When choosing a pipette, laboratories often compare factors such as volume range, ease of use, price, and brand.
Before comparing individual product specifications, however, there is one fundamental question to consider:
What type of liquid will you be dispensing?
Two pipettes may cover the same volume range, yet perform differently depending on their operating principles. Air displacement pipettes are widely used for general aqueous solutions, while positive displacement pipettes may be better suited to liquids with properties that differ significantly from water, such as viscous or volatile liquids.
In this article, we compare air displacement and positive displacement pipettes, explain how each system works, and explore which types of liquids and laboratory workflows they are best suited for.
The Best Pipette Depends on the Application
There is no single pipette that is best for every laboratory or every experiment.
The most expensive pipette, or the model with the most features, is not necessarily the most appropriate choice for the work being performed.
Factors to consider when selecting a pipette include:
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The physical properties of the liquid
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The required dispensing volume
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The number of dispensing operations
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Single-channel or multichannel operation
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Fixed-volume or variable-volume operation
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Required accuracy and reproducibility
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Contamination-control requirements
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Operating costs, including dedicated consumables
Among these factors, the properties of the liquid are particularly important when choosing between different pipetting mechanisms.
A free-flowing aqueous solution and a highly viscous liquid such as glycerol do not behave in the same way during aspiration and dispensing.
Understanding the difference between air displacement and positive displacement systems is therefore an important first step in choosing the most appropriate pipette.
What Is an Air Displacement Pipette?
In an air displacement pipette, a cushion of air separates the piston inside the pipette from the liquid inside the tip.
As the piston moves, it changes the pressure within this air cushion. The resulting pressure difference draws liquid into the tip or dispenses it from the tip.
Most conventional micropipettes used in laboratories operate using this principle.
Air displacement pipettes are generally well suited to liquids such as:
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Aqueous solutions
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Buffers
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Culture media
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Diluted samples
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Laboratory reagents with properties similar to water
One of the main advantages of air displacement pipettes is their versatility. They are available in a wide range of volumes and formats and can typically be used with standard disposable pipette tips.
This makes them a practical choice for routine liquid handling and general laboratory use.
However, because an air cushion is present between the piston and the liquid, dispensing may be affected by factors such as viscosity, volatility, density, foaming, and temperature.
What Is a Positive Displacement Pipette?
In a positive displacement pipette, a piston located inside a dedicated tip or syringe comes into direct contact with the liquid.
Unlike an air displacement system, there is no air cushion between the piston and the sample.
Because the piston directly aspirates and dispenses the liquid, positive displacement pipettes are less affected by the expansion or contraction of an air cushion and by vapor released from the liquid.
Positive displacement pipettes may be suitable for liquids such as:
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Viscous liquids such as glycerol
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Oils
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Volatile liquids such as ethanol
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High-density liquids
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Foaming liquids
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Certain corrosive liquids
This mechanism can be advantageous when the liquid behaves very differently from water and stable dispensing is difficult with a conventional air displacement pipette.
However, positive displacement systems commonly require dedicated piston tips or syringes rather than standard pipette tips. Their consumable costs may therefore be higher.
Chemical compatibility also varies between products.
A positive displacement mechanism does not automatically make a pipette suitable for every chemical or solvent. The specifications of both the instrument and its dedicated consumables should always be reviewed before use.
Air Displacement vs. Positive Displacement
The main differences between the two systems can be summarized as follows:
| Comparison | Air Displacement | Positive Displacement |
|---|---|---|
| Space between piston and liquid | An air cushion is present | The piston directly contacts the liquid |
| Primary use | General liquid handling | Liquids with challenging physical properties |
| Suitable liquids | Buffers, culture media, diluted samples, and aqueous solutions | Viscous, volatile, high-density, and foaming liquids |
| Consumables | Standard pipette tips | Dedicated piston tips or syringes |
| Influence of liquid properties | May be affected | Generally less affected |
| Consumable cost | Typically easier to control | Often higher |
| General versatility | High | Stronger for specific applications |
A positive displacement pipette is not simply an upgraded version of an air displacement pipette.
For routine dispensing of aqueous solutions, an air displacement pipette may be easier to use and more economical, with a wider range of compatible consumables.
When liquid properties have a significant effect on dispensing performance, however, a positive displacement pipette may provide more stable handling.
The important question is not which mechanism is universally better, but which one is better suited to the liquid and workflow.
What Happens When Dispensing Viscous Liquids?
Viscous liquids such as glycerol, oils, and concentrated protein solutions flow more slowly than water.
They require more time to enter and leave a pipette tip.
When a viscous liquid is handled with an air displacement pipette at the same speed as an aqueous solution, several problems may occur:
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The full set volume may not be aspirated
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The liquid may continue moving upward after aspiration
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Liquid may remain inside the tip
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Dispensing may take longer
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Results may vary between operators
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Reproducibility may decrease
When using an air displacement pipette for a viscous liquid, aspiration should generally be performed slowly, followed by a short pause to allow the liquid to enter the tip.
Dispensing should also be slow enough to allow the liquid to leave the tip as completely as possible.
Reverse pipetting or a wide-bore tip may improve handling in some cases.
When viscosity is high and reliable dispensing remains difficult despite adjusting the technique, a positive displacement pipette may be worth considering.
What Happens When Dispensing Volatile Liquids?
