Everything you need to know about the different types of pipettes
Written by Tom Bentivegna16. April 2021
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.
Table of contents
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 vs. positive displacement pipettes
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.1
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.2
Because adjustable volume air displacement pipettes are usually the first choice of labs, the following chapters will focus on this pipette type.
How to choose a new pipette
Searching for the right pipette can be confusing. To help you with this decision, we have outlined a few things to bear in mind when selecting a new pipette.
Manual vs. electronic pipettes
The first thing you should decide is whether you need a manual or an electronic pipette. Manual pipettes are widely used and are great tools, but if you have the budget, then an electronic pipette will pay for itself in the long term, because it offers five significant advantages:
- Improved ergonomics – minimizing the risk of repetitive strain injuries, as there's no plunger to press down to move the piston, and no need to twist knobs or the plunger to adjust the volume.
- Better precision and accuracy – as they are less operator-dependent. You can also save pipetting protocols to reduce manual errors and prevent operator-to-operator variability.
- Multiple pipetting modes – avoiding the need for separate repeater, dilutor and titrator pipettes.
- Straightforward calibration – with built-in gravimetric performance validation. Simply enter the dispensed versus target volume, and it will self-calibrate. Many electronic pipettes also offer calibration reminders.
- Integrated pipetting protocols – with pre-set or custom programs to minimize operator errors. Watch the video tutorials on our pipetting programs to see some examples.
If you're now thinking "That's all very well, but electronic pipettes are complicated, require constant charging and are difficult to clean," then let us tell you more about the advantages of using electronic pipettes.
The volume you transfer can significantly influence the reliability of pipetting. As explained above, you should always choose the smallest pipette capable of handling the required volume as the accuracy of adjustable volume air displacement pipettes decreases with the set volume.
Reliability can also be negatively affected if the volume is accidentally changed during the pipetting procedure. You should therefore choose a pipette with a mechanism designed to avoid inadvertent volume changes. Another common problem is tips that loosen, leak, or fall off, so selecting a pipette with tips specifically designed for it is better than one that uses universal tips.
Filling microplates with a single channel pipette can quickly become a very tedious and error-prone task. Using multichannel pipettes allows you to transfer multiple samples at once, increasing efficiency and preventing errors and repetitive strain injuries. Multichannel pipettes can even be used for transfers of samples between different labware formats, if you buy one with adjustable tip spacing.
The number of channels you require, and if you need adjustable tip spacing, depends on the labware types you're planning to use. The table below lists the most common pipette formats, and the labware types they're best suited for:
|Pipette format||Labware type|
|8 channel pipette||Transferring samples from reservoirs or 96 well plates to entire columns of a 96 well plate|
|12 channel pipette||Transferring samples from reservoirs or 96 well plates to entire rows of a 96 well plate|
|16 channel pipette||Transferring samples from reservoirs or 384 well plates to entire columns of a 384 well plate|
|4 channel adjustable tip spacing pipette||Transferring samples from different labware formats to entire rows of a 12 well plate or entire columns of a 24 well plate|
|6 channel adjustable tip spacing pipette||Transferring samples from different labware formats to entire rows of a 24 well plate|
|8 channel adjustable tip spacing pipette||Transferring samples from different labware formats to entire columns of a 96 well plate|
|12 channel adjustable tip spacing pipette||Transferring samples from different labware formats to entire rows of a 96 well plate|
When choosing your pipette, you should make sure that it's lightweight, well-balanced and fits comfortably into the hand, for both left- and right-handed users. Additionally, tip loading and ejection forces should be as low as possible to reduce the strain on operators.
How to use a pipette
The very first thing you should do is make sure that you choose the right pipette for your experiment. As explained above, an adjustable volume air displacement pipette is most accurate when pipetting at its maximum capacity, so you should always choose the smallest pipette capable of handling the required volumes.
If your experiment allows, you should also make sure that the pipette, tips and liquids are equilibrated to room temperature before you begin your work. This will prevent temperature-dependent expansion or contraction of the air cushion inside of the pipette tip, which can negatively impact the accuracy and precision of your results.
