What is a membrane filter?
A. A membrane filter is a matrix with channels which act as a screen and retain particles larger than the filter on the surface of the membrane. Membrane filters allow the retention of sub-micron particles and organisms.
Filtration definations and relative pore size
||0.1 - 5.0 micron
|Nanofiltration (NF), Reverse Osmosis (RO)
Note that pores have not been observed in RO or NF membranes using a microscope.
What is the difference between hydrophilic vs hydrophobic membranes?
A. Hydrophilic filters posses an affinity for water and can be wetted with almost any liquid. Hydrophilic membranes inlcude:
- Silver Metal
- Polyethersulfone (PES)
- Glass Fiber
- Polycarbonate Track Etch (PCTE)
- Polyester (PETE)
- Mixed Cellulose Esters (MCE)
- Cellulose Acetate
Hydrophobic filters lack an affinity for water and are best suited for venting applications. Example of hydrophobic filters are polypropylene membranes and PTFE (Teflon®), either laminated or unlaminated.
How is the performance of a filter measured?
A. Design and material selection determines the performance of a filter. Three important measures of filter performance are flow rate, throughput and bubblepoint, defined as follows:
Flow Rate: Determines the volume of liquid or air that will flow through the filter at a fixed pressure and temperature. This is usually displayed as ml/minute/cm^2.
Throughput: Describes the dirt handling capacity of a filter. Namely, how long the liquid will continue to flow through the membrane before the membrane clogs. The lower the flow rate and throughput, the longer it takes the researcher to complete the analysis.
Bubble point: A test to determine the integrity and pore size of a filter. The differential pressure at which a steady stream of gas bubbles is emitted from a wetted filter under specific test conditions. The bubble point test measures the largest pore. Bubble point is generally determined using water or an alcohol (methanol or isopropynol) and is displayed as PSI.
What variables affect the performance of a filter?
A. Viscosity: The viscosity of a liquid determines its resistance to flow; the higher the viscosity, the lower the flow rate and the higher the differential pressure required to achieve a given flow rate.
Porosity: The flow rate of a membrane is directly proportional to the porosity of a membrane, eg. the more pores, the higher the flow rate.
Filter Area: The larger the filter area, the faster the flow rate at a given pressure differential and the larger the expected filter throughput volume prior to "clogging for a given solution."
How are pore sizes rated?
A. A pore size rating is determined by the diameter of the particle that it can be expected to retain with a defined, high degree of efficiency. The rating is stated in nominal or absolute terms.
How is pore size determined?
A. The pore size of a filter, normally stated in micrometers (µm), is determined by the diameter of a particle that is retained by the filter. This is determined using a challenge organism and/or bubble point testing.
What is the difference between nominal and absolute pore size ratings?
Q. What is the difference between nominal and absolute pore size ratings?
A. Nominal pore size rating describes the ability of the filter to retain the majority of the particles at the rated pore size and larger (60-9%). Glass fiber filters and screen filters are a good example of nominally rated filtration.
Absolute size rating describes the pore size at which a challenge organism of a particular size will be retained with 99.9% efficiency under strictly defined test conditions. Most membrane filters are rated as absolute terms.
What organisms are used to determine pore size?
What is a bubble point test?
A. A test to determine the integrity and pore size of a filter. The differential pressure at which a steady stream of gas bubbles is emitted from a wetted filter under specific test conditions. The bubble point test measures the largest pore.
What is a KD (kiloDalton)?
A. KD, or kD is the abbreviation for kiloDalton and is equal to 1,000 Molecular Weight Cut-Off (MWCO). A solution having a molecular weight of 1,000,000 would be equivalent to 1,000 KD. The table listed below gives a general relationship between kiloDalton to Microns (micrometers), Nanometers, and Angstroms:
As a general rule, choose a membrane with a pore size (MWCO) that is less than half of the compound of interest.
Can I filter aqueous solutions through a polytetrafluoroethylene (PTFE) hydrophobic membrane?
A. Yes, but the membrane requires pre-wetting with alcohol (like Isopropanol or Methanol) to establish flow with reasonable pressure differentials. We often use a 60/40 solution (60 Isopropanol/ 40 water) to prewet the membrane.
Which membrane is recommended for size characterization analysis?
A. The polycarbonate track-etch (PCTE) membrane is recommended for size characterization analyses. The pores of this membrane are exceptionally uniform and are offered as low as 0.01um in size.
We would like to filter fluids and wonder which membrane to use for cytology?
