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Short answer
Ultracentrifugation is a laboratory separation method spinning biological samples from 60,000 to 150,000 RPM to generate forces between 100,000 × g and 1,020,000 × g. Unlike a standard laboratory centrifuge, an ultracentrifuge operates under high vacuum with active refrigeration to isolate macromolecular complexes, viral vectors, ribosomes, and nanoscale vesicles.
Definition and scope
Ultracentrifugation extends sedimentation analysis beyond the physical limits of standard benchtop and high-speed centrifuges. Where conventional benchtop centrifuges clear cells and coarse debris below 25,000 × g, ultracentrifugation generates centrifugal fields sufficient to overcome thermal diffusion in liquid suspension.
First developed by Theodor Svedberg in the 1920s to demonstrate that proteins exist as defined, uniform macromolecules, ultracentrifugation encompasses two core operating modes:
- Preparative ultracentrifugation: Isolates, fractionates, and harvests bulk biological materials—including viral vectors, plasmid DNA, ribosomes, and extracellular vesicles—for downstream biochemical or clinical analysis.
- Analytical ultracentrifugation (AUC): Utilizes optical absorption and Rayleigh interference detection within the spinning chamber to record sedimentation boundaries in real time. AUC measures sedimentation coefficients (expressed in Svedberg units, where 1 S = 10⁻¹³ s), molecular masses, and equilibrium dissociation constants without solid matrices or fluorescent labels.
How it works or how it is used in practice
The core ultracentrifugation principle relies on centrifugal acceleration acting on suspended particles. When spun, each particle experiences an outward centrifugal force counterbalanced by buoyant forces and solvent friction. Particle migration velocity depends on mass, buoyant density, hydrodynamic shape, and solution viscosity.
Vacuum staging and thermal management
Spinning a rotor at speeds approaching 100,000 RPM generates severe aerodynamic drag. Operating in ambient air would quickly overheat the rotor, denaturing biomolecules and creating turbulent currents that remix sediment bands.
Commercial systems such as the Thermo Scientific Sorvall WX+ and Beckman Coulter Optima series incorporate multi-stage vacuum systems (combining mechanical roughing pumps with oil diffusion or turbomolecular pumps) to evacuate the chamber below 0.7 Pa (5 microns). Systems dwell at an intermediate speed (typically 4,000 RPM) during pump-down to stabilize temperature and pressure before ramping to set speed. Thermoelectric or hermetic refrigeration maintains chamber temperature between 0 °C and 40 °C within ±0.5 °C.
Differential ultracentrifugation
Differential ultracentrifugation applies sequential spins of increasing g-force and duration. Low speeds clear unbroken cells and nuclei; intermediate speeds pellet mitochondria and lysosomes; and an extended run (such as 100,000 × g for 90 to 120 minutes) pellets microsomes, viruses, and ribonucleoproteins at the tube bottom.
Density gradient centrifugation methods
Density gradient centrifugation provides high-resolution fractionation through a supporting column of graded liquid density:
- Rate-zonal centrifugation: The sample is layered as a thin zone atop a pre-formed shallow gradient (such as 5% to 20% w/v sucrose). Under centrifugal force, particles migrate down the tube at rates determined by size and mass. The run must stop before target zones pellet at the bottom. Swinging-bucket rotors are required to prevent tube-wall collisions.
- Isopycnic (equilibrium) centrifugation: The gradient spans densities encompassing the buoyant density of target particles. Particles migrate until the medium density equals their buoyant density (ρ_particle = ρ_medium). Once at equilibrium, particles remain in stationary bands indefinitely. Fixed-angle and vertical rotors excel here because short radial pathlengths minimize run times.
Types or related concepts
The choice of density gradient medium determines separation efficiency, sample integrity, and downstream purification steps:
| Medium | Chemical Nature | Density Range (g/mL) | Primary Applications | Key Practical Characteristics |
|---|---|---|---|---|
| Sucrose | Disaccharide carbohydrate | 1.00 to 1.32 | Rate-zonal separation of proteins, ribosomes, and membranes | Inexpensive and chemically inert; high viscosity and hyperosmotic at concentrations above 30% w/w. |
| Cesium Chloride (CsCl) | Inorganic alkali salt | 1.00 to 1.90 | Isopycnic banding of plasmid DNA and viral particles | Forms self-generating continuous gradients; high ionic strength denatures sensitive enzymes and capsids. |
| Iodixanol (OptiPrep) | Non-ionic iodinated dimer | 1.00 to 1.32 | Purification of AAV, lentivirus, and extracellular vesicles | Maintains iso-osmotic conditions across all densities; non-toxic to cells; low viscosity accelerates pelleting. |
| Percoll | PVP-coated colloidal silica | 1.00 to 1.13 | Separation of intact cells, peroxisomes, and subcellular organelles | Forms self-generated gradients spontaneously; low osmolality (≤ 25 mOsm/kg); requires 1.5 M NaCl (9:1 v/v) for physiological osmolarity. |
For benchtop processing of whole blood, researchers often use Ficoll and Percoll, whereas macromolecular viral fractionations require iodixanol or CsCl.
