Depth Filtration vs. Surface Membrane Filtration
Whatman are leaders in separation technology. The Whatman range of multiwell devices is centred around an extensive range of filter microplates and associated solid bottom collection/storage plates. In order to specify the most effective filter microplate an understanding of the filtration process is required.
Filter media have different properties that affect the performance of the filter in microplate applications. When selecting a filter media, the following properties are considered.
Depth Filtration vs. Surface Membrane Filtration
A depth filter traps contaminants both within the thickness of the filter and on the surface of the filter. Cellulose filter papers and glass fibres are depth filters with no defined pores, unlike a membrane that is a screen filter with a defined pore structure. Cellulose filter papers and glass fibres consist of a matrix of intertwined fibres in which particulates may be trapped in as well as on the surface of the filter. Since a depth filter does not have defined pores it is characterised not in terms of pore size, but in “particle retention”.
The advantage of a depth filter is that since entrapment and adsorbtion occur within the upper fraction of the media, there is a considerable surface area available for filtration. Also, because of its random matrix of fibres filter paper retains a large percentage of particulate. The disadvantages of a depth filter are that in the case of a sudden surge of differential pressure, as can occur when a vacuum is suddenly applied, the filter media will slough off fibres or particles during the filtration period. Also particulates trapped within the matrix can be forced through the matrix and contaminate the filtrate. In many applications a depth filter is used as a prefilter to clean a sample. A good example is the Whatman glass fibre filter paper GF/B with a thickness of 675 um and mean pore size of 1.3µm.
A screen membrane traps particulates larger than its rated pore size on its surface. Particulates smaller than the specified pore size may either pass through the membrane or may be captured within the membrane. The advantages of a screen membrane are that rigid particulates can form a porous cake on the surface of the screen membrane and effectively improve the throughput of the screen. Also there is little risk of the filtrate being contaminated. The major disadvantage of a screen membrane is that the filtration process is slow compared with a depth filter
Membrane filters are used for critical applications such as sterilising and final filtration. An example is the PVDF membrane with 0.2µm pores size.
A “combination filter” combines different membrane pore sizes or combines depth media and membrane filters to create a serial filtration units. These multi-layer filters can offer an economical alternative to using individual prefilters and final filters.
A double filter membrane can have unique characteristics not achieved by either of the constituent single membranes. The filter membrane must be capable of retaining liquids during the application. Hydrophobic filters may be appropriate. Examples of such situations are:
- Cell capture prior to assay
- Removal of bacterial bio-load
- Retention of precious liquids
Chemical Compatibility is defined as the ability of a filter media to resist select chemicals, to prevent damage to the pore structure and the filter material. This also prevents the shedding of particles or fibres To select the proper filter media and microplate housing, the compatibility of the filter with the fluid must be established. Temperature, concentration, and length of exposure time affect chemical compatibility.
The materials used in the manufacture of filter media are carefully chosen for their resistance to a wide range of chemical solutions. An understanding of the compatibility between the fluid to be filtered and the filter elements is essential.
Hydrophilic vs. Hydrophobic
Hydrophilic filters possess an affinity for water. Hydrophilic filters can be wetted with virtually any liquid and are the preferred filter media for aqueous solutions. Hydrophobic filters repel water. A hydrophobic filter will not wet in water but will wet in low surface tension liquids such as organic solvents. Once a hydrophobic filter has been wetted out by an organic solvent, aqueous solutions will pass through.
Both hydrophobic or hydrophilic GF/C and PVDF filters are available.
The pore size of a filter is the diameter of the particle that is retained by the filter. Pore sizes are measured in micrometers (µm). Pore size ratings refer to the size of the particle or organism retained by the filter. Porosity is the percentage of all of the open spaces (pores) in the membrane. Generally membranes are 50% to 90% open space.
For microplate applications a nominal pore size is quoted. A nominal pore size rating describes the ability of the filter to retain 60 to 98% of the particulates that are equal to or greater than the rated size. Process conditions such as concentration of contaminant have a significant effect on the retention efficiency of filters.
Note: Nominal Pore Size Ratings vary widely among different filter manufacturers.
Some of the more common molecular biology particles filtered and their size are:
- Microbial cells 10µm to 0.3µm
- Viruses 0.3µm to 0.02µm
- Blood cells 8.0µm to 3.5µm
- Yeast’s 4.0µm to 0.6µm
- Proteins 0.5µm to 0.0005µm
Extractables are contaminants that elute from the filter media or device that may adversely affect the quality of the filtrate. These contaminants may include wetting agents in the filter media, manufacturing debris, sterilisation residuals, adhesives or additives in polymer or housing components, colorants, mold release agents, etc.
Polypropylene contains total additives of 0.5% or less. Leaching out of extractables occur at temperatures over 50°C. The type and amount of extractables vary with the type of liquid being filtered. Extractables can affect filtration in almost every application. In cell culture they can kill cells. In microbiological analysis they can affect the recovery of microorganisms. Glass fibre is chemically inert.
