Sterlitech™ Membrane Filter: Frequently Asked Questions

Q. What is the maximum temperature for the different filter membranes?

Q. What membranes are hydrophilic?

Q. Why is my flow rate low? Little or no liquid is seen to pass through the membrane, what could be the problem?

Q. What membrane has been used to prevent water vapor from passing but would allow regular air to pass?

Q. How is porosity determined on Sterlitech membranes?

Q. What membrane is best for applications where low extractables are required?

Q. I need a 0.45um membrane for bacterial analysis.  Which one should I use?

Q. I would like to filter fluids and wonder which membrane to use for cytology?

Q. What is a KD (kiloDalton)?

Q. What is the maximum temperature for the different filter membranes?

A. The maximum operating temperatures for Sterlitech filter membranes are listed below.

• Sterlitech Silver Metal - 427°C
• Sterlitech Ceramic - 350°C
• Sterlitech Polycarbonate Track Etch - 140°C
• Sterlitech Polyester - 140°C
• Sterlitech Nitrocellulose (MCE) - 130°C
• Sterlitech Nylon - 180°C
• Sterlietch Polyethersulfone (PES) - 180°C
• Sterlitech Polypropylene - 82°C
• Sterlitech Cellulose Acetate - 135°C
• Sterlitech PTFE (Laminated) - 130°C
• Sterlitech PTFE (Unlaminated) - 260°C
• Sterlitech PVC Data not available

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Q. What membranes are hydrophilic?

A. The following is a list of membranes we offer that are hydrophilic:

• Silver Metal
• Polyethersulfone (PES)
• Glass Fiber
• Polycarbonate Track Etch (PCTE)
• Polyester (PETE)
• Mixed Cellulose Esters (MCE)
• Nylon
• Cellulose Acetate
• PVC Silica-Free

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Q. Why is my flow rate low?  Little or no liquid is seen to pass through the membrane, what could be the problem?

A. There could be a number of things going on to restrict the flow through the membrane.

• First you want to take the original flow rate into consideration, for example a 25mm polycarbonate membrane with a 0.05 micron pore size and a flow rate of 0.4 ml/min/cm2 is only going to get about 1.2 ml of liquid through a minute.  This amount decreases with smaller pore size.

• Another thing to consider is if the liquid is being properly prefiltered.  With such small pore sizes, 0.05-0.01, you need to filter in a step down manner so as not to plug the pores.  Step down means a series of filters with decreasing pore sizes.  This can even happen with DI water, bacteria or other particles can be introduced to the process and cause problems with blinding or plugging the filter.

This should eliminate most problems with flow through the membrane; other issues need to be evaluated on an individual basis.

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Q. 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 our 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:

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Q. How is porosity determined on Sterlitech membranes?

A. The porosity of Sterlitech membranes is determined from flow rate and bubble point data correlated to particle retention.

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Q. What membrane is best for applications where low extractables are required?

A. The membrane with the lowest extractables is the Polycarbonate Track Etch (PCTE) membrane.  Some of the other features PCTE offers are:

• Capture of samples on a flat, smooth, glass-like surface, with even distribution in one plane.
• All particles larger than the actual pore size are captured on the membrane surface.
• Lowest, non-specific binding.
• Free of contaminants.  PCTE membranes are produced under Class 100* conditions in critical manufacturing steps.
• Precise pore size and narrow pore size distribution provide for accurate separation, or fractionation, of samples by size.
• Negligible adsorption and absorption of filtrate.
• Low extractables.
• No sloughing or particle shedding.  PCTE membranes are integral plastic films.  Non-staining, providing an exceptional background of sample observations.
• Biologically inert.
• Optically transparent in most pore sizes.
• Exceptionally low tare weights, non-hygroscopic, and low trace element level.
• Superior strength.
• Excellent chemical resistance and thermal stability.

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Q. I need a 0.45um membrane for bacterial analysis.  Which one should I use?

A. Sterlitech offers a broad spectrum of Sterlitech mixed cellulose esters, MCE filters including black nitrocellulose filters specifically manufactured and tested for quality assurance testing in food and beverage analysis.  These filters, both plain and gridded, are certified to meet specifications listed in APHA Standard Methods.  The uniform microporous structure of these filters provides the fastest flow rates and highest throughputs available in a membrane filter.

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Q. I would like to filter fluids and wonder which membrane to use for cytology?

A.  Cytology:  cells are removed from the body, then stained and examined under a microscope.  The trained physician or cytotechnologist is able to detect the presence of malignancy. 

Use either the 5.0 um or 8.0 um polycarbonate membrane.

Membrane Characterisitics Advantage in Diagnostic Cytology

• 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.
• Smooth, Flat Surface - High cell visibility.  Improved morphologic resolution.  Surface capture.
• "Thinness" - Easy mounting.  Immediate microscopic examination.
• Low absorption and adsorption - Improved contrast.  Greater cell isolation.  Easy mounting.
• Non-staining - Improved contrast.  Simpler microscopic analysis routine.
• Transparency - Simpler microscopic analysis routine.
• Chemical Resistance - Unaffected by conventional cytologic fixatives and stains.
• Strength - Less critical handling techniques needed.

Equipment needed:

25 or 47mm filter holder

Stainless steel forceps

5.0um - 25mm 0r 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 50-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% alcohol

10% buffered formalin

IMucolexx

Diagnostic procedures

For optimeum 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 5ml specimen or smaller volume provides a good sample it it is properly mixed before filtration.

The 8-um 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

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Q. 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 havnig 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.

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