Today's Centrifuges Offer Options For Every Research Need

COMPACT UNIT: Accurate Chemical and Scientific’s Saturn Micro24 microcentrifuge Centrifuges are a little like cars: For most biological laboratories they are an essential piece of equipment, but they may chug along for years with little service and less thought. When it comes time to buy a new one, make and model choices abound. With centrifuges, as with cars, capacity and features vary, but you can likely find one to meet your needs and your budget. Centrifuges work by spinning a sample-filled rotor about its axis, creating an outward centrifugal force. Exposed to this gravitational pull, or g-force, more dense particles move faster, allowing the researcher to remove denser material as it forms "pellets" at the base of the tube. Alternatively, gradient centrifugation uses media that gradually increase in density toward the bottom of the tube, causing particles to settle where their density is identical to that of the surrounding liquid. In clinical settings, centrifugation is commonly used to separate cells from surrounding plasma. It also has shed light on cellular processes, since the various cellular compartments (such as the nucleus, Golgi apparatus, lysosomes, mitochondria, and membranes) can be purified using the technique. Other applications include isolation of viruses, DNA, RNA, proteins, and lipids. Analytical centrifuges, capable of high speeds often in excess of 100,000 rpm, can measure a particle's molecular size and shape, sedimentation rate, and compressibility. To follow the diffusing molecules, analytical centrifuges use optical measuring systems. LOW PROFILE: IEC’s CL3 centrifuge All of these options require careful analysis of future research protocols-not to mention future budget restrictions-under which the instrument will operate. Still, one should not overlook the obvious, advises Bob Snapka, an associate professor of radiology at Ohio State University. "Be sure that the model you are considering has rotors with a suitable number of sample holes. If every experiment you do involves, say, 24 samples, and you can only get a 20-place rotor for your centrifuge, then every step that requires centrifugation will have to be done twice in order to complete the experiment. It can double the time required for each experiment. Better to have a 24- or 48-sample rotor to start with." A quick look at company catalogs reveals a wide variety of rotors that can handle sample tubes of all shapes and sizes. Another factor to consider is the time it takes for a centrifuge to reach the set speed. Those that can go from 0 to 30,000 rpm rapidly endear themselves to researchers with short-run experiments. "Slow-starting centrifuges make it difficult to tell exactly how long you have been spinning, [since] the timer starts when you press the start button," observes Cliff Luke, a molecular biologist at the Institute for Animal Health in Berkshire, United Kingdom. To help get around that problem, Beckman Instruments Inc. of Fullerton, Calif., incorporates the switched-reluctance drive (SRD)-licensed from Switched Reluctance Drives Ltd. in Harrogate, U.K.-into its Avanti J-25 and Avanti J-30I series high-performance centrifuges. The SRD induces high torque at low speeds to afford high rates of acceleration and deceleration, according to Patricia Silver, centrifuge product marketing manager at Beckman. "The net result is much less time per centrifuge run while maintaining even the most sensitive gradients and pellets," she says. Another feature that is being offered by more and more companies-and welcomed by researchers-is the brushless motor. Traditional centrifuge models have carbon "brushes" to rub against motor windings, producing current. The dust produced complicates cleanup and gets into the works, causing wear and tear and frequent maintenance. By contrast, a brushless induction system inflicts no physical contact on the motor. Instead, the system uses a magnetic field to induce the current. "It is much more reliable over the long term . . . and much quieter. [Noise] is often listed as a major factor in centrifuge selection," reports Edie Davis, continuous-flow centrifugation specialist at Heraeus Instruments Inc. of South Plainfield, N.J. Heraeus has incorporated the technology into all of its centrifuge models. But the drive system isn't the only source of friction