Differential centrifugation. Fractionation is a separation process in which a certain quantity of a mixture (gas, solid, liquid, 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.ifferential centrifugation. Fractionation is a separation process in which a certain quantity of a mixture (gas, solid, liquid, 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.