Pipettes are essential laboratory tools for scientists but, with so many available options, knowing which one to choose can create confusion. What are the differences between the various types of pipettes? What attributes to look for when purchasing a new one? How do positive and air displacement pipettes work? How to use pipettes correctly, and ensure that they stay reliable for years to come? This comprehensive article will cover all these questions and many more.
What is a pipette?
Pipettes are one of the most extensively used lab tools. They are designed to measure small amounts of liquid and/or transfer it between containers. They’re available in a wide variety of types, from simple serological and Pasteur pipettes to more complex instruments containing a body, with a plunger and piston. The latter are often referred to as micropipettes, of which there are several kinds, with many different features. All these will be covered below.
Different types of pipettes and how they work
Air displacement pipette
Air displacement pipettes have an air cushion between the piston and the sample. To aspirate liquid, the piston first descends, pushing out a volume of air corresponding to the required volume of liquid, before moving up again and creating a partial vacuum that will be filled by the liquid.
Positive displacement pipette
In positive displacement pipettes, the piston is in direct contact with the sample, so is part of the disposable tip, not the pipette itself, to avoid cross-contamination. The aspiration process is similar to air displacement pipettes: the piston moves up, creating a partial vacuum that is filled by liquid.
Labs typically use air displacement pipettes, as they’re less expensive and well suited to most liquids, including viscous and volatile liquids – as long as the correct technique and pipette tips are used (see section The influence of the right pipette tip). However, if you regularly need to pipette very viscous or very volatile liquids, a positive displacement pipette might be better suited to your needs. Positive displacement tips are more costly because of the integrated pistons but, as there’s no air cushion inside the tip, volatile liquids can’t evaporate and viscous samples are pushed out more efficiently. In addition, positive displacement pipettes are more accurate when pipetting hot or cold samples, as they are not affected by temperature variations.
Adjustable vs. fixed volume pipettes
As the name suggests, adjustable volume pipettes cover a certain volume range, while fixed volume pipettes are designed to pipette a specific volume only. When purchasing a pipette, laboratories usually opt for adjustable volume pipettes, because they offer greater flexibility and can be used for many different applications. However, their accuracy is highest at the maximum pipetting volume, and it decreases slightly with the pipetting volume, so a fixed volume pipette may be more appropriate for some applications.
Because adjustable volume air displacement pipettes are usually the first choice of labs, the following chapters will focus on this pipette type.