Image Credit: Sartorius Group

Laboratory filters are used to remove particulates from samples in laboratory-scale applications. They consist of a filter medium and housing or holder that constrains and supports the filter media in the sample’s path.

Types of Laboratory Filters

There are several basic types of laboratory filters.

• Membrane filters are thin, polymeric films that contain thousands of microscopic pores. The size of the pores determines the size rating of the membrane. Typically, membrane filters are used in the quantitative separation or filtration of suspended matter from liquids and gases. Pre-filters are often placed upstream from membrane filters to reduce particulate loading and allow the membrane to operate more efficiently.
• Centrifugal filters are suitable for small-scale laboratory separations such as protein or nucleic acid desalting and concentration. These devices use centrifugal force to drive liquid through the filter. Increasing the centrifuge speed (G force) increases the pressure across the filter. Fouling is minimized by design features that cause the G force to reach the filter at an angle, sweeping accumulated molecules from the surface.

• Syringe filters consist of a filter element and housing assembly and are used in applications where a sample must be filtered before entering a syringe.

• Specialized and proprietary filters that are made from a variety of porous papers are also available.

 
Specifications

Selecting laboratory filters requires an analysis of both physical specifications and performance specifications.

Physical specifications for laboratory filters include:

• Sample size - Sample size, the maximum amount that filters can accept, is especially important for centrifugal filters and syringe filters.
• Pore size - Pore size indicates whether particles of a specific size are retained with an efficiency rate less than 100%, typically 90% – 98%. Rating methods vary widely among manufacturers.
• Filtration area - The effective filtration area (EFA) is the total, usable filter area. As a rule of thumb, the larger the filter area, the faster the flow rate at a given pressure differential and the larger the throughput volume prior to clogging.
• Filter shape - Most filters are circular or rectangular in shape.

Performance specifications for laboratory filters include:

• Flow rate - Flow rate determines the volume of a liquid that flows through the filter at a fixed pressure and temperature.
• Hold-up volume - Hold-up volume is the maximum volume of a sample that a filter can retain.
• Particle retention size
• Porosity
• Maximum pressure

Laboratory filters also vary in several other ways including:

• Configuration - Configurations usually consist of a filter element, a housing element, or both a filter element and housing assembly.
• Sample type - Filters that are used with solid, liquid or gaseous sample types are commonly available.
• Filter paper measurement type - There are two filter paper measurement types: qualitative and quantitative. Qualitative filter papers have an ash content that is ten times higher than quantitative filter papers.
• Filter medium material - Common filter medium materials include cellulose, cellulose acetate, nitrocellulose, and regenerated cellulose; ceramic, carbon, and glass fiber materials; polytetrafluoroethelene (PTFE), polyvinylidene fluoride (PVDF), and polyvinylidene chloride (PVDC); and polypropylene (PP), polysulfone (PSU), and polyethersulfone (PES).
• Housing material - Housing materials for laboratory filters include acrylics, plastic acrylics, modified acrylics, and polypropylene (PP).

Standards

AFR 78-24 - CENTRIFUGALS, SEPARATORS AND FILTERS