Sample Preparation by Filtration

Filtration is a separation technique used to concentrate or purify substances based on their physical or chemical properties. It is a simple and routine method used in many laboratories to remove insoluble particles from solutions and to prepare samples for analysis. Filtration is used to reduce sample complexity, improve clarity of viscous samples, and reduce background signals resulting in increased signal-to-noise ratios in analytical tests.  

• Common Filtration Applications
• Types of Filtration Processes & Procedures

Depending on the filtration method applied, particles or molecules are separated based on properties such as size, shape or charge. The liquid that passes through the filter is called the ‘filtrate’ and the collected or retained material is the ‘retentate’ or ‘residue’.

• Reverse osmosis (ionic separation) separates ions or molecules using a semipermeable membrane or barrier. Applied pressure overcomes osmotic pressure and forces solvent to move from a high solute concentration to a low solute concentration. Reverse osmosis rejects a high percentage of organic matter, other than particulates, and >99% of salts. The typical membrane rating is based on sodium chloride retention (<0.001 µm, <100 Daltons)
• Ultrafiltration (macromolecule separation) separates particles and dissolved molecules from fluids based on particle size. Ultrafiltration is used for concentration, fractionation, desalting, and buffer exchange. The typical rating is a nominal molecular weight limit (NMWL) between 1-1000 kDa.
• Microporous filtration (microfiltration) is used for particle retention/exclusion and sterilization because it separates/removes particles and biological entities, such as bacteria and cells, based on particle size. Pore sizes are typically between 0.025-10 µm and are rated as nominal (~98% retention) or absolute (100% retention of the size equal to the pore size rating).
• Clarification filters are used for pre-filtration and particle analysis because they retain/remove large particles, aggregates, and debris based on size. They may be used as a primary filtration step before microfiltration.  Clarification filters typically have pore size ratings >5 µm. retained.