ANALYTICAL & LABORATORY EQUIPMENT
PICK THE PERFECT PIPETTE
Air displacement or repetitive pipette? Kathrin Friedrich off ers guidance on how to make the right choice for your application
E
very day, various liquids and volumes are dispensed in the laboratory. T e instruments used for this purpose are a key element in the lab. T eir choice determines whether an experiment can be carried out successfully and reproducibly. Pipettes and dispensers are necessary
tools. High-quality dispensing units enable precise and fast work and support the user in every application. But there are technologies and instruments to choose from. How can a user fi nd the right one for their application? T e most common distinction is
between positive displacement and air displacement systems. T e two systems, which function very diff erently, allow the user to work with almost any liquid without any loss of accuracy or reproducibility. But how do these two systems diff er? And what are their advantages and disadvantages? Here, we compare the two functional principles, show which properties should be considered, and will help a user make the right choice for the respective application.
TECHNICAL DIFFERENTIATION T e air displacement principle is used mainly for pipettes and enables precise dispensing with fast and easy handling. By moving the piston in the pipette
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www.scientistlive.com High-quality pipettes enable precise and fast work
shaft up and down, a negative or positive pressure is created. T is causes liquid to be aspirated into or expelled from the tip. However, the liquid is always separated from the inner piston and the pipette shaft by an air cushion and does not come into contact with the pipette itself when handled correctly. T is enables contamination-free yet simple handling with the highest precision. But this is also where the application limits of the system lie. Media that strongly infl uence the air cushion can aff ect the accuracy of the result. For example, liquids with high viscosity or highly volatile media make precise work diffi cult. Meanwhile, the positive displacement principle is often used for repetitive pipettes or specialised positive displacement pipettes. In contrast to air displacement systems, the piston of the positive displacement pipette is in direct contact with the liquid to be pipetted. T e piston aspirates the liquid through a vacuum and wipes the walls of the tip clean during dispensing – down to the last drop, which clearly leaves the tip. T is principle ensures highly reproducible results
regardless of the liquid and the infl uence of an air cushion. T e piston, which is built into the device in air displacement models, is located in the tip cylinder in positive displacement models and must always be replaced to ensure contamination-free operation. Depending on the model, this can result in more time needed to change the tip and higher consumable costs. In summary, air displacement pipettes
are the right choice for fast serial pipetting whereas positive displacement pipettes are better for viscous or volatile fl uids.
MAKE THE RIGHT CHOICE T e criteria used to select the optimal device are as varied as the diff erent models on the market. T ere is often a large selection of devices even within a group and there are usually many diff erent models that all work according to the same principle. T e properties of the medium used
are often the determining factor in the selection process between both functional principles. But other requirements of the
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