in centrifuges. The spinning rotor must contend with air resistance. Beckman's Friction Reduction System lessens air resistance on the spinning rotor, cutting back the heat output of the centrifuge. That in turn reduces the amount of refrigeration power necessary to keep the rotor cool. It's also good for the rotor; friction can add to the downtime of the instrument, explains Silver. The feature comes standard with all of Beckman's high-speed/high-performance centrifuges. Varying ambient temperature-and the warming that results from a high-speed rotor-can wreak havoc on heat-sensitive samples. "Ventilated centrifuges have fans which circulate cooler air into the centrifuge to dissipate heat, [but] it is physically impossible to remove all heat without refrigeration," says Ursula Jablonski, centrifuge product manager at Brinkmann Instruments Inc. of Westbury, N.Y. One common solution is to operate centrifuges in designated cold rooms, but this method has drawbacks. Condensation is no friend to the exposed electrical gear of a centrifuge, and "traffic in and out makes it difficult to maintain a constant temperature," Jablonski explains. BROAD CAPABILITIES: Beckman’s Optima XL-A and XL-1 Series ultracentrifuges Centrifuges not only can prepare samples for further analysis, but also can be effective analysis tools in their own right. Beckman's Optima XL-A and XL-1 analytical centrifuges can characterize molecular mass, size, and shape-and they require no standards. The dual optical systems of the XL-1 provide sensitivity at low concentrations and selectivity for spotting individual components in a mixture, according to company literature. Ultracentrifuges reach speeds in excess of 60,000 rpm. The higher centrifugal force they generate makes possible tough separations of particles like lipoproteins, viruses, and plasmid DNA. Beckman's Optima K ultracentrifuge series uses an air-driven, thermoelectrically cooled drive system that significantly reduces heat output and drive wear, claims Silver. When units run at speeds of 120,000 rpm and with centrifugal forces of up to 802,000 times the force of gravity, heat generation and wear can be a pretty serious problem. On the other end of the scale, several companies offer general-purpose centrifuges that are ideal for common separations and suited for a wide variety of other uses. Accurate Chemical and Scientific Corp. of Westbury, N.Y., offers its Saturn6, an inexpensive instrument with 10 speed settings to 3,800 rpm. It is ideal for situations in which one needs a basic instrument and is on a tight budget-such as in physicians' offices or school classrooms, according to Rudy Rosenberg, president of Accurate. The company also offers the Saturn Micro24, with speeds to 13,000 rpm. It is suitable for research labs and designed for 2 ml or smaller sample tubes. For the physician's office, Medical Equipment Resources of Minneapolis sells its Physician's Centrifuge, a compact, portable unit for handling urine or fecal sedimentations and blood separations. These single, fixed-speed units have a heavy-gauge aluminum housing. Heraeus's Contifuge 28 RS, with speeds to 28,000 rpm, and Contifuge 17 RS, with speeds to 17,000 rpm, are both refrigerated. They are useful for quantitative pelleting; preparation of samples for chromatography, separation, or concentration of cell debris; particle-size analysis of packed pellet samples; and other applications. Both models are fitted with a frequency-controlled induction drive, which ensures quiet and vibration-free centrifugation, according to company literature. GENERAL-PURPOSE: The Eppendorf 5416, marketed by Brinkmann Instruments Brinkmann markets the Eppendorf line of centrifuges in North America, including the general-purpose models 5416 (with speeds to 15,000 rpm) and the refrigerated 5403 (with speeds to 15,000 rpm). The instruments feature a variable-speed, brushless motor; an automatic rotor detection system that senses the rotor being used and displays the maximum permissible speed; and digital display of time, speed, and temperature. Brinkmann also offers a series of interchangeable fixed-angle and swing-bucket rotors. The Centra-EC series and Centra-CL4 models-sold by International Equipment Co. (IEC) of Needham Heights, Mass.