Caitlin Smith has a B.A. in biology from Reed College, a Ph.D. in neuroscience from Yale University, and completed postdoctoral work at the Vollum Institute.
Centrifugation is an indispensable separation and concentration tool for many areas of research. High-speed centrifugation (approximately 20,000 to 40,000 RPMs, or 20,000 to 100,000 x g, or higher) is especially valuable in the biosciences for applications such as concentrating cells in suspension, isolating and separating cell membranes from cytosolic contents and purifying and isolating genetic material.
Many researchers have performed these tasks. But use alone does not necessarily equip a researcher to purchase a rotor. Given the infrequency of such a purchase, it is especially important to make the right choice for your lab or research facility. Here are some points to keep in mind.
Know your expected use
Rotors typically cost from a few hundred to a few thousand dollars. MIDSCI's 4x500 ml swing-out rotor lists for about $1,700, plus $900 for the required carriers, according to product manager Elise Ambrose; the company's 6x85 ml angled high-speed rotor lists for about $2,100.
That's not break-the-bank expensive, but it's enough that you want to pick a rotor that will last and serve your needs. So as you shop for rotors, consider current and future applications and how those constrain your options.
Max speed. Rotors are designed to spin safely and effectively up to a specific maximum speed or force, denoted as a number of revolutions per minute (RPMs) or a multiple of the force of gravity (g). In general, floor-model centrifuges can generate more force and spin samples of larger volumes, but benchtop centrifuges are catching up, so check the specifications.
Another way to rate a rotor is by its “k-factor,” which represents efficiency based on the rotor geometry and RPMs. “The lower the k-factor, the more efficient the rotor will be in pelleting [a] sample,” explains Randall Lockner, centrifugation marketing manager at Beckman Coulter Life Sciences. “Because of innovations in rotor design, a rotor with a lower k-factor may be able to run your application faster, even if it has lower maximum RPM than a comparable rotor.”
Sample volume. Rotors capable of spinning liter-sized samples differ from rotors designed for microliter-sized samples. That’s obvious, but it’s important to consider future sample volume needs. Sample volume is closely related to vessel type.
Vessel type. Most rotors are designed to hold tubes or bottles, and some can hold microplates. Plus, it often is possible to purchase adaptors to spin different types of vessels. “Tube or microplate compatibility, coupled with the ease of switching out buckets and adapters” is one of the most important factors in choosing a rotor, says Ambrose. “Many customers want to spin 15- and 50-ml tubes, and plates, so ease of use becomes key.”
Refrigeration. Many experiments require refrigeration of samples, especially on longer runs. If you are using a centrifuge that will keep samples cool, make sure the rotor is rated to the temperatures you plan to use.
Biocontainment. Some rotors provide built-in protection against biohazardous materials. These usually use a lid that seals down over the samples with a greased, rubber O-ring, but other features may enhance a rotor’s biocontainment capability. For example, Thermo Fisher Scientific offers an auto-lock feature that improves both safety and ergonomics (as it requires fewer parts and less handling). “The rotor clicks onto a spindle, which captures it,” says Phil Hutcherson, global product manager at Thermo Fisher Scientific. “There are no worries that the rotor will come off.” Another rotor-biosafety feature involves “canister systems that allow lab personnel to transfer biocontained samples to or from a biosafety hood to the centrifuge, without needing to install or remove the entire rotor,” says Lockner.
Rotor material. Rotors traditionally are made of metal, such as anodized aluminum or titanium. Today, researchers also can choose rotors made from a carbon-fiber material. Although metal rotors remain a great choice, carbon-fiber rotors offer several advantages. They are lighter (a bonus if you anticipate having to move the rotor in and out frequently), less vulnerable to corrosion and tend to have longer lifetimes—not to mention longer warranties.
Match configuration to application
Rotors come in four basic configurations. The most common is the fixed-angle rotor, so-called because it contains sample holes bored through the rotor material at an angle (which cannot be adjusted). Fixed-angle rotors are commonly used for pelleting cells or other materials from a suspension.
The second most common configuration is the swinging-bucket rotor, which has evenly spaced holders for sample buckets or containers radiating out from the center. The buckets are attached to the rotor by pivots that allow the bucket to swing outward when the rotor is spinning.
Swinging-bucket rotors are used for such tasks as separating large-volume samples at low speeds and separating by mass or size via gradient centrifugation. “Swinging-bucket rotors have longer path lengths than fixed-angle rotors, which results in better band resolution for rate-zonal and isopycnic separations,” says Lockner. “High-performance swinging-bucket rotors can also be used for pelleting large particles, cells, cell components, as well as fractionating whole blood.”
Two less common types of rotors include the vertical (or near-vertical) tube rotor, and the continuous-flow rotor. Vertical tube rotors have short path lengths that lead them to separate components efficiently. “Near-vertical tube rotors, as the name indicates, have a slight angle that offers a compromise between very short path length and pellet accessibility” near the bottom of the tube, says Lockner.
Continuous-flow rotors are useful for large-volume samples such as from bioreactors or fermentors. Sample is pumped into the centrifuge, which isolates the solid fraction in a pellet while allowing the liquid fraction to flow into a separate container for collection. Also used in vaccine research and production, continuous-flow rotors are advantageous when you have “a large amount of starting material with a small number of cells,” says Hutcherson. “It allows you to separate out the cells quickly without having to use ‘20-something’ containers.”
Do some maintenance
Given the forces they experience, rotors should receive regular maintenance. If nothing else, performing the recommended maintenance tasks for your rotor will help to ensure that it functions smoothly for its maximum intended lifetime.
Generally, rotors should be inspected regularly for any signs of deterioration, preferably with each use. “Routinely inspect [for] signs of deterioration, such as small cracks or pitting, that the lid is not bent or damaged and that the rotor is seated onto the motor shaft properly,” says Peter Will, product line manager for centrifugation at Labnet International (a Corning Life Sciences company).
In addition, the metal pivots of swinging-bucket rotors should be greased regularly, as should the sealing O-ring of biocontainment rotors. Regular cleaning with water or just a mild detergent, and immediately cleaning up spills, are important to prevent corrosion by salts from buffers or solutions (especially for metal rotors).
After cleaning a rotor, let it dry upside-down to prevent moisture from collecting in the cavities. Of course, following the rotor manufacturer’s instructions to the letter is always best, to preserve the warranty.
Still, the unpredictable does sometimes happen, even with the appropriate rotor selection, operation and maintenance. So choosing a rotor with a good warranty is a wise move, says Ambrose. Luckily, high-quality rotors and warranties are designed to guard against the unpredictable, so you stand an excellent chance of choosing a rotor that will serve you well for years to come.