Binding is the ability of a substances to interact with the filter media. Binding is desirable when an assay calls for the nucleic acid or protein binding on the filter. For example the natural hydrophobic PVDF membrane has high molecular DNA or protein binding. Other high binding filters are cellulose nitrate and nylon. However, binding can also be undesirable if proteins indiscriminately bind to a filter during the filtration of a proteinaceous solutions This can result in the loss of active ingredients during filtration. Hydrophilic PVDF has low protein binding. Other low binding filters are polypropylene and cellulose acetate
In addition to the generic ability to bind, the membrane must have the capability of immobilising the component of choice specifically without interference from others.
The membrane should immobilise the molecule in such a manner that orientation and confirmation result in optimal biomolecular activity.
Thermal Stability is the maximum temperature that the filter media and microplate remain stable. Thermal stability is important when considering sterilisation by autoclaving Polystyrene cannot be autoclaved but polypropylene can. There is a relationship between chemical compatibility and thermal stability. Many types of filter media are compatible with a chemical at room temperature, but not at elevated temperatures. Most filter media are stable up to 100°C when exposed to aqueous solutions. If organic solvents are used the max. temperature could be as low as 50°C.
Flow rate and throughput are two important performance factors that are affected by the following variables.
This is the difference between the pressure on each side of the filter. A high pressure on one side forces the filtrate through the filter to the lower pressure on the other side. This occurs during filtration in a vacuum manifold. As the filter begins to clog, differential pressure increases.
This determines a liquid’s resistance to flow. The higher the viscosity of a liquid (at a constant temperature and pressure), the lower the flow rate and the higher the differential pressure required to achieve a given flow rate.
This is the measure of all of the open spaces (pores) in the membrane. Generally, membranes have 50 to 90% open space. Flow rate is directly proportional to the porosity of the membrane. More pores equal higher flow rate.
This is not just a factor of pore size. Other factors are porosity, pore density, tortuous path, and hydrophobicity. e.g. Hydrophobic PP has a slow flow rates to prevent analyte breakthrough when a vacuum is applied.
Selecting a Filter Microplate
As mentioned earlier a clear understanding of membrane characteristics is essential to identify the right filter for assay and process applications. To summarise, the characteristics to consider when selecting filter multiwell devices are:
- Retention Capabilities Binding characteristics
- Maintenance of Flow Rates Pore Size
- Filtration Efficiency Chemical Compatibility
The filter MUST:
- Bind component of choice (high specificity)
- Bind component in a biologically active state
- Be compatible with detection mode (high sensitivity)
- Have tensile strength to withstand the rigors of the protocol.
Technical Reference – Filter Types
The different groups of filter paper types offer increasing degrees of purity, hardness and chemical resistance. Whatman quantitative filter papers have extremely high purity for analytical and gravimetric work.
Membrane filters allow the efficient retention
Glass Microfibre (GMF) Filters
Glass Microfibre filters are manufactured
Whatman cellulose filter papers exhibit
Multigrade GMF 150 combines two filters
Filter Selection Guide
Users should verify compatibility based upon experimentation with a specific filter under actual-use conditions.
Flow Rate is directly related to pore size (1 = Low, 5 = High).
PTFE – Polytetrafluoroethylene
PVDF – Polyvinylidene Fluoride
GMF – Glass Microfibre
General Comments: Wide range available. Typically used as absorbtive or adsorbtive wicking media and prefilters. Physically weak but available with chemical binders. Good particulate retention.
|Basic Principles of Microfiltration|
Filters for the Pulp and Paper Industries
Hytrex® Filters Fill Multiple Needs for the Pulp and Paper Industry
The pulp and paper industry has several applications for Hytrex cartridge filters that are often overlooked. First, Hytrex filters can be used for filtration of bleaching chemicals (methanol, chlorate, and sulfuric acid) prior to mixture. These chemicals are mixed to form chlorine dioxide, the primary chemical in the paper bleaching process. Any contaminants can cause spotting or other flaws in the final paper product.
Hytrex filters can also offer effective protection for high-pressure spray nozzles. These nozzles are used at several critical points in the paper making process. One such area is the drying section of the paper machine, where the paper is blotted by a series of felts that are continuously cleaned by high-pressure spray nozzles. If a nozzle plugs, and one portion of the felt is not sufficiently cleaned of paper fibers, paper quality is compromised.
High-pressure water showers are also used to clean wire screens following the head box of the paper machine. In this application, a mixture of 99% water and 1% wood fiber flows onto a moving screen to form a continuous sheet of paper. The purpose of the screen is for dewatering at a specified rate. It is important to keep the wire screen clean to avoid the risk of an off-quality product or paper breakage. Generally, filters with a micron rating of one-sixth the diameter of the nozzle orifice are suggested.
A more critical application for Hytrex filters is filtering the raw materials for paper coatings. Raw materials such as binders, pigments, and water all contribute contaminants to the coating process.
A large pulp and paper plant in western Canada has selected Hytrex cartridge filters as an affordable alternative to absolute rated filters for chemical filtering, pump seal protection and felt showers. Excellent performance and reasonable price dictated the change to Hytrex filters.
Periodic tightening of the paper market means that economical filtration alternatives, like Hytrex filters, are the best choice for the cost conscious engineer. For absolute filtration, Selex® depth filters are also an affordable option for high quality paper production.