Fraction (chemistry)

Fraction (Latin fractio, fracture, fraction ') in chemistry is a subgroup of substances in a mixture, regardless of the state of aggregation. The substances of a group are similar to each other than the rest of the mixture. The percentage of a fraction of a mixture can be expressed as molar fraction, mass fraction or gases likely than volume fraction.

Examples are the CO2 fraction in the exhaled air, the O2 fraction of the inhaled air ( FiO2, → ventilation), the light oil fraction of petroleum, in biochemistry, the protein fraction of the milk or organelle fractions in the processing of biological tissues as well as soluble proteins on the one hand and insoluble membrane fractions on the other hand subfractionation of organelles ( or generally membranous systems).


In the extraction of plant or animal material often fall mixtures of several substances. Likewise, in the chemical synthesis often results in several fabrics next to each other. The separation of these materials is referred to as fractionation, thereby to obtain more or less clean (uniform) fractions, each containing only one substance, in the ideal case, which is therefore chemically defined. To separate the fractions are different chemical (eg precipitation) and physical (for example, fractional crystallization, fractional distillation, fractional sublimation, fractional resolution, fractional extraction, fractional sedimentation, flotation and fractional fractional electrophoresis) methods. In the gas chromatography, column chromatography or thin-layer chromatographic separation of mixtures several fractions incurred, and then contain individual substances or group of substances separately.

Analytical separation

For the analytical separation ( fractionation) usually only a small quantity ( typically < 1 mg) of the material mixture separated, for example by gas chromatography. Often, the separation method is combined with an analytical method. An example of this is the combination of gas chromatography and mass spectrometry (GC -MS).

In macromolecular chemistry, the separation of polymers different degrees of polymerization into fractions nearly uniform molecular mass plays a role.

Preparative separation

In the preparative separation ( fractionation) is a mixture of substances in significant amounts (typically several mg, g, kg or tonnes) by fractional distillation (example: separation of crude oil into petroleum fractions such as gasoline, diesel fuel, fuel oil, base oils for lubricants, bitumen ), fractional crystallization (separation of solid mixtures ) or by column chromatography (often in the organic chemistry laboratory, rare in the art) separately.