While pipetting, there are a few additional things to keep in mind:
- Pre-wet the pipette tip by aspirating and dispensing the nominal volume 3 times to humidify the air cushion.
- Always hold the pipette at an angle not exceeding 20 degrees to ensure that the hydrostatic pressure inside the tip stays consistent.
- When immersing the tip into a liquid, it shouldn't be more than 2-3 mm below the surface, to minimize the amount of liquid retained on the outside of the tip.
- Following each dispense, there may be a liquid droplet clinging to the end of the tip. To remove it, you should perform a touch off.
- When you're repeat dispensing multiple aliquots, you should discard the first and last dispense of the series, as they have the largest associated volume errors.
The influence of the right pipette tip
It’s not just using the right pipetting technique that has an impact on the accuracy and precision of your results, but also using the right tip. For example, for low surface tension or viscous solutions, low retention tips produce significantly better results than standard tips. To learn more about when to use which tip, check out our comprehensive article on how to choose the right pipette tip for your application.
How to pipette viscous and volatile liquids
Pipetting viscous and volatile liquids with an adjustable volume air displacement pipette can be challenging but mastered with the right technique and pipette tip. Viscous liquids should be aspirated and dispensed slowly using reverse pipetting. Using this technique, a larger volume than needed is aspirated, which compensates for the retained liquid adhering to the inside of the tip. Low retention tips are the ideal option for viscous liquids, and for very high viscosity liquids, or those that tend to foam, wide bore tips are recommended.
When pipetting volatile liquids, be sure to pre-wet the tip and use fast pipetting speeds for both aspiration and dispensing to minimize the effects of evaporation. Do not pause unnecessarily between aspiration and dispensing and use reverse pipetting to further reduce the effect of evaporation on the actual volume to be delivered.
Pipettes are usually calibrated with distilled water, at room temperature. It may be useful to recalibrate them when pipetting liquids with different physical properties (specific gravity and vapor pressure).
How to prolong the lifespan of your pipette
Besides correct use, the proper storage and cleaning of your pipettes – as well as regular calibration and maintenance – are crucial to ensure that they will yield reproducible results for many years.
Never lay a pipette down on the bench. Instead, store it vertically on a stand. This will ensure that any liquid residue trapped inside the pipette body drains out, and it prevents piston misalignment or lubricant accumulation on one side of the pipette. As pipette tips may retain liquid residues, you should always eject them when you finish pipetting. If not, this residual liquid can evaporate into the pipette body. Last but not least, you should always set your pipette to its maximum volume (if you're using a manual pipette), to allow the spring to return to its least stressed position.
Always consult the operating manual of your pipette before cleaning it. It often contains detailed information about the chemical compatibility of your pipette with common cleaning agents, and tells you how to disassemble and reassemble it.
Cleaning the outside of the pipette should be part of your daily routine. Simply wipe it with a lint-free cloth lightly soaked with 70 % ethanol.
Cleaning the interior of your pipette is a more complex and time-consuming process but can usually be done by the operator for single channel pipettes. First, you have to take the pipette apart. Depending on whether you're cleaning it as a matter of routine, or because it has been contaminated, you'll need to clean the components not just with distilled water, but also with a suitable decontaminant. Afterwards, you should check the components for visible damage, let them air dry, and lubricate the piston, before reassembling the pipette. Finally, briefly check the pipette’s functionality by performing a leak test and volume validation.
Calibration and service
The last aspect that can increase the lifespan of your pipette is regular calibration and service. You should have it calibrated and serviced every 6 to 12 months to ensure that it remains accurate and precise, and that potential problems are detected and addressed before costly repairs or replacement becomes inevitable. Performing routine checks on a regular basis is also recommended, so that you can be confident in your results between calibrations.
You now know all the tips and tricks to find out which pipette suits you best, how to correctly use it and how to make sure that it serves you faithfully for many years. If you found this blog post helpful, don’t forget to subscribe to our newsletter for more inspiring articles on lab topics: