Silver Membranes

Q. What is the shelf life of silver metal membranes?

A. There is no predetermined shelf life for the silver membrane. However, the formation of extraneous compounds is possible over time.

Q. What if my membrane is slightly discolored?

A. Although the silver metal membrane is 99.97% pure silver, the formation of extraneous compounds is possible over time. For example, silver can become tarnished, especially when the environment contains certain emissions as described below. To minimize contamination of the membrane, leave it in sealed packs. Silver compounds may form on the surface which are primarily cosmetic imperfections and do not affect the pore structure or membrane filtration performance. Examples of colored compounds that can form on the surface of the silver metal membrane are:

• Ag₂S (black)
• Agl (yellow)
• Ag₃PO₄ (yellow)
• Ag₂CrO₄ (dark red)
• AgCl (dark brown)
• Ag₂O (dark brown)
• AgBr (light yellow)

The most common compounds that form on the silver metal membrane are Ag2S and AgCl. AgCl is a photosensitive salt that can be removed by flushing the membrane with an ammonia solution. Typically, just a brief soak or dip in the ammonia solution will dissolve AgCl. Ag2S is a very stable compound and is very difficult to remove from the membrane without altering the structure. A flush with methyl or ethyl alcohol can be used to remove some of the other compounds.

These compounds should not be confused with the natural grayish white appearance of the silver metal membrane surface. This appearance is due to the microporous structure of the media which reflects light in a manner different than polished silver. The slight difference in color between the two sides of the membrane is due to the manufacturing process and is most noticeable on 3 and 5 micron pores sizes.

Q. What NIOSH Standards are Silver Membranes specified for?

A. National Institute for Occupational Safety and Health (NIOSH) - used for industrial hygiene in foundries, glass plants, quarries, mines, ceramic manufacturing - Methods using 0.45 µm, 25 mm:

N6011 (Bromine & Chlorine) - http://www.cdc.gov/niosh/docs/2003-154/pdfs/6011.pdf
N7500 (Silica, Crystalline) - http://www.cdc.gov/niosh/docs/2003-154/pdfs/7500.pdf
N7501 (Silica, Amorphous) - http://www.cdc.gov/niosh/docs/2003-154/pdfs/7501.pdf
N7504 (Vanadium Oxide) - http://www.cdc.gov/niosh/docs/2003-154/pdfs/7504.pdf
N7505 (Lead Sulfide) - http://www.cdc.gov/niosh/docs/2003-154/pdfs/7505.pdf
N7506 (Boron Carbide) - http://www.cdc.gov/niosh/docs/2003-154/pdfs/7506.pdf
N9000 (Asbestos, Crysotile) - http://www.cdc.gov/niosh/docs/2003-154/pdfs/9000.pdf

Q. Can silver metal membranes be cleaned?

A. Sterlitech™ Silver Membranes: Cleaning Guidelines

Sterlitech’s Silver Metal Membrane Filters can be cleaned and reused repeatedly. The membranes should be cleaned immediately after each use, and handled carefully to avoid membrane punctures and tears. Various cleaning procedures can be used, depending on the nature, type, and degree of contamination. There are four effective cleaning methods outlined below, that can be used to clean Sterlitech’s silver metal membrane filters.

Chemical Cleaning

Immerse in a strong alkaline solution, a solvent, or an acid. Do not immerse in nitric acid, sulfuric acid, or cyanide solutions.

Ignition Cleaning

Place the silver metal membrane filter in a laboratory muffle furnace for approximately ½ hour to effectively remove organic contaminants from the membrane. Do not exceed the following temperatures.

Combination Cleaning

A combination of chemical and ignition cleaning may be the best method to completely regenerate the membrane. Immerse in a 10 percent concentration of hydrofluoric acid for ten minutes, followed by ignition cleaning in a muffle furnace, following the guidelines given above. This can allow for reuse of membrane up to 10 times.

Ultrasonic Cleaning

Low intensity ultrasonics can be used to lean the silver metal membrane. The cleaning intensity and time will depend on the degree and type of contamination encountered. Do not use high intensity ultrasonics.

Q. What are the specifications for the silver metal membranes?

A. Excellent Chemical and Thermal Properties: Sterlitech™ Silver Metal Membranes have the chemical inertness and high thermal stability of pure silver. The membranes resist alcohols, fuels, and other hydrocarbons including halogenated hydrocarbons, natural and synthetic oils, alkalies, cryogenic fluids, photo-resists, ethers, propellants, oxidizers, esters, most organics and acids making them ideal for industrial hygiene tests. Silver Metal Membranes provide excellent filtration performance at temperatures up to 427°C (800°F). The membranes can be autoclaved, steam or hot air sterilized a number of times without losing effectiveness.

Q. What are the advantages of silver metal membranes over conventional membranes?

A. Sterlitech™ Silver Metal Membranes are 99.97% pure, thus unique in their ability to withstand extreme chemical and thermal stress, making them ideal for applications involving aggressive fluids and/or high temperatures. In addition, the purity of the membranes enables users to easily exclude and account for the few chemicals that react with silver, thus making test results and filtering operations more precise than available through traditional membranes.

Q. What are the Silver Membrane lead times?

A. We carry most diameters and pore sizes of Sterlitech™ Silver Metal Membranes in stock and will generally ship within one or two days.

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.

Note that pores have not been observed in RO or NF membranes using a microscope. 

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)
• Nylon
• 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.

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.

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."

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.

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.

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.

A.

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.

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.

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.

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.

A. 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.

• 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
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
IMucolexx

Diagnostic procedures

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

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.

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.

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:

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.