Cells are removed from the body, then stained and examined under a microscopy. The trained physician or cytotechnologist is able to detect the presence of malignancy.
Use of either the 5.0 um or 8.0 um polycarbonate membrane.
- Pore Structure and Porosity
Minimum clogging by red blood cells and protein. Well preserved cellular morphology. High recovery rate. Rapid filtration with low pressure. Surface capture. No cover slip edge sealing.
High cell visibility. Improved morphologic resolution. Surface capture.
Easy mounting. Immediate microscopic examination.
- Low absorption and adsorption
Improved contrast. Greater cell isolation. Easy mounting.
Improved contrast. Simpler microscopic analysis routine.
Simpler microscopic analysis routine.
Unaffected by conventional cytologic fixatives and stains.
Less critical handling techniques needed.
25mm or 47mm filter holder
Stainless steel forceps
5.0um - 25mm or 47mm PCTE membranes
General Procedure - procedure may vary
Most body fluids contain blood in various amounts.
Collect them in a container with an anticoagulant.
One recommended using polycarbonate membranes is EDTA (0.1 molar)
For each 5-ml specimen generally 1 ml of EDTA solution is sufficient; for specimens of higher blood content, 1 ml of EDTA for each 20-ml specimen is recommended.
Fixatives or preservatives recommended are:
30 to 50% alchol
10% buffered formalin
For optimum results employ a limited amount of fluid when preparing specimens that exhibit high cell concentrations. If too much fluid is used with cellular specimens, such as endometrial washing, ascites, gastrics, pleurals, sputums, and urines, the cells crowd and clog the filter. A 5-ml specimen or smaller volume provides a good sample if it is properly mixed before filtration.
The 8um pore size is recommended for filtering most body fluids.
For samples of higher probability of small cancer cells or fewer cancer cells, such as CSF or baby urine, the 5um pore size is recommended.
um = micrometer = micron
Which membranes are recommended for gravimetric analysis?
A. We have several membranes to recommend for gravimetric analysis.
- Mixed Cellulose Esters (MCE) Membrane Filters, Plain: In gravimetric analysis using ashing techniques, (MCE) Nitrocellulose filters yield a residue of less than 0.045% of their initial weight. They are hydrophilic with a non-cytotoxic wetting agent extractable level of less than 4% of their weight.
- Polycarbonate Track-Etch Membranes (PCTE) - 25mm: Polycarbonate Track-Etch or our Polyester Track-Etch (PETE) membranes are two membranes that offer exceptionally low tare weights, are non-hygroscopic, and exhibit extremely low absorption and adsorption losses.
Since these membranes are non-hygroscopic, they are particularly well suited for gravimetric analysis. They do not require drying when used directly out of the package. If they are wet, they can be dried rapidly and will not pick up moisture from the air during weighing.
- Analytical Filter Funnels: Funnels are available complete with low hold-up polypropylene support pads, gravimetric analysis is one of the ideal applications for this product. Each package of 12 units comes with an extender that adapts the unit for side-arm flask filtrations. Optional No. 8 rubber stoppers may be ordered for filtering with flask.
- Glass Fiber Filters & Prefilters: Glass fiber filters without binders are recommended for analytical and gravimetric determinations.
How does Epifluorescent microscopy work?
Q. How does Epifluorescent microscopy work?
A. Epifluorescent microscopy uses a UV-visible light source and specific filters to excite fluorescent stains added to microbiological specimens to aid in identifications and enumeration.
Black Polycarbonate membranes are ideal for epifluorescent microscopy, since they retain bacteria on the surface of the membrane and provide a non-distracting background to view fluorescence against.
What membrane has been used to prevent water vapor from passing, but would allow regular air to pass?
A. The common membranes used for gas/air filtration are hydrophobic Polypropylene and PTFE Membranes. They both inhibit the flow of water vapors (hydrophilic) while allowing regular air molecules (such as oxygen) to pass.
For the PTFE membrane the water intrusion pressure (which is inversely related to pore size) is greatest with the smaller sizes:
Water Intrusion of PTFE
|Pore Size (um)
||Water Intrusion Pressure (psi)
What is the function of a binder in the glass fiber filter?
A. The binder's purpose is to increase strength and dirt-loading capacity while decreasing fiber slough. They are used for filtration of long duration under pressure. Binders are acrylic material added to the borosilicate glass fibers.
Our TCLP glass fiber filters have a pore size of 0.7um and are available. Glass fiber filters without binders are autoclavable and have a maximum temperature of 500°C.