Air displacement or repetitive pipette? Kathrin Friedrich offers guidance on how to make the right choice for your application
Every day, various liquids and volumes are dispensed in the laboratory. The instruments used for this purpose are a key element in the lab. Their 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 find the right one for their application?
The most common distinction is between positive displacement and air displacement systems. The two systems, which function very differently, allow the user to work with almost any liquid without any loss of accuracy or reproducibility. But how do these two systems differ? 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.
The air displacement principle is used mainly for pipettes and enables precise dispensing with fast and easy handling. By moving the piston in the pipette shaft up and down, a negative or positive pressure is created. This 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. This 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 influence the air cushion can affect the accuracy of the result. For example, liquids with high viscosity or highly volatile media make precise work difficult.
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. The 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. This principle ensures highly reproducible results regardless of the liquid and the influence of an air cushion. The 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 fluids.
Making the Right Choice
The criteria used to select the optimal device are as varied as the different models on the market. There is often a large selection of devices even within a group and there are usually many different models that all work according to the same principle.
The properties of the medium used are often the determining factor in the selection process between both functional principles.
But other requirements of the application must also be considered.For example, it is necessary to check how often a delivery must be repeated, which vessels are used or which volume is required. A device must therefore optimally support users in their work; it must be intuitive to operate, deliver accurate volumes and meet the necessary requirements.
So, decision criteria include selection based on: the medium used; the number of deliveries; the vessel used; and the volume range needed.
The overview left summarises the criteria described here and therefore serves as a decision-making aid.
Kathrin Friedrich is with BRAND