What are membrane filters?
Membrane filters act as barriers for separation of dissolved solutes, colloids, or fine particulates from a solution or gas via pressure gradient. Typically membrane filters are made from microporous polymeric, ceramic, or metallic materials that form a complex network of microchannels — depending on the desired application.
Whatman™ membrane filters are differentiated by material, sterility, pore size, and strength. They have a broad application range that includes air sampling and diagnostics. Since there are many membrane filter material options you may need assistance in finding the optimal filter for your application.
What are the different types of membrane filtration?
Membrane filters can be used for many filtration processes, and are classified into four categories based on the membrane pore size:
Sometimes called reverse osmosis, hyperfiltration is the separation of materials less than 0.001 μm in size. Hyperfiltration utilizes the smallest possible pore size. Applications include separating monovalent salts from water, which is used in seawater desalination.
Filters out larger molecules such as sugars and proteins while allowing smaller molecules like monovalent salts to pass through
Used to extract materials in the 0.001 to 0.1 μm range, such as proteins or colloids from a solution
Used for sterilization by removing particulates ranging in size from 0.1 to 10.0 μm. This larger pore size stands to extract microbes from a solution while allowing salts, sugars, protein etc. to pass through.
What materials are membrane filters made from?
Membrane filters are made from synthetic microporous materials. These materials have certain characteristics that make them ideal for different filtration applications. Below are some of the main membrane filtration composite materials and applications:
Anopore™ is a novel material comprising of a high-purity aluminum oxide matrix that forms a honeycomb structure. Anopore™ membranes exhibit low levels of protein binding, minimal autofluorescence, are nontoxic, and support cellular growth. When wet, the membrane is transparent, which makes Anopore™ excellent for pre-microscopy applications. Retained particles do not need to be transferred to another surface before microscopic examination.
Cellulose acetate membrane filters have very low levels of protein binding. This minimizes sample loss when filtering protein-based aqueous samples.
Cellulose acetate provides high chemical and heat resistance, which makes these filters ideal for liquid scintillation cocktail preparation or for filtration of hot liquids or gasses.
Also known as nitrocellulose, cellulose nitrate membrane filters provide sub-micron pore size distribution and low extractable levels. This allows for very fine particle retention for recovery or pre-microscopy applications. These filters have broad applications in the filtration of particles ranging from 0.1 µm to 12.0 µm.
Mixed cellulose ester
Composed of cellulose acetate and cellulose nitrate. Mixed cellulose membranes have a more uniform surface than pure nitrocellulose filters and can come with grids to aid with colony/cell/particulate counting.
Nylon membranes are suitable for filtering most aqueous solutions and organic solvents. They are hydrophilic, which removes the need for wetting agents that sometimes risk being extracted when filtering aqueous solutions. The membranes are flexible, durable and tear-resistant.
Polyethersulfone membrane filters feature low protein binding, are hydrophilic and stable in alkaline pH. This makes them ideal for biological samples and tissue culture applications.
Polyamide membranes are made from pure polyamide polymer, and are considered to be a universal filter for clarification. They are mechanically resistant and have high wet/dry strength. Applications include: filtration of aqueous and organic mobile phases, vacuum degassing, and filtration of tissue culture media, microbiological media, buffers, and solutions.
PTFE membrane filters offer high chemical stability. This makes them suitable for use with aggressive organic solvents, strong acids, and alkalis. They are often used in HPLC analysis for sample filtration.
What considerations are important when selecting a membrane filter?
When choosing a membrane filter paper, it’s important to consider several key factors:
- What level of particle retention is required? This will determine your required pore size.
- Is your solution to be filtered aqueous or organic? If so, consider hydrophilic membranes.
- Does your filtration process require high wet strength? If so, consider polyamide or nylon membrane filters.
- Does your filtration process need to support biological samples? If so, make sure your filter has low protein binding.
For more information on the different types of filter paper, read our extensive guide to Whatman™ filter paper or contact our specialists to discuss your needs.