Centrifuge Calculators

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Centrifugation uses centrifugal force to separate particles in a solution based on their size, density, and shape. The relative centrifugal force (RCF, measured in × g) is the effective force experienced by particles, calculated from the rotor radius and rotation speed (RPM). Centrifugation is one of the most widely used techniques in biological and chemical laboratories for separating cells, organelles, proteins, nucleic acids, and viruses. Types range from low-speed clinical centrifuges (300–3,000 × g) to ultracentrifuges (up to 700,000 × g) for separating small macromolecules and viruses.

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RCF Formula

RCF (× g) = 1.118 × 10⁻⁵ × r × N²

r = rotor radius (cm); N = rotation speed (RPM). Example: 4000 RPM, rotor radius 12 cm: RCF = 1.118 × 10⁻⁵ × 12 × 4000² = 1.118 × 10⁻⁵ × 12 × 16,000,000 = 2146 × g ≈ 2150 × g.

To convert × g to RPM: RPM = √(RCF / (1.118 × 10⁻⁵ × r)).

Types of Centrifugation

  • Low-speed (300–3,000 × g): Pellet cells, cell debris; clinical blood separation
  • High-speed (3,000–50,000 × g): Pellet mitochondria, bacteria, yeast; harvest cell lysate insoluble fraction
  • Ultracentrifugation (50,000–700,000 × g): Pellet ribosomes, viruses, membrane vesicles; requires titanium rotors, vacuum-sealed chamber

Differential Centrifugation

Sequential centrifugation at increasing speed to separate organelles by size/density: 600 × g → nuclei, large debris; 10,000 × g → mitochondria, lysosomes; 100,000 × g → ribosomes, microsomes (ER fragments); supernatant after 100,000 × g = cytosol fraction.

Density Gradient Centrifugation

Equilibrium (isopycnic) gradient: particles sediment to their buoyant density in CsCl, sucrose, or iodixanol gradient — separates by density regardless of size. Rate-zonal gradient: separates by size and sedimentation coefficient (S value) in a shallow gradient — used for RNA/DNA/protein size separation.

Glossary

RCF (Relative Centrifugal Force)
The centrifugal force expressed as a multiple of gravitational acceleration: RCF = 1.118×10⁻⁵ × r × N²; rotor-independent measure of centrifugation force; always use × g (not RPM) in protocols.
Differential Centrifugation
Sequential centrifugation at increasing speeds to separate cell components by size: 600×g (nuclei) → 10,000×g (mitochondria) → 100,000×g (ribosomes, microsomes) → cytosol supernatant.
Isopycnic Centrifugation
Equilibrium density gradient centrifugation where particles sediment to their buoyant density; uses CsCl or iodixanol; separates DNA, RNA, and viruses by density regardless of size.

Frequently Asked Questions

RPM (revolutions per minute) describes how fast the rotor spins — it varies with rotor size, giving different centrifugal forces for the same RPM. RCF (relative centrifugal force, × g) describes the actual force experienced by particles: RCF = 1.118 × 10⁻⁵ × r × N², where r = rotor radius in cm and N = RPM. A small microcentrifuge (r = 5 cm) at 13,000 RPM gives RCF = 1.118×10⁻⁵ × 5 × 169,000,000 ≈ 9,450 × g. Always specify RCF (× g) in protocols — not RPM — because RCF is rotor-independent and ensures reproducibility across different instruments.

Differential centrifugation sequentially pellets cell components based on size and density: (1) 600 × g, 10 min → pellet: nuclei and unbroken cells; supernatant: everything else. (2) 10,000 × g, 10 min → pellet: mitochondria, lysosomes, peroxisomes; supernatant: ribosomes, ER, cytosol. (3) 100,000 × g, 60 min → pellet: ribosomes, microsomes (ER/Golgi vesicles); supernatant: cytosol. For clean organelle preparations, combine differential centrifugation with density gradient (sucrose, Percoll) to remove contaminating organelles of similar size.

Ultracentrifugation (>50,000 × g, typically 100,000–700,000 × g) separates: ribosomes (~80S eukaryotic; separated into 40S and 60S subunits); viruses (pellet at 100,000 × g); membrane vesicles and exosomes; lipoprotein fractions (VLDL, LDL, HDL at different densities in KBr gradient); DNA/RNA by CsCl isopycnic gradient (separates by base composition, G+C content). Requires high-strength rotors (titanium or carbon fiber), vacuum chamber to reduce heat from air friction, and temperature control. Preparative ultracentrifuges can hold multiple tubes; analytical ultracentrifuges measure sedimentation kinetics in real time.

Density gradient centrifugation uses a gradient of increasing density to separate particles by buoyant density (isopycnic) or sedimentation rate (rate-zonal). Isopycnic (equilibrium): CsCl or iodixanol gradient; centrifuge until particles reach their buoyant density (density where they float); separates DNA, RNA, viruses by density regardless of size. DNA with higher G+C content has greater buoyant density in CsCl. Rate-zonal: sucrose or glycerol gradient; particles sediment through the gradient at different rates based on their sedimentation coefficient (S value, which reflects mass, shape, and density); used to separate ribosomal subunits, mRNA, and proteins by size.