Microcentrifuge Buyers Guide

 Microcentrifuge Buyers Guide

microcentrifuge (aka microfuge) is one of those basic pieces of equipment that lab personnel use all the time but don’t really think much about. It lasts for years, needs little if any maintenance and does its job often at the push of a button. Let’s face it: They’re taken for granted. Yet when it comes time to purchase a new one, because you’re starting up or expanding a lab, perhaps, you’re best off giving it some thought. Not all microcentrifuges are created equal.

What kinds of tubes?

Perhaps the first thing to consider is: What type of samples are you planning to run?

Standard microcentrifuge tubes are 1.5 to 2 ml, come in a variety of shapes—straight or curved walls, for example, with screw tops or pop tops, and sometimes with spin columns inside—and are historically what microcentrifuges were designed for.

Standard microcentrifuges also can spin smaller tubes, such as 0.5, 0.4 or 0.2 ml varieties, but require tube adapters to reduce the size of the rotor’s holes, says Peter Will, product manager for centrifugation products at Labnet International Inc., Corning Life Science’s equipment business unit.

Many microcentrifuges can handle these and other formats—such as 5-ml tubes, cryotubes, HPLC tubes, PCR strips, microtiter plates and hematocrits (capillary tubes)—by using specialized rotors designed to accommodate them. And at least one combination—the Eppendorf 5430 centrifuge equipped with F-35-6-30 rotor—can even handle 15- and 50-ml conical tubes.

Of course, not every microcentrifuge allows for interchangeable rotors. Typically, an entry-level model will come with a single, dedicated rotor, whereas higher-end models allow for choices. In fact, says Will, “one of the greatest differentiating features [between microcentrifuges] is how many options of interchangeable rotors a customer can choose from.” Labnet’s top-of-the-line Z216 MK, for example, can accommodate seven different rotors, including one that can accommodate both standard and 0.5-ml tubes and another for PCR strips.

It’s also important to keep in mind that rotors generally are not interchangeable between different models. So it’s always best to think about the system as a combination of the microcentrifuge and rotor(s).

Speed and capacity

In addition to the type of samples they can hold, rotors differ in the number of samples they can hold. There are rotors that are compatible with Eppendorf’s 5430, for example, that will handle a maximum of 24, 30 or 48 standard tubes.

But number of samples is not the only differentiating factor. Of the two 24-tube rotors, one is fixed-angle with a PTFE (Teflon) coating for high chemical resistance; the other is a swinging-bucket rotor, which is useful for density gradient centrifugation and size separation, for instance. Both have aerosol-tight lids. (These are the only rotors of these types compatible with this instrument. Some others have aerosol-tight lids, and others do not.)

But if you had to choose just one rotor for standard tubes (and didn’t require a PTFE-coated rotor), why not just order the 48-tube rotor? The costs of the 48- and 24-tube fixed-angle rotors are not significantly different, you get higher throughput when you need it, and you can always run fewer tubes when you don’t.

Brenda Rickert, Eppendorf’s territory manager for Minnesota and North Dakota, provides one reason: speed.

“The rotors are rated for different speeds,” Rickert explains, adding, “The 48 is rated lower than the 24” (18,210 × g, compared with 30,000 x g, to be precise). The rotors have chips in them that communicate with the instrument, preventing them from being spun faster than they are rated.

Of course, the centrifuges themselves are rated for top speed, as well, and it’s important to make sure the instrument you purchase can meet the needs of your applications. “There’s a really wide market that uses centrifuges that are running at 17,000 x g or 21,000 x g—you’ll find quite a few protocols that would actually ask for that,” says Ileana Price, Thermo Fisher Scientific’s product manager for centrifugation.

For example, QIAGEN’s QIAquick Spin Handbook notes, “All centrifugation steps are carried out at 17,900 x g (13,000 rpm) in a conventional tabletop microcentrifuge at room temperature.” On the other hand, Eppendorf has a technical note on its web site indicating, “Centrifugation at 30,000 x g in plasmid DNA precipitation allows better recovery rates and shorter centrifugation times.”

Chill out

Ambient-temperature microcentrifuges are ventilated to allow the heat generated during a run to dissipate. Some instruments are kept in a cold room—although Rickert warns that some shouldn’t be, because their electronics might not do too well there over time. In fact, several vendors, including Eppendorf and Beckman Coulter, explicitly market units as “cold-room compatible.”

There is another option, but it comes at a cost. For samples that need to be maintained at a specific temperature, or if you are using the unit constantly, choose a refrigerated microcentrifuge, says Price.

Many ventilated models also can be ordered as refrigerated units, but some models come only as refrigerated units. As a general rule-of-thumb, users can expect to pay between about $1,650 and $3,000 for non-refrigerated units, and from $5,200 to $7,000 for refrigerated systems, says Will. Rotors tend to add $600 to $1,200 to the cost of the instrument, he adds.

The mini

If all you need to do is quickly get the sample to the bottom of the tube (as opposed to doing a separation), you might want to consider a mini centrifuge. These low-cost cousins tend to accommodate only six to eight tubes, have a much smaller footprint than microcentrifuges and are fixed-speed (maxing out at about 5,000 to 7,000 rpm), says Will. Labnet sells one version of its mini that accommodate microarray slides and one exclusively for PCR plates.

If nothing else, a mini centrifuge can save some wear and tear on your more full-featured units. There’s a cost benefit, too: Basic units can be had for less than $200.

But for more general, versatile use, you’re going to need a full-featured microcentrifuge. Consider what and how many samples you need to run, how fast you need to run them, and whether that’s likely to change. Think about safety—do you need a containment lid (perhaps one that’s transparent, so you can see a spill) or a unit that’s biosafety-certified? Do you need a refrigerated or cold-room-compatible unit? Does it matter how quiet it is during operation, its height above the bench or the display’s visibility from across the room? (If so, you might want to see one in action).

And of course, think about your budget. But remember to amortize—they last.

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