Where it matters for labs, clinics or supply decisions
Ultracentrifugation is critical across modern biological workflows:
- Viral vector purification: Producing clinical-grade adeno-associated virus (AAV) and lentiviral vectors requires separating full, genome-containing capsids from empty capsids. Isopycnic density gradient ultracentrifugation resolves this slight buoyant density difference, frequently working alongside upstream tangential flow filtration.
- Extracellular vesicle isolation: Isolating exosomes from plasma or culture supernatant relies on differential ultracentrifugation with iodixanol or sucrose cushions to eliminate soluble albumin contamination.
- Rotor geometry and protocol conversion: Protocols specify run times for exact rotor models. Converting between rotors requires the clearing factor (k-factor):
k = [253,300 × ln(r_max / r_min)] / (RPM / 1000)²
To convert run time from Rotor 1 (k₁) to Rotor 2 (k₂), use the formula t₂ = t₁ × (k₂ / k₁). This preserves yield, as explained in our guide to centrifuge k-factor calculations. Operating at high g-forces also mandates strict centrifuge rotor care and cycle logging to prevent metal fatigue.
Common misunderstandings
- Misunderstanding: Higher RPM always means faster pelleting. Rotational speed alone does not determine clearing time. Rotor geometry and liquid pathlength determine the k-factor; a rotor at 70,000 RPM with a low k-factor pellets faster than an 80,000 RPM rotor with a longer radial path.
- Misunderstanding: Rate-zonal and isopycnic separations use the same rotors interchangeably. Rate-zonal runs require swinging-bucket rotors to prevent wall impaction. Isopycnic runs are faster in fixed-angle or vertical rotors with shorter pathlengths.
- Misunderstanding: Percoll is ready for mammalian cells out of the bottle. Undiluted Percoll has an osmolality under 25 mOsm/kg H₂O. Layering cells directly into unadjusted Percoll causes osmotic lysis. Preparing Stock Isotonic Percoll (SIP) by mixing 9 parts Percoll with 1 part 1.5 M NaCl or 10× buffer is mandatory before dilution.
- Misunderstanding: Analytical ultracentrifugation requires crystal samples. Unlike X-ray diffraction, AUC characterizes macromolecules natively in liquid solution.
Frequently asked questions
What is centrifugation short answer?
Centrifugation is a physical separation technique that uses centrifugal acceleration to separate particles suspended in liquid according to size, shape, and buoyant density.
What is density gradient centrifugation?
Density gradient centrifugation is a method where samples migrate through a medium with increasing density, separating components into distinct bands based on sedimentation velocity (rate-zonal) or equilibrium buoyant density (isopycnic).
What is a preparative centrifuge?
A preparative centrifuge is designed to separate and recover bulk quantities of biological samples, such as viral particles, organelles, and proteins, for downstream experimental or therapeutic applications.
What is ultracentrifugation and what are its applications?
Ultracentrifugation is high-speed centrifugation exceeding 60,000 RPM and 100,000 × g under vacuum and refrigeration. Major applications include viral vector purification, exosome harvesting, ribosomal isolation, plasmid banding, and analytical macromolecular characterization.
Sources
- Beckman Coulter. An Introduction to Ultracentrifugation and Optima XPN Series User Guide. Beckman Coulter Life Sciences. https://www.beckman.com (Checked November 8, 2026).
- Griffith, O. M. Techniques of Preparative, Zonal, and Continuous Flow Ultracentrifugation. Beckman Instruments. https://www.beckman.com (Checked November 8, 2026).
- Thermo Fisher Scientific. Sorvall WX+ Series Ultracentrifuges Instruction Manual (Manual 50145792). Thermo Fisher Scientific Inc. https://www.thermofisher.com (Checked November 8, 2026).
- Cytiva. Percoll and Percoll PLUS Instructions for Use (Document 71-7144-00). Cytiva Life Sciences. https://www.cytivalifesciences.com (Checked November 8, 2026).
