ultracentrifuge

An ultracentrifuge is a high-speed centrifuge designed to generate very large relative centrifugal forces (RCF) for separating components of biological and chemical samples that differ in density or size. The instrument uses rotors inside a sealed chamber, often under vacuum or with cooled, dry gas to minimize air friction and heat buildup. Tubes containing the sample are loaded in a balanced pair and spun at high speeds; rotor types include fixed-angle rotors, where tubes sit at a steep angle, and swinging-bucket rotors, where tubes swing outward to a horizontal orientation during spinning.

Rotor speeds typically reach tens of thousands of revolutions per minute; RC forces commonly exceed 100,000 g and can reach several million g depending on rotor radius. The relative centrifugal force is calculated as g = 1.118×10^-5 × r × (rpm)^2, where r is the mean radius in centimeters.

Applications include purification and analysis of biomolecules such as proteins, nucleic acids, ribosomes, viruses, lipoproteins, and extracellular vesicles; density gradient ultracentrifugation (using sucrose, cesium chloride, or iodixanol) separates components by buoyant density. Analytical ultracentrifugation (AUC) uses optical detection to study sedimentation velocity or equilibrium, providing information about molecular weight, shape, and interactions; preparative ultracentrifugation concentrates or purifies samples.

The ultracentrifuge industry traces to Theodor Svedberg, whose work on colloids and macromolecules led to the development of high-speed instruments; the unit “Svedberg” (S) expresses sedimentation coefficients. Safety and maintenance are critical: rotor imbalances, mechanical failure, and explosive rotor burst risk; units are built with multiple containment features and require regular inspection and training.