Fractionation is a separation process in which a certain quantity of a mixture (gas, solid, liquid, enzymes, suspension, or isotope) is divided during a phase transition, into a number of smaller quantities (fractions) in which the composition varies according to a gradient. Fractions are collected based on differences in a specific property of the individual components. A common trait in fractionations is the need to find an optimum between the number of fractions collected and the desired purity in each fraction. Protein fractionation generally refers to the process of isolating, identifying and characterizing various proteins present in a sample. However, the analysis of proteomes is usually hindered by the vast amounts of proteins, especially since the larger, more abundant proteins tend to inhibit the signal of lower abundance proteins. Incidentally, the lower abundance proteins are usually the more interesting proteins in the group. Fractionation makes it possible to isolate more than two components in a mixture in a single run. This property sets it apart from other separation techniques.