Filtration is the mechanical separation of small particles or molecules through a filter. For sample preparation and processing of substances, the samples in dissolved or gaseous state are actively pressed through a defined membrane by pressure or suction or passively filtered by gravity (filtration of liquids). The membrane material as well as the pore size define the grade of the filter. The retention capacity of a filter are important characteristics of its separating ability. Glass fiber filters, filtration papers in the form of membrane filters or pleated filters, and syringe filters are common filter types.