Jeffrey Perkel has been a scientific writer and editor since 2000. He holds a PhD in Cell and Molecular Biology from the University of Pennsylvania, and did postdoctoral work at the University of Pennsylvania and at Harvard Medical School.
If there’s one device that’s ubiquitous in the life-science laboratory, it’s the handheld pipette. Used to dispense anywhere from about 0.1 microliters up to 10 ml, pipettes are like opinions: Everybody’s got (at least) one.
And they all look more or less alike. Indeed, most if not all micropipette manufacturers have adopted the same basic aesthetic: a long, tapered barrel with a handle and buttons at the top and a window indicating the selected volume on the side. Researchers grip them like a dagger, blade-down.
Yet despite their similarity, all pipettes are not created equal. Nor are they inexpensive—a typical set of three single-channel manual pipettes costs about $1,000. So it makes sense to consider your needs before choosing your next pipettor. Here we break down some of the key variables you should think about before your next purchase.
Manual vs. electronic
The first variable users should consider is daily pipetting workload, says Suvi Berghall, pipetting systems product manager for Thermo Fisher Scientific --basically, how much pipetting do you think you’ll need to do and how complex your protocols are. In other words, will you be performing steps like mixing and multiple reagent additions over and over? If the answer is yes, consider an electronic pipette.
Instead of the traditional thumb-controlled pipetting button and manual volume adjustment, electronic pipettes feature a digital interface, like a cell phone’s, to adjust volume, and a motorized piston for aspiration and dispensing. They also enable useful preset programs, such as a multidispense mode, in which users might aspirate 1,000 μl and dispense it out 100 μl at a time, and a mixing mode.
Electronic pipettes offer a number of tangible benefits. They generally are more precise than manual pipettes, Berghall says, as the piston’s motion is not subject to user-to-user variation. They also are faster and more reproducible. Plus, all those saved pipetting motions and button pushes translate into improved ergonomics.
That said, electronic pipettes are also considerably more expensive than their manual counterparts. Thermo Fisher’s single-channel manual pipettes cost about $300 apiece, but an electronic pipette will run you about $700. Eppendorf’s single-channel manual pipettors cost $348, says Melinda Sheehan, the company’s North American product manager for liquid-handling products; electronic pipettes start at $730.
Single vs. multichannel
For day-to-day pipetting chores, a standard, single-channel pipette is your go-to liquid handler. But if you find yourself working extensively with microtiter plates, consider a multichannel pipette, too.
According to Sheehan, multichannel pipettes—Eppendorf offers 8- and 12-channel versions—offer both ergonomic and accuracy improvements. The ergonomic benefit arises from simple math: A 12-channel pipettor requires one-twelfth the pipetting (and time) of a single-channel pipette.
As for accuracy, every pipetting operation increases the likelihood of a mistake, such as dispensing the wrong volume, missing a well or introducing an air bubble. With a multichannel pipette, Sheehan says, “you significantly cut down the chance of error.”
Fixed vs. variable volume
Labs that routinely perform the same task over and over should consider fixed-volume pipettes.
On the plus side, says Sheehan, fixed-volume manual models are generally the most accurate pipettes you can buy. (Fun fact: “[Variable-volume] pipettes are always the most accurate at the higher end of the volume range,” Sheehan says. That means a 10- to 100-μl pipette will dispense 100 μl more accurately than a 100- to 1,000-μl one. But neither will be as consistently accurate as a fixed-volume model, she says.)
On the other hand, fixed-volume pipettes are, obviously, fixed, meaning you most likely will also need a variable-volume pipettor, as well. Furthermore, not every volume is readily available. Eppendorf offers its Reference® 2 single-channel, fixed-volume pipettes in the following sizes: 1, 2, 5, 10, 20, 25, 50, 100, 200, 250, 500, 1,000, 2,000 and 2,500 μl. Gilson’s PIPETMAN® L Fixed line includes the same options, as well as 300 μl, 400 μl and 5000 μl models (and omitting the 2,500 μl one). But neither company offers, say, a 7.5-μl instrument—though Eppendorf might be willing to custom build such a device, Sheehan says, assuming the quantity needed was high enough.
