If you’ve ever been frustrated with solvent dripping from a pipette tip, or a film of residue inside the tip that refuses to purge, you’re not alone. Some liquids need a specialized tool to accurately pipette, including volatile, viscous, hot and cold liquids. In this article, Anna Tucker, field marketing manager at Gilson, discusses common problems pipette users encounter handling these materials, and the solutions that positive displacement offers.
Q: How common is positive displacement pipetting?
A: Many labs have a use for a positive displacement pipette. However, research tells us that only some of our customers are familiar with it as a concept. They know that they have problems with certain liquids, but they don’t know that there’s a solution.
Q: What kinds of problems are these labs facing?
A: Chiefly, they lose accuracy while dispensing volatile liquids, including many common solvents (e.g., acetone), because these leak out of the tip. These tools similarly struggle with viscous liquids like glycerol and blood that cling to pipette tips or introduce air bubbles, and liquids with low surface tension that leave behind residue. Hot and cold liquids can also affect the accuracy of air displacement pipettes.
Q. How does positive displacement overcome these challenges?
A: We developed a positive displacement pipette, Gilson’s MICROMAN E that uses a disposable capillary piston to maintain physical contact with the sample. Most other pipettes work on the principle of air displacement, meaning a column of air separates the sample from a much shorter piston attached to the plunger.
Replacing the air column with a physical piston delivers liquids more accurately and precisely, dramatically reducing systematic error in challenging solvents and viscous liquids.
Q: How does the risk of contamination compare with filter tips?
A: Filter tips offer valuable control of aerosols for air displacement pipetting, but the only completely foolproof way to prevent contamination of your sample and of your pipette is with positive displacement because, thanks to the disposable piston, sample and device are totally isolated from one another.
Many medical and forensics labs already consider this quality indispensable for controlling cross-contamination, as do labs handling radioactively labeled materials that could contaminate their tools.
Q: What is the cost difference between positive- and air displacement pipetting?
A: That depends on a few factors. Up front, expect air displacement pipette tips to be less expensive. Down the line, some labs have found that the cost of inaccurately pipetting expensive reagents adds up quickly and outweighs the premium on disposables. I have met customers that in a single 96-well plate are using a $1000 worth of reagents. So, if you are overpipetting by 4%, then instantly that’s a $40 loss in reagent costs. In extreme cases, inaccuracy upstream forces labs to repeat experiments, and that incurs additional costs that are difficult to calculate.
That said, we have customers who are so convinced of the benefits of positive displacement that they use them across the board, and we also have customers that just keep one on hand as a specialized tool. Both approaches can be economically smart.
Q. What difference will a user notice handling a positive displacement pipette? Will he or she need special training?
A: If you’ve used an air displacement pipette, you can use a positive displacement pipette. Disposable capillary pistons are pre-assembled and racked just like regular pipette tips. MICROMAN E has the Quick Snap feature, so you press the pipette into the boxed tip, and it seats into place like with any other pipette. You press the push button until you feel and hear a slight click, and you’re ready to go. You don’t need to purge, and when you eject the tip, the piston ejects with it. All this makes MICROMAN E at least as easy to use as regular pipettes.
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