Pipettes: A Tutorial
What is a pipette?
A pipette is a very precise tool for the accurate and precise transfer of liquids. These tools were developed for the medical, pharmaceutical, chemical and solution handing laboratories, but their usage has spread to various other industries likebeverage, food, etc. Pipettes have become very popular because of the measuring accuracy of solutions, in the various industries that need to measure accurate volumes from microliter (µL) to millilitre (mL).
There are various types of pipettes available for different applications and different types of applications. Each one will be useful for accurately measuringspecific volume and may even be specialized to work with a specific type of solution. With the advancements in pipetting technology, measurements with a pipette is easy enough that even users with little or no prior experience can easily handleit.
There are two major types of pipettes: Air Displacement and Positive Displacement (Figure-1). The basics of pipetting are in the shared term – displacement. A pipette will move liquid by pulling the liquid in and then pushing it back out of the pipette. Pipettes can be categorized by how they cause that displacement, and their applications can be different based on that technology.
The general principle of using a pipette is that the user will set a volume to aspirate (pull in) and dispense (push out). The liquid is drawn into an attached pipette tip, which is uniquely chosen for the volume being handled. These actions are controlled by a plunger – the button at the top of the pipette – and a piston. The more complex principle is the difference in how the piston is controlled and how it affects the liquid.
Air Displacement Pipettes
Air Displacement pipettes are almost just as they sound. The liquid is pulled in and pushed out through the use of displaced air. Each of these pipettes has an internal piston connected to a plunger. For an air displacement pipette, the plunger directly pushes out a volume of air equal to the liquid that will be aspirated. In essence, the piston pushes out air and replaces it with liquid.
Figure -1 Diagrams of positive and air displacement pipettes
Positive Displacement Pipettes
Positive Displacement pipettes work according to direct displacement. Unlike air displacement pipettes, the piston is contained in the pipette tip instead of the pipette. The reason for the difference is that liquid is pulled in by pulling the piston up and creating a vacuum.
Positive displacement pipettes offeranadvantage over air displacement pipettes. The Air displacement can be affected by liquids with high vapor pressures or densities, and positive displacement pipettes will be unaffected. Since the piston is in direct contact with the solution, it can accurately measurehigh viscous liquids.
The modern pipettes can be classified based on the number of available channels and majorly categorized into single-channel and multichannel pipettes. Single-channel pipette is one of the most essential and utilized pipettes in the laboratory which can be used to transfer 0.1 mL to 10 mL.
Figure-2: Sigle and multichannel pipettes
Multichannel pipettes can measure accurately small amounts of liquids and multiply it many fold into a single device with multiple heads (typically in the number of 4, 6, 8 or 12). These pipettes are the essential ones used in microbiology/cell culture labs to handle 96 well plates.