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Please see our Refrigerated Laboratory Centrifuge section to find manufacturers that sell these products
At their most basic, centrifuges are little more than a motor that spins around, effectively pushing objects out with a force exponentially proportional to their distance from the center of rotation. Some centrifuges sit on a desk and do little more than force liquid to the bottom of small tubes or microtiter plates at the push of a button. Others take up considerable floor or bench space and hurl objects at millions of times the force of gravity (g) in order to separate them according to slight differences in molecular weight.
Centrifuges are used across the gamut of life sciences, chemistry, and physics laboratories, in polymer and nanoparticles materials research, even in the food and beverage industry. Unprotected, they could send heavy metal saucers, bottles of liquid, or slices of glass through the lab at high speed, or allow the air to become contaminated with radioactive, toxic, or infectious fluids. Yet they tend to be among the lab’s underappreciated workhorses, lasting for generations—there when you need them, and ignored when you don’t.
Today’s centrifuges, of course, are far more than a motor, and come with a host of safeguards. They are available in many different sizes, speeds, and configurations, with a variety of sample holders and features. Here we look at refrigerated centrifuge options for the modern research lab.
Cold enough for you?
There has been a shift over the last five years or so “where more researchers are starting to see the benefits of spending the extra money to buy a refrigerated centrifuge,” says Matt Lieber, Eppendorf’s product manager for centrifugation. He cites at least two arguments against merely sticking a centrifuge in a cold room.
The first is that operating a centrifuge itself generates heat. And while the outside may be held at 4°, the bowl containing the samples won’t be. “That means you’re not really spinning at as low a temperature as you think.” While this may not be a huge concern working with nucleic acids, “you take a big risk that proteins will denature.”
Second, “if you ever pull that centrifuge back out of the cold room, you’re going to have condensation on a lot of the electrical components,” he says, “and that is obviously a risk.” To alleviate such concerns, there are some small and low-speed centrifuges that are explicitly marketed as being “cold room compatible.”
But small and low-speed centrifuges are also the only ones that will even have a nonrefrigerated model to choose from. Typically faster centrifuges (other than microcentrifuges) will generate so much heat that without refrigeration the samples would become overheated, notes Randy Pawlovich, director of product management for Beckman Coulter centrifugation.
Most of the work in a refrigerated centrifuge is done at 4 °C. Most centrifuges will claim a lower temperature range of –20 °C, but not all. The rating may pertain to all conditions, or it may be the result of a single rotor being tested in a 70 °C lab in low humidity, cautions Pawlovich. Similarly, the temperature may be sensed in the bowl, or in the rotor, or both. “Unless specs are well-defined they can be misleading.” He suggests checking the specs and, if tight temperature control is important, asking a lot of questions.
Place, speed, and size
The world of centrifuges is divided up in different ways. The first distinction most vendors make is typically by where it’s to be located, that is, benchtop vs floor models. Benchtops are generally more compact and have a smaller capacity than their standalone cousins. These are each themselves in turn subcategorized, yet the lines are not always clear.
The closest thing there is to a hard and fast rule is what’s spun in them, at least in the benchtop category, notes Lieber. “Microcentrifuges” are those in which 1.5-mL tubes are typically spun, whereas in “general purpose” (GP, or sometimes just plain “centrifuge”) units you’re looking at mostly 15- and 50-mL conical tubes. Of course, this is not to say that microcentrifuges can’t run 50-mL conicals (which some can) or that GPs aren’t used for smaller tubes (which they are).
Dan Hanle, commercial manager for centrifugation at Thermo Fisher, considers GPs to be at the top of a three-tiered benchtop hierarchy, with the “small benchtops,” which spin at slower speeds, in the middle. Assigning specific speed ranges to these categories, too, is a nebulous exercise. For example, Thermo Scientific GPs quote maximum rpms between 15,200 and 23,300, depending on the model and rotor, while small benchtops have top speeds between 4500 and 16,000 rpm.
In addition, vendors such as Thermo Fisher, Beckman Coulter, and Hitachi offer a benchtop “ultracentrifuge” which, because of the unique functionality, is typically compared with other ultracentrifuges rather than other benchtops (see below).
In addition to the ultracentrifuges with top speeds from 30,000 to perhaps 150,000 rpm, Hanle categorizes floor-standing centrifuges as “superspeed” or “high-speed,” with maximum speeds between about 10,000 and 30,000 rpm, and “large-capacity” or “low-speed” models topping out at between 6000 and 10,000 rpm. The latter are often used to spin down liters of material for applications such as blood banking and bioprocessing.
In the past, an ultracentrifuge was typically anything capable of generating a force of over 100,000 × g, says Pawlovich, but for many years, Beckman Coulter has had a high-performance centrifuge generating up to about 110,000 × g. “After that you have a whole lot of things that have to change in the systems to be able to push the performance above that. We go over a million × g in ultracentrifuges—so it goes up almost an order of magnitude in the force that it can apply.” Explains Hanle,“You have to trade off torque for high speed—you can’t make a super speed do the high end work of an ultra.”
The vessel
The capabilities of a centrifuge are a combination of instrument and rotor, and there are several factors that should be taken into consideration when selecting that combination, notes Peter Will, product line manager for Labnet International, a Corning Life Sciences company. These include “how many tubes will be spinning in one given run, if the rotor requires an aerosol-tight cover, and if a fixed angle or a swing-out rotor is needed.”
Except for the very basic models, most centrifuges will accommodate several different rotors, some of which may be specific to a given model. Especially for super speeds, notes Hanle, it’s not uncommon for the inventory of rotors to cost as much as the instrument itself.
Choosing the appropriate rotor/centrifuge combinations will often involve a user going through a decision tree—perhaps helped along by charts, web-based apps, or a salesperson’s expertise—with nodes that include things like the type and number of sample containers, total capacity, and maximum g-force. Fixed-angle rotors are often rated for higher speeds, for example. Swinging-bucket rotors, on the other hand, can be particularly versatile because “they generally have one or two or three types of buckets that will fit on the arms,” notes Lieber. “And then in those buckets you can put in a number of different adapters for different types and sizes of vessel.”
New and improved
Many vendors are more than happy to tout their green credentials. Eppendorf’s ECO shut-off software feature turns the compressor off after a period of nonuse, for example. Beckman Coulter invokes a partial vacuum in its higher-speed centrifuges that reduces air drag.
Table 1 – Providers of refrigerated laboratory centrifuges
Others tout ergonomics and ease of use. Labnet’s microcentrifuge tube rotors are uniquely designed so that caps of tubes can be readily accessed, making the tubes easy to remove after a run, for example, while several manufacturers are designing their benchtop centrifuges with lower profiles and easier-to-use latches, to make it simpler to change rotors and load samples. Thermo Fisher’s new Lynx platform has onboard video tutorials, accessible on the touchscreen.
Carbon fiber technology has allowed Thermo Fisher to make larger and faster rotors than is possible with machined metal.
Whether for microplates, hematocrits, PCR tubes, or liter-sized bottles, fast or slow, –20 or +20 °C, there is a refrigerated centrifuge/rotor combination for the project. All you have to do is look and ask.
A list of refrigerated laboratory centrifuge providers is given in Table 1.
Josh P. Roberts has been a full-time biomedical science writer for more than a decade. After earning an M.A. in the history and philosophy of science, he went through the Ph.D. program in molecular, cellular, developmental biology, and genetics at the University of Minnesota, with dissertation research in ocular immunology; e-mail: [email protected]
Please see our Refrigerated Laboratory Centrifuge section to find manufacturers that sell these products