools. Selecting filter materials based on compatibility reduces the risk of problems such as slow or ineffective filtration and contamination from extractables, while maximizing filtration efficiency.
