Membrane Filters
Introduction
Membrane filters or “membranes” are microporous plastic films with specific pore size ratings. Also known as screen, sieve or microporous filters, membranes retain particles or microorganisms larger than their pore size primarily by surface capture. Some particles smaller than the stated pore size may be retained by other mechanisms.
Advantec membranes are produced by three different processes. Mixed Cellulose Esters and Cellulose Acetate, are reverse phase solvent cast membranes, where controlled evaporation or removal of the complex solvent system forms the porous structure. Both hydrophilic and hydrophobic PTFE are made by a patented process where the membranes are stretched biaxially to form the porous structure. Polycarbonate membranes are track etched.
Performance Characteristics of Advantec Membranes
- Strong: Advantec membranes are monitored for both burst (longitudinal) and tensile (lateral) strength.
- Chemically and biologically clean: As part of a comprehensive quality program, only high purity reagents and raw materials are used to produce Advantec membranes. Once cast, the membranes are handled in a class 1000 clean room to minimize ambient contamination. While some membranes require a small amount (0.1–3 weight %) of an aqueous wetting agent, Cellulose acetate has the lowest aqueous extractables (0.1 weight %). All Advantec membranes are Triton-free.
- Thin membranes with high porosity: Uniformly thin membranes (typically 150 μm) with high porosity (about 80%) provide high gas and liquid flow per unit area. High porosity also provides high surface area for adsorption or binding
- Thermostable: All Advantec membranes can be sterilized by autoclaving. Operating temperatures of up to 180°C can be achieved depending upon the membrane polymer (see individual membrane specifications for details). Advantec membranes exhibit minimal shrinkage at elevated temperatures
Quick Guide to Selecting Membrane Filters
- Determine what liquid or gas will be filtered
- Check which membranes are chemically compatible (following and appendix)
- Determine the maximum pore size required to achieve the results you want
- Check the membrane specifications for any unusual process conditions that might otherwise limit your choice of membrane (e.g. temperature)
MEMBRANE COMPARISON
| Membrane polymer |
Sample applications |
General compatibility |
Hydrophilic | Hydrophobic | Pore size range available (μm) | ||||||||
| 0.1 | 0.2 | 0.45 | 0.8 | 1.0 | 3.0 | 5.0 | 8.0 | 10 | |||||
| Mixed Cellulose Esters (MCE) |
– General purpose Microbiology – Particle Analysis |
Aqueous solutions | ✓ | ||||||||||
| Cellulose Acetate | – General filtration – Cytology -Binding studies |
Aqueous solutions | ✓ | ||||||||||
| Coated Cellulose Acetate | – Clarify solutions Prefilter | Aqueous solutions | ✓ | ||||||||||
| Hydrophilic PTFE | – HPLC solutions Clarify or sterilize aqueous – organic mixtures |
Aqueous and organic solutions | ✓ | ||||||||||
| Hydrophobic PTFE | – Gas venting – Clarify or sterilize strong acids or solvents |
Non-aqueous solvents | ✓ | ||||||||||
| Hydrophobic PTFE with supported PP net |
|||||||||||||
| Polycarbonate | – Microscopy Beverage testing | Aqueous solutions | ✓ | ||||||||||
Mixed Cellulose Esters (MCE)
- Composition: Mixed cellulose esters including cellulose nitrate and cellulose acetate, also known as nitrocellulose
- High porosity provides superior flow rates
- High protein binding can be blocked by pretreatment or utilized in applications
- High purity: Triton-free
- Autoclavable: Withstands autoclaving temperatures up to 130°C without adversely affecting bubble point, flow rate or microbiological recovery
- Rapid wetting time: <3 seconds to wet a 47 mm diameter disc with aqueous 1% methylene blue
APPLICATIONS
- Standard membranes for many laboratory applications including filter sterilizing biological fluids, microbiology, contamination analysis and air monitoring
- Can be transparentized to view collected particles
- – using compatible liquid (immersion oil, toluene),
OR
– select Opticlear membranes for the “hot block”acetone vapor method - Gridded filters available for quantifying microbial growth
- Available non-sterile or sterilized by ethylene oxide (EtO)
Cellulose Acetate
- Composition: Mixture of cellulose triacetate and diacetate
- Characteristics: Low static charge and high strength
- Autoclavable: Withstands autoclaving temperatures up to 130°C without adversely affecting bubble point, flow rate or microbiological recovery
- Clean: Lowest aqueous extractables (0.1 wt%) of all Advantec membranes
- Relative to MCE (Mixed Cellulose Esters, Nitrocellulose): – improved solvent resistance to low molecular weight alcohols
– better heat resistance
– lower protein binding
APPLICATIONS
- Enhanced recovery of fastidious gram positive organisms
- Filtration of enzyme solutions
- Diagnostic cytology
- Receptor binding studies
Hydrophobic PTFE
- Properties: Thin, highly porous, behaves as an absolute retentive membrane
- Inert to most chemically aggressive solvents, strong acids and bases
- Operating Temperature Range: -120 ~ 260ºC
- Autoclavable
APPLICATIONS
- Sterilize gases: traps aqueous aerosols
- Air and gas venting: allows gases to pass freely while blocking aqueous liquids, protect vacuum pumps and critical samples
- Sterilize and clarify strong acids and many other solvents incompatible with other membranes
Hydrophobic PTFE with Supported PP Net
- Properties: Thin, highly porous, behaves as an absolute retentive membrane
- Supported: polypropylene laminated to one side to improve handling
- Inert to most chemically aggressive solvents, strong acids and bases
- Thermostable: can be used up to 100ºC
- Operating Temperature Range: -35 ~ 130ºC
- Autoclavable
APPLICATIONS
- Sterilize gases: traps aqueous aerosols
- Air and gas venting: allows gases to pass freely while blocking aqueous liquids, protect vacuum pumps and critical samples
- Sterilize and clarify strong acids and many other solvents incompatible with other membrane
Hydrophilic PTFE
- Characteristics: Maximum chemical and pH resistance
- High flow rates with minimal aqueous extractables (<0.3 wt%)
- Optically clear when wet with water
- Non-supported
- Thermostable: can be used up to 100ºC
APPLICATION
- Ideal for HPLC and other mixtures of aqueous and organic solvents
Note: Hydrophilic PTFE membrane filters are not autoclavable.
Coated Cellulose Acetate
- Composition: Cellulose acetate cast onto a non-woven polyester support
- Characteristics: Non-fiber releasing
- Low protein binding relative to nitrocellulose
- Low static charge matrix with enhanced chemical compatibility to low molecular weight alcohols
- Autoclavable
APPLICATION
- Use as a clarifying filter or prefilter
Polycarbonate
- Characteristics: Low non-specific binding and optically translucent, extremely uniform, cylindrical pores
- Thin screen-type membranes minimize entrapment within the filter structure; resulting in surface capture of particles on the membrane
- Stable: excellent chemical resistance, good thermal stability, non-hygroscopic and extremely weight stable
- Autoclavable: at 121º C, 30 min.
APPLICATIONS
- Epifluorescence microscopy: available in black for this method
- Electron microscopy: smooth surface is ideal for observing captured particles
- Light microscopy: easily transparentized for optical illumination
- Beverage and sterility testing