Air cushioned vs. positive displacement
Most standard micropipettors feature a so-called “air-displacement” design. They have a cushion of air between the piston and the sample, and as the piston rises, so too does the cushion, creating a vacuum that is filled by the sample.
That architecture works well for water and aqueous solutions, says Sheehan, but less so for liquids of differing densities, viscous solutions and volatile compounds—not to mention, use in any environmental condition that might alter the size of the air cushion. In those situations, a positive-displacement pipette is a better option.
Positive-displacement pipettes use special tips in which the piston (which, unlike in air cushion models is part of the consumable itself) comes into direct contact with the solution, like a syringe. “Positive displacement doesn’t work based on density or mass; therefore, it’s not affected [by environmental factors and liquid properties],” Sheehan explains. “You’ll be pipetting a true volume rather than a mass associated with a volume.”
Another (ahem) positive feature: Positive-displacement pipettes often enable an ergonomically friendly multidispense mode. Both Thermo’s Finnpipette™ Stepper Pipetter and Eppendorf’s Repeater® Plus enable users to aspirate a large volume and dispense it in smaller steps. (These pipettes are sometimes called “stepper” pipettes for this reason.)
Ergonomics
You’ll be using your pipette all day, every day, so consider its ergonomic properties carefully. For instance, says Berghall, how heavy is the pipette, how much force is required to operate the pipetting button, and how much force is required to attach and eject a pipette tip.
In all these variables, today’s pipettors far surpass their predecessors. A whitepaper (PDF) on Rainin’s new ergonomic LTS™ LiteTouch™ Tip Ejection System says LTS pipettes require just 0.6 kg (5.9N) to eject a tip, compared to 4 kg (39.2N) with standard designs. According to Sheehan, Eppendorf’s original 100- to 1,000-μl Research pipette required 6.8N of force to push the aspirate/dispense button to the first stop; today’s Research Plus takes just 2.9N. (By comparison, other pipettes tested in an Eppendorf Technical Report (PDF) required between 4.9 and 11.5N.)
The new Thermo Scientific™ F1-ClipTip™ Pipetting System is designed specifically to reduce the force of tip attachment and ejection, says Berghall. Most pipettors use friction to attach a tip; as a result, users often forcefully pound the pipette into the tip to make sure it is seated and properly sealed. The F1-ClipTip system features mechanical clips that lock the tip in place with a light touch, while a sealing ring ensures a complete seal in every channel.
According to Berghall, the force required to eject a ClipTip tip is up to 75% lower than friction-based tips. “There’s a huge ergonomic benefit, particularly for those who are doing a lot of pipetting,” she says.
That said, ClipTip tips only work with Thermo Scientific’s F1-ClipTip pipettes, which means users must buy their tips through the manufacturer. Sheehan advises users to consider consumables prior to making a purchase, especially if they anticipate needing specialty pipette tips. “If they want a specialty filter tip and [the manufacturer] doesn’t make that tip, they can’t buy it. They don’t physically fit.”
Cleaning and maintenance
Single-channel pipettes usually can be serviced in the lab, though many labs outsource that task. For day-to-day work, Berghall recommends users clean their pipettes with a 70% ethanol wipe-down. Some pipettes also can be autoclaved, but others, including electronic pipettes, cannot. (You can usually disassemble and autoclave the lower part of a single-channel pipette’s barrel, in any event.)
Because of their more complex design, multichannel pipettes usually must be serviced by trained service providers. But what if they break? “If you over-aspirate in one channel or drop it on the floor and one channel snaps off, with some pipettes you have to replace the entire manifold,” Sheehan says. Eppendorf, though, allows users to clean, maintain and replace individual channels.
And there are other variables, too. Some pipettes feature barcodes or RFID tags for traceability; others sport volume adjustment locks, left-and-right-handed controls, and advanced ergonomic designs.
The bottom line is this: Choose your pipette carefully -- you'll be relying on it for years to come.
Image: The Thermo Scientific™ Matrix Multichannel Electronic Pipetter.