by Caitlin Smith
A microcentrifuge, also called a microfuge, is an important piece of lab equipment; it is used to spin small (2 ml or less) liquid samples at high speeds (generally tens of thousands times g-force). Centrifugation of small samples is important for many biological applications, such as pelleting nucleic acids or proteins from solution, microfiltration of small aqueous samples or simply to gather those last precious drops of liquid into the bottom of the tube. Most often, microcentrifuges are small, compact benchtop centrifuges with a small footprint, available in refrigerated or non-refrigerated (ventilated) models. Here are some factors to consider when choosing a microcentrifuge for your lab.
Consider capacity and sample size
The capacity of a microcentrifuge—the maximum number of sample tubes it can hold in one run—and the size of the sample tubes are concerns directly related to how much of your sample you need to process at one time or in one day. In other words, what kind of throughput capacity do you need? Many microcentrifuge rotors are optimized to spin 18 to 24 1.5 ml or 2.0 ml tubes. Many manufacturers offer different rotors or rotor adaptors that can be switched out to accommodate different types or sizes of tubes. Consider whether you’ll need any unusual tube sizes and ensure that the microcentrifuge model you choose includes adaptors for these (or at least offers them separately).
The Thermo Scientific “dual” rotor is designed to accommodate both the 1.5 ml and the 2.0 ml tube sizes in Thermo Scientific’s ventilated and refrigerated models. “When you need to run a non-standard tube, you have another row of holes to fill instead, and you’re ready to go,” says Maurizio Merli, senior product manager for benchtop centrifuges at Thermo Fisher Scientific. “No adapters, no extra time required.” Elizabeth Murphy, product specialist at Cole-Parmer, which also offers Thermo Scientific microcentrifuges, says the “dual-row rotor eliminates the need for adapters for either 1.5 or 2.0 ml tubes. These Thermo Scientific microcentrifuges are truly versatile, with many other rotor options available.”
Also consider whether you need the ability to spin other types of sample containers, such as plates or larger tubes. Such flexibility can be found in Eppendorf’s crossover or hybrid microcentrifuge, the 5430 (and refrigerated 5430R). “It has the footprint of a microcentrifuge, spins all microcentrifuge tubes, and in addition it can also spin PCR plates and 15 to 50 ml conical tubes,” says Lars Borrmann, group marketing manager for centrifugation and sample prep at Eppendorf. “Lab space is at a premium, and so to be able to have this versatility on your lab bench—that’s unique.” The 5430/5430R spins up to 50 ml conical tubes using a fixed-angle rotor, and spins PCR plates using a swing-bucket rotor. The fixed-angle rotor also can be used to spin 10 to 50 ml Oak Ridge tubes and cryo and HPLC vials. The crossover model is advantageous to researchers who sometimes need to use a multipurpose centrifuge and would benefit from the convenience of having it on their benchtop. Eliminating the hassle and waiting time required with sign-up sheets (often used for common lab or facility equipment) can save valuable time and money.
Consider safety
Spinning hazardous materials is sometimes necessary for the successful execution of an experiment. Clinical or medical laboratories also may need to process blood or other tissue samples using microcentrifugation. Hazardous samples are especially unsettling to work with because occasionally sample tubes break—and they may contain radioactive labels or contagious biohazardous substances such as bacteria, HIV or other viruses. In this situation, a rotor lid that fits well enough to ensure user protection and that is certified for use with biohazardous materials is a wise choice.
Generally, rotor lids are made from either plastic or metal; both choices have advantages and disadvantages. Plastic lids have the advantages of being transparent and preventing corrosion. The Thermo Scientific ClickSeal plastic rotor lid enhances the company’s microcentrifuge safety as well as ease of use (see below). “It is transparent,” Merli says, “so you can see if a tube broke during the run before you open the lid.” However, it might be difficult to see a broken tube through the lid if the break is on the bottom of the tube. Because the ClickSeal lid is made from polyphenylsulfone (PPSU), “it is very resistant to mechanical stress and temperature variations,” says Merli. “PPSU has many interesting applications; it’s used to manufacture silverware for prisons, because it’s so resistant that you can’t crack it to get a sharp cutting surface to use as a weapon.” Merli notes another advantage of PPSU: It is corrosion-free, unlike metal, which can corrode over time.