Volatile liquids, including ethanol and certain organic solvents, can also be difficult to dispense with an air displacement pipette.
Vapor from the liquid can enter the air cushion inside the tip and change its volume and pressure.
This may result in:
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Liquid dripping from the tip
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Incomplete aspiration
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Inconsistent dispensing volumes
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Changes in performance during repeated dispensing
Because a positive displacement system does not contain an air cushion between the piston and the liquid, it is generally less affected by vapor pressure.
This can make it a useful option when volatile liquids are handled frequently.
However, organic solvents may affect the materials used in pistons, tips, seals, or syringes.
Always verify chemical resistance before use.
Air Displacement Performance May Be Improved by Adjusting the Technique
Handling a viscous or volatile liquid does not always mean that a laboratory must immediately switch to a positive displacement pipette.
Depending on the properties of the liquid and the required performance, adjusting the air displacement technique may improve results.
Allow the Pipette, Tips, and Liquid to Reach a Similar Temperature
Temperature differences between the pipette, tip, and liquid can cause the air cushion to expand or contract and may affect the dispensed volume.
This can be particularly relevant when working with reagents immediately after removing them from refrigerated storage.
When experimental conditions allow, place the pipette, tips, and liquid in the same environment long enough for their temperatures to stabilize.
Aspirate and Dispense Slowly
Viscous liquids need additional time to move through the tip.
Operate the plunger slowly and consistently rather than allowing it to return suddenly. After aspiration, pause briefly before removing the tip from the liquid.
During dispensing, allow enough time for the liquid to leave the tip.
Pre-Wet the Pipette Tip
Pre-wetting involves aspirating and dispensing the same liquid several times before measuring the final volume.
This helps condition the inner surface of the tip and stabilize the environment within it.
For volatile liquids, pre-wetting may help saturate the air cushion with vapor and reduce changes in volume or dripping.
Use Reverse Pipetting
In reverse pipetting, more liquid than the set volume is aspirated, while only the intended volume is dispensed.
This technique can be useful for viscous or foaming liquids.
However, excess liquid remains inside the tip, so the available sample volume and disposal procedure must also be considered.
Select a Tip Appropriate for the Liquid
Wide-bore tips may be useful for viscous samples or materials that are sensitive to shear forces.
Low-retention tips may also be considered when liquid tends to adhere to the inner surface of a standard tip.
The tip should therefore be reviewed together with the pipetting technique and the pipette mechanism.
When Is an Air Displacement Pipette a Good Choice?
An air displacement pipette may be the more practical option when:
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You primarily handle aqueous solutions
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You routinely dispense buffers or culture media
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You need one pipette for a wide range of general laboratory tasks
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You want to use standard disposable pipette tips
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You want to control consumable costs
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You need a broad choice of volume ranges or channel configurations
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Liquid properties are unlikely to have a major effect on dispensing
When most of the liquids used in a laboratory behave similarly to water, air displacement pipettes are often a practical standard choice.
When Should You Consider a Positive Displacement Pipette?
A positive displacement pipette may be worth considering when:
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You frequently handle viscous liquids such as glycerol or oils
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You dispense volatile liquids
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You work with high-density liquids
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You handle liquids that foam easily
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Dripping or residual liquid frequently occurs with air displacement pipettes
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Results vary significantly between operators
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Dispensing performance is more important than the cost of dedicated consumables
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You want to reduce the risk of liquid entering the pipette body
If adjusting the operating technique does not provide stable results, reviewing the pipetting mechanism itself may be necessary.
Consider Frequency of Use and Operating Costs
Positive displacement pipettes can make challenging liquids easier to handle, but they may require dedicated consumables.
The purchase price of the pipette is therefore not the only cost to consider.
Laboratories should also review:
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The cost of dedicated tips or syringes
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The number used during each experiment
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Monthly or annual consumption
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Available volume ranges
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Consumable availability
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Required inventory levels
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Cleaning and maintenance procedures
The most practical approach may differ between a liquid that is handled only several times per year and one that is dispensed every day.
Rather than replacing every pipette with a positive displacement model, laboratories may use air displacement pipettes for general tasks and place positive displacement pipettes only in workflows involving challenging liquids.
The Best Pipette Is the One That Matches the Liquid and Workflow
The best pipette is not necessarily the most expensive model or the one with the longest list of features.
The appropriate choice depends on the physical properties of the liquid, the required volume, frequency of use, expected dispensing performance, and ongoing consumable costs.
Air displacement pipettes are generally well suited to routine handling of aqueous solutions such as buffers, culture media, and diluted samples.
Positive displacement pipettes may be a stronger option for viscous, volatile, high-density, or foaming liquids that behave differently from water.
Start by checking whether any of the following problems occur with your current pipette:
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Liquid drips from the end of the tip
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Aspiration volumes are inconsistent
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Liquid remains inside the tip
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Results vary between operators
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Viscous liquids take too long to dispense
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Reproducibility is difficult to maintain with certain liquids
When these problems cannot be resolved through technique adjustments alone, reviewing the operating principle of the pipette may be an important next step.
WATSON’s Micro Dropper uses a positive displacement system and can dispense a range of liquid volumes without requiring electricity.
When reviewing liquids that are difficult to handle with conventional air displacement pipettes, consider not only the volume and features of the instrument, but also the dispensing mechanism itself.