How low you go depends on the application
Scientists often try to protect hard-to-get samples by working with smaller amounts of them. What is considered “small,” though, varies. As Stefan Overkamp, field marketing manager, automation and genomics, at Beckman Coulter (Indianapolis, IN) says, what is small “kind of depends on the actual application you are looking at.” He adds, “A low volume in DNA extraction from blood is not necessarily considered a low volume in ultrahigh throughput screening, but in general, ‘low volume’ in liquid handling refers to volumes below 1 microliter.”
At the University of Texas at Austin’s Institute for Cellular and Molecular Biology, for example, Arthur F. Monzingo, manager of the macromolecular crystallography facility, uses the Crystal Phoenix from Art Robbins Instruments (Sunnyvale, CA) in preparing crystallization experiments. The smallest volumes that he uses are 0.1 microliters. When asked about the key challenges in using liquid handling to transfer such small volumes, he says that they are the “accuracy of the volume, accuracy of the location of the transfer, and speed.”
For an overview of dispensing sizes, David J. Edwards, senior director, mass spectrometry marketing, life sciences, and applied group, at Agilent Technologies (Santa Clara, CA), says, “Lipidomics and metabolomics studies generally work in the milliliter range, while proteomics research can focus primarily in the microliter range.” Other areas use even smaller samples.
According to Edwards, “Drug discovery and compound screening can be in the pico- to nanoliter ranges and require special types of liquid handlers, such as acoustic dispensers for ultra-low-volume dispenses.” He adds, “A recent trend we have also seen is the need to dispense very small volumes that contain only single cells for genomics analysis.”
Transfer tech tips
“Classic liquid handling—aspiration and dispense—can work down to volumes of a few hundred nanoliters, but that requires heavy optimization,” Overkamp explains. “The key to success is a combination of ultra-precise pipetting heads, high-quality pipette tips, and software that allows the control of the pipetting action in ultimate detail and precision.”
As scientists need to work with even smaller volumes, that requires more advanced technology. “If you go below this volume range, say to the single- or two-digit nanoliter area, or even down to picoliters, other technologies need to be applied,” Overkamp says. “These can be simple pin tools, acoustic impulse techniques, piezo technologies, or pressure systems with precise valves.” He adds, “None of these technologies allow for classic liquid handling though, so often a combination with classic liquid handlers is needed.”
Beckman Coulter focuses on classic liquid handling. With the company’s hardware, pipette tips, and control software, says Overkamp, “customers have been able to get good pipetting results down to several hundred nanoliters.” Plus, he adds, “If lower volumes are required in an assay, we can offer complete automated solutions by integrating dedicated low-volume devices into our Biomek workstations.”
The characteristics of the liquid must also be considered, because “the type of liquid being moved can dramatically influence the accuracy of the liquid transfer at different transfer volumes,” Edwards explains. “For example, nanoliter transfers of glycerol won’t be handled as easily or as accurately as waterbased reagents.”
Hudson Robotics (Springfield, NJ) concentrates on technology that transfers volumes down to low microliter quantities. At all volumes of liquid transfer, though, how it gets done affects the outcome. “We have found that slight differences in technique can make all the difference,” says Alan H. Katz, chief scientific officer at Hudson. “For example, many people use a pre-aspiration of air followed by a blowoff to maximize accuracy, but at low volumes, this can be a problem.” He adds, “When there is air in your system and you press your plunger, you are competing between releasing liquid and compressing air. All sorts of factors come into account—for example, the exact geometry of a particular tip, how wet or dry it is, how deeply it sits in the liquid—and the results can randomly vary, and are often not what you are looking for.”
Other factors impact the results as well. “We have also found that precision of low-volume pipetting is improved by over-aspirating and returning some of the liquid to the source before going to your destination. This minimizes effects from backlash,” Katz explains. “Our SOLO automated pipettor is designed to take these factors into account.”
Low volume, high throughput
Scientists and technicians don’t only go to low volumes to save sample, because sometimes they use such volumes to quickly process many samples. “Some researchers work in the world of 96-well microplates, which hold up to roughly 300 microliters of liquid,” says Katz. “When they add ‘small’ quantities of reagents, they are usually talking about single-digit microliters.” He adds, “High-throughput screening groups that use high-density microplates, like 1,536 wells and beyond, can barely fit a microliter into a well, so they focus on nanoliter volumes.”
To work with such small volumes, two statistical factors become extremely important. The first is precision, and a lab manager should know a device’s coefficient of variance. Accuracy— transferring the intended volume—is also key. As Katz points out, “You can be very precise but have lousy accuracy—like trying to hit the bull’s-eye and always missing in exactly the same spot, like hitting dead center of the ‘20’ on a dart board.”
To get precision and accuracy, a scientist must rely on the liquid-handling supplier. “It’s important to make sure your equipment is well calibrated so precision and accuracy are optimized,” Katz recommends.
Also, you might want a platform that provides lots of flexibility. As an example, Edwards says, “Our Bravo is a versatile and flexible platform for a wide range of tip heads that can handle single to 384 tips with transfer volumes of 500 nanoliters to 250 microliters.”
With the right liquid-handling platform that’s running in top form, you can transfer lots of low volumes at high levels of performance.
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