Ultracentrifugation is a technique that spins samples at extremely high speeds, exceeding 100,000 revolutions per minute, to separate particles based on their size, shape, and density. At those speeds, the centrifugal force can reach over 800,000 times the force of gravity, powerful enough to pull apart molecules that would never separate under normal conditions. It’s used across biology, medicine, and even nuclear engineering.
How It Works
A standard centrifuge works like a salad spinner: it pushes heavier things outward while lighter things stay closer to the center. An ultracentrifuge does the same thing, just with vastly more force. High-speed centrifuges top out around 15,000 to 30,000 rpm. Ultracentrifuges blow past that range, generating forces so intense they can separate individual proteins, pull viruses out of a liquid sample, or sort cholesterol-carrying particles by subtle differences in density.
To put the forces involved in perspective: a one-gram imbalance at rest becomes the equivalent of 250 kilograms at 500,000 times gravity. That’s why ultracentrifuges require precision balancing, vacuum chambers to eliminate air friction (which would generate destructive heat at those speeds), and heavy armored containment in case something goes wrong.
The Two Main Types
Ultracentrifuges fall into two broad categories, each designed for a different purpose.
Preparative Ultracentrifugation
This type is about physically collecting separated material. You load a biological sample, spin it, and then extract the fractions you want. There are two common approaches:
- Differential centrifugation uses repeated rounds of spinning at increasing speeds. Each round pellets progressively smaller particles at the bottom of the tube. First spin collects whole cells, the next collects large organelles, and later spins pull down smaller structures like ribosomes.
- Density gradient centrifugation layers the sample on top of a gradient medium, typically sucrose or iodixanol solutions that get progressively denser from top to bottom. During spinning, each particle migrates to the point in the gradient that matches its own density, forming distinct bands you can then extract with a needle or pipette.
Sucrose density gradients remain the most widely used technique for purifying viruses, for example. Researchers working with retrovirus particles have used continuous iodixanol gradients (10 to 30%) to separate functional virus particles from defective ones, something cruder methods can’t achieve.
Analytical Ultracentrifugation
Rather than collecting material, analytical ultracentrifugation (AUC) watches particles as they move through the sample in real time. The instrument shines light through the spinning sample and tracks how fast particles sediment, how they diffuse, and how concentrated they are at different points in the tube.
Modern AUC instruments use three optical systems. UV absorbance optics detect how much light the sample absorbs, working best at moderate concentrations. Interference optics measure changes in how light bends as it passes through regions of different density, allowing measurements at higher concentrations. Fluorescence optics detect tagged molecules and can track particles at concentrations as low as a few nanomolar, orders of magnitude more sensitive than the other two methods.
From a single well-designed experiment, AUC can reveal a particle’s size, its diffusion rate, its rough shape, whether it interacts with other molecules, and how many different species are present in the mixture. That combination of information from one run is hard to match with other techniques.
Rotors: Fixed-Angle vs. Swinging-Bucket
The rotor is the spinning component that holds the sample tubes, and its geometry matters. Fixed-angle rotors hold tubes at a set angle (commonly 15 to 35 degrees from vertical). Particles hit the outer wall of the tube and slide down, making these rotors efficient for pelleting. Swinging-bucket rotors let the tubes swing outward until they’re horizontal during the spin, so particles travel straight down the length of the tube. This produces cleaner separation bands in density gradient work.
The choice between them depends on the application. In lipoprotein research, for instance, swinging-bucket rotors reduce interference from certain particles that would contaminate results in a fixed-angle setup, because the vertical separation path makes it easier to remove unwanted fractions without disturbing the bands you want.
What Ultracentrifugation Is Used For
In biology and medicine, ultracentrifugation separates subcellular components that are too small and too similar in size to isolate any other way. Common targets include ribosomes (the cell’s protein-building machinery), membrane vesicles, different classes of lipoproteins (the particles that carry cholesterol in your blood), DNA, and viruses. Vaccine production, for example, relies heavily on ultracentrifugation to purify viral particles from the cell cultures used to grow them.
Outside the life sciences, ultracentrifugation has a completely different application: isotope separation. Gas centrifuges spin uranium hexafluoride gas at high speed. The heavier uranium-238 isotope drifts toward the outer wall of the spinning cylinder, while the lighter uranium-235 concentrates near the center. By running the output through cascades of many centrifuges in series, engineers can progressively enrich the uranium-235 concentration. This process is mechanically demanding. Early efforts during the Manhattan Project were plagued by vibrations at critical speeds, seal failures, and motor breakdowns, but the technology eventually matured into the primary method of uranium enrichment used today.
A Brief Origin Story
The ultracentrifuge was developed in 1925 by Theodor Svedberg, a Swedish chemist who realized that spinning mixtures fast enough would let him calculate the molecular weight of proteins by measuring how quickly they moved outward. His work earned him the Nobel Prize in Chemistry in 1926. The unit used to describe how fast a particle sediments, the Svedberg (S), is named after him. You’ll see it in terms like “70S ribosome” or “28S ribosomal RNA,” where higher numbers mean the particle sediments faster.
Safety Considerations
The forces involved make ultracentrifuges genuinely dangerous if something goes wrong. A rotor spinning at 100,000 rpm contains enormous kinetic energy, and a failure (from metal fatigue, improper balancing, or corrosion) can be catastrophic. Labs track rotor usage in logbooks, retire rotors after a set number of runs, and inspect them for scratches or corrosion before every use.
Before starting a run, operators check that tubes are properly balanced, O-rings are intact, vacuum grease is fresh, the rotor is correctly seated on the drive hub, and tubes aren’t overfilled. If the sample is infectious or radioactive, loading and unloading happens inside a biological safety cabinet using aerosol containment tubes. Once the run begins, operators stay nearby until the instrument reaches full speed and confirms the run is stable.