Metal rotor lids have the advantage of strength, compared with plastic lids. It is possible for a plastic rotor lid to break, which of course is dangerous when using hazardous samples. Though metal rotor lids can corrode over time, proper rotor and lid maintenance helps to ward off corrosion, along with an anodized coating that protects the metal.
Consider ease of use
Ease of use sounds like an obvious factor in choosing any lab equipment, but this can make a big difference for researchers who use a microcentrifuge every day, perhaps multiple times a day. More than just making your day easier, greater ease of use can translate into smoother protocols, fewer errors, better results and fewer repetitions. Murphy notes that generally it is good to consider flexibility, reliability and throughput. “Look for a microcentrifuge that has many interchangeable rotors so that a single unit performs many functions. Select a refrigerated centrifuge for high-throughput labs, temperature-sensitive samples—for example, DNA, RNA, proteins—and areas with high ambient temperatures due to geography or equipment,” she says. “Programmability is also important when multiple users are handling the equipment throughout the day.”
Ergonomic features: You also may look for features that are designed to make using the microcentrifuge easier and less physically stressful. For example, many of Eppendorf’s models use the company’s OptiBowl feature, which enables its microcentrifuges to spin without a rotor lid to reduce handling steps. “You can spin without the rotor lid, and the unit is still whisper quiet,” says Borrmann. “That means for standard applications you don’t have to use the lid anymore.” For hazardous materials, Eppendorf’s aerosol-tight QuickLock™ metal rotor lids require no pressing down, but rather a quarter turn for safe lid closure. For ease of use in the non-refrigerated models, the centrifuge lid opens automatically when finished, allowing you to see that the run is complete even from across the lab.
Thermo Scientific microcentrifuges are made as easy to use as possible with the ClickSeal lid, “our innovative rotor lid locking system, where you just put the lid in place and snap it closed with one finger,” says Merli. “Then to remove it, simply press a button. No more turning handles so many times a day.” Merli also believes ease of use should include time away from the microcentrifuge. “The display of Thermo Scientific microcentrifuges employs big white LEDs for a bright, highly visible display that can be seen meters away,” he says. “[Because] you can easily read how much time is left, you can monitor the status of your unit at a glance while you keep working.”
Spin your tube with columns and filters attached: Another feature that can increase ease of use for researchers who use their microcentrifuge for microfiltration is the ability to spin tubes with filters attached, for example when using miniprep spin column kits. “One of the problems when you work with these kits is that in the elution step you have to fit that little filtration column into a microcentrifuge tube, and you have to spin them with open tube lids,” says Borrmann. “That’s a problem when you spin the tubes with open lids because the tube lids can shear off. You can either cut off the lids or turn the tubes in a special orientation that usually reduces the capacity of the rotor.” Researchers who use these kits regularly may want to consider microcentrifuges such as Eppendorf’s models 5424 and 5430, which offer a kit rotor with a raised rim that prevents open tube caps from shearing off. Another example is MIDSCI’s new 24-tube Prism. “The rotor is designed to hold all standard 1.5/2.0 ml tubes, both conical-bottom and skirted, and the snap-on lid offers space for microfilter filters that fit into the tops of the tubes,” says Mike Degenhart, vice president of MIDSCI. “Our new friction-fit lid latching system combines reliability and safety with easy operation.”
For major pieces of lab equipment, so many ease-of-use features are also related to safety—
especially when the equipment spins at tens of thousands times g-force. All these factors are important to keep in mind, no matter what model you are considering. Something as simple as a secure lid could save your data, or even your life. “Think of it as the simplest way to achieve the highest safety conditions in microcentrifugation,” Merli points out. “Like fastening the seat belt on your car: It’s a two-second thing that can save your life if something goes wrong along the way.”
The image at the top of this article is Eppendorf's Microcentrifuge Model 5430 R.