-are designed specifically for clinical settings. The 6,000-rpm Centra-CL4 can spin up to 48 blood and urine samples at a time in safety-sealed buckets, according to product literature, while taking up just 18.5 inches of bench space. The 4,000-rpm Centra-EC series offers a range of models to fit a clinical lab's specialized needs. The list includes refrigerated or ventilated models; units that can accompany different numbers and types of sample tubes; and devices of various sizes. PROGRAMMABLE: IEC’s PR-7000M centrifuge IEC has replaced its Clinical centrifuge line with the CL-2, which runs at speeds to 8,500 rpm. It offers some new features, including a cover interlock, an electronic timer, variable speed control, and a variety of aerosol containment options. The timer starts when the rotor reaches 95 percent of the set speed instead of at the beginning of the run, eliminating the headache of estimating how long it takes the centrifuge to get up to speed. Other IEC centrifuges include the Centra-CL3, a low-profile instrument with speeds to 8,500 rpm; and the large-capacity, programmable models PR-7000M and B-22M, with speeds to 6,900 rpm and to 22,000 rpm, respectively. Cole-Parmer Instrument Co. of Vernon Hills, Ill., sells fixed- and variable-speed centrifuges. The fixed-speed E-17250 models spin at 3,500 rpm. They are outfitted with a molded-in rotor cover and rotor collar gasket that seal the rotor off, protecting against aerosols and splashes. Its variable-speed centrifuges, the E-02550 models, which can operate to 5,500 rpm, feature an automatic braking system that stops the rotor in 38 seconds and is capable of run times up to 60 minutes. The interlocking lid is designed not to open until the rotor has stopped moving. The company also sells a large-capacity, variable-speed model, the E-17451, which spins to 5,000 rpm. Researchers with programmable centrifuges on their wish lists might consider Cole-Parmer's programmable universal centrifuge, with speeds to 14,000 rpm. Users can store the value of the centrifugal force to be applied as well as the speed, time, acceleration, and braking rate for up to 10 runs. The instrument also includes a rapid-cool mode that can pre-cool the rotor in one-fifth of the usual time, according to the product literature. Cole-Parmer's high-speed universal centrifuges, with speeds to 17,000 rpm, have an automatic imbalance detector that prevents damage to the samples by gently bringing the rotor to a stop. A radius-correction feature calculates the exact g-force, taking the size of the sample tubes into account. Beckman's general-purpose centrifuges include its GS-6 series, with speeds to 6,400 rpm. The units have capacity up to 3 liters and can accommodate every popular-size tube, according to the product literature. The company also offers its slightly less expensive small-capacity GS-15 series. Units in this series use only 16.6 inches of bench space and spin at 15,300 rpm. Large-volume users may want to consider Beckman's J-2 series, which can handle up to 3 liters of volume and speeds to 21,000 rpm. These models are ideal for routine applications such as cell separations, ethanol precipitations, immunoassays, and spin columns, according to Silver. Devices in the Avanti J-25 series are available with a system to automatically identify rotor characteristics. In addition to routine applications, the J-25, which spins to 25,000 rpm, can handle plasma membrane and virus separations, says Silver. The Avanti J-30I, with speeds to 30,000 rpm, is capable of differential centrifugation of subcellular organelles and viruses. Microcentrifuges (or microfuges) are the ideal solution to limited bench space. They take up less space than their larger cousins, sometimes less than 10 inches. These centrifuges generally offer the same capabilities as general-purpose machines, but with less sample capacity. Consider the Biofuge fresco, sold by VWR Scientific Products of West Chester, Pa. The unit, which spins to 13,000 rpm, is the smallest refrigerated centrifuge in the world, according to the company brochure. It is equipped with an induction drive that minimizes noise and vibration. VWR also offers the nonrefrigerated version, the Biofuge pico, which comes standard with a 24-place fixed-angle rotor, suitable for sample sizes up to 2 ml. On both models, the ergonomic design makes the machines easier to use, even for researchers wearing laboratory gloves. REFRIGERATED: Brinkmann Instruments sells the Eppendorf 5403 centrifuge. The Eppendorf Micro Centrifuge model 5415C (with speeds to 14,000 rpm)-marketed by Brinkmann-accepts disposable spill guards to protect the machinery from contamination, which simplifies decontamination and instrument cleaning, according to Jablonski. The Eppendorf 5417C, which spins to 14,000 rpm, offers a choice of four interchangeable rotors, a variable-speed brushless motor, and an adjustable-speed momentary spin button, which helps ensure that delicate samples won't be damaged during quick spins. The 5417R is the refrigerated version. IEC's Micro-MB centrifuge features a high-speed motor capable of speeds to 14,000 rpm. It also offers a momentary spin-control option for short spins, and a user-activated brake that provides a quick-braking action or a slow, gentle coast mode. With the right components, the Micro-MB can also be used to determine packed cell volume using the Microhematocrit Method. IEC's Micromax, with speeds to 15,000 rpm, is capable of high-speed centrifugation of 24 1.5-ml to 2-ml samples, or 48 microtubes. Cole-Parmer sells fixed- and variable-speed microcentrifuges with a three-position switch for "pulsing," continuous operation, or a time cycle from one to 10 minutes. The Eppendorf variable- and fixed-speed microcentrifuges reach maximum speed in 10 seconds, while the digital variable-speed microcentrifuge (E-17353, which spins to 13,200 rpm) accelerates to maximum in seven seconds, according to the company. At a slight 8.5 inches wide, Beckman's Microfuge Lite takes up little bench space. It comes with an 18-place rotor that can spin 1.5-ml or 2-ml tubes to 13,500 rpm, while the Microfuge R-the refrigerated version-has a 24-place rotor. Centrifugal vacuum concentrators use a vacuum to draw off solvent and dry multiple samples. The centrifugal force helps prevent solvent from bubbling over as it evaporates. Savant Instruments Inc. of Holbrook, N.Y., offers a variety of SpeedVac instruments. The company sells a series of instruments with features to go with aqueous or organic solvents; differing sample capacities; and low, medium, or high boiling solvents. Savant's automated integrated systems include a concentrator, a 4-liter cold trap to collect the solvent as it is removed, and an oil-free Teflon vacuum pump. Savant's line includes three systems: Two automated Environmental SpeedVacs can handle all solvents, while the Model ISS110 is designed primarily for aqueous/organic mixtures. Savant's DNA SpeedVac is a dedicated concentrator for drying of small volumes of methanol/ethanol precipitates of DNA and RNA. Three drying rates allow the researcher to optimize evaporation. For radioactive samples, Savant trap kits-attached to the exhaust port-are designed to block radioactive free radicals. SELECTED SUPPLIERS OF CENTRIFUGES: Accurate Chemical and Scientific Corp. Beckman Instruments Inc. Brinkmann Instruments Cole-Parmer Instrument Co. Heraeus Instruments Inc. International Equipment Co. Medical Equipment Resources Savant Instruments Inc. VWR Scientific Products When considering the array of models and features, a researcher ought not to lose sight of the need for a mechanic-normally the company-that will service the instrument. Philip Schwartz is project manager at Affinity BioReagents Inc. in Golden, Colo., which sells antibodies, recombinant proteins, kits, and other reagents for basic research purposes. "A little over a year ago, we were in the process of expanding our lab and in need of an all-purpose refrigerated centrifuge," he recalls. "Several models looked okay, but [one company's rep] really went the extra mile for me by putting together a 'fuge/rotor package that matched our needs perfectly and at the right price. In the year that we have had the unit we experienced a cooling problem from the refrigeration condenser. This was immediately reported to [the company's] service department. . . . One day later [the company] was taking care of the instrument. I do not think it gets much better than this!" With service, budget, and an array of features to consider, deciding on a centrifuge to meet your needs is a complicated task. But a model that keeps your lab running smoothly over its lifetime is worth its weight in gold. James Kling is a science writer based in Bellingham, Wash.

Today's Centrifuges Offer Options For Every Research Need

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