Even when the same pipette, the same tip, and the same volume setting are used, pipetting results can still vary depending on how the liquid is aspirated.
Two factors that are easy to overlook are aspiration speed and tip immersion depth.
The plunger may be released too quickly.
The tip may be placed too deeply into the liquid.
Or the tip may be held so close to the surface that it begins to aspirate air.
These small differences can lead to bubbles, liquid adhering to the outside of the tip, and inconsistent aspiration. Their effects can become especially noticeable when working with low volumes or viscous liquids.
Stable pipetting therefore requires more than simply pressing the plunger to the first stop and allowing liquid to enter the tip. The speed of aspiration and the position of the tip should both be controlled consistently.
What Happens When Liquid Is Aspirated Too Quickly?
With a standard manual pipette, liquid enters the tip as the plunger returns from the depressed position.
If the user suddenly releases the plunger or allows it to snap back, the liquid may rush into the tip too quickly. This can make the flow unstable and may cause air bubbles or splashing inside the tip.
When air bubbles enter the tip, part of the aspirated volume may consist of air rather than liquid. As a result, the tip may not contain the full intended sample volume.
Rapid aspiration can also cause liquid to move farther up the tip than necessary. If liquid reaches too high inside the tip, the risk of contamination or liquid entering the pipette body may increase.
Fast aspiration may appear to save time. However, if it causes bubbles, incomplete aspiration, or repeated transfers, it can reduce overall efficiency.
The plunger should therefore be returned smoothly rather than released suddenly.
Consistency Matters More Than Aspiration Speed Alone
There is no single aspiration speed that is ideal for every liquid. Water-like solutions and viscous samples do not enter a pipette tip at the same rate.
What matters across all applications is consistency.
One aspiration may be slow, while the next is rushed.
Different users may return the plunger at different speeds.
The operator may gradually work faster as the procedure continues.
These differences make it harder to maintain the same aspiration conditions from one transfer to the next.
Improving pipetting reproducibility is not simply about working as slowly as possible. It is about selecting a speed that matches the liquid and reproducing that motion consistently.
Why Viscous Liquids Should Be Aspirated More Slowly
Viscous liquids do not flow as quickly as water. If the plunger is returned too rapidly, the liquid may not be able to follow the piston movement immediately.
The plunger may have reached its starting position even though the full intended volume has not yet entered the tip. If the tip is then removed from the liquid immediately, aspiration may be incomplete.
With viscous samples, it is important to return the plunger slowly and allow enough time for the liquid to move into the tip. After the plunger has returned, a short pause before removing the tip from the liquid can also help stabilize aspiration.
In this context, “slowly” does not simply mean reducing working speed. It means matching the pipetting motion to the flow behavior of the liquid.
Reverse pipetting or Low Retention Tips may also be useful for some viscous samples. However, even an appropriate technique or tip cannot perform as intended if aspiration is too fast for the liquid.
Immersing the Tip Too Deeply Can Increase Liquid on the Outside
The tip opening must remain below the liquid surface during aspiration. However, deeper immersion is not always better.
When the tip is inserted too deeply, a larger section of its outer surface comes into contact with the liquid. After the tip is removed, some of that liquid may remain attached to the outside.
Even if the correct volume has been aspirated inside the tip, liquid on the outer surface may also be carried into the receiving vessel. This can affect the actual transferred amount.
The liquid on the outside of the tip may also touch the rim or wall of another vessel, increasing the possibility of unintended sample transfer or contamination.
The tip should therefore be immersed deeply enough to remain securely below the liquid surface, but not farther than necessary.
If the Tip Is Too Shallow, It May Aspirate Air
Shallow immersion can create the opposite problem.
If the tip opening is positioned too close to the liquid surface, it may leave the liquid during aspiration because of hand movement or a falling liquid level. At that point, the pipette begins drawing in air instead of sample.
This is especially important when only a small amount of liquid remains in the vessel or when several aspirations are performed from the same container. The liquid level falls each time, so a tip position that was suitable at the beginning may become too shallow later.
Small wells and narrow vessels can also make the tip position more difficult to see.
For this reason, users should not only check the tip position before aspiration. They should also remain aware of whether the tip opening stays below the liquid surface throughout the aspiration step.
The Appropriate Immersion Depth Depends on Volume and Vessel
There is no single immersion depth that is suitable for every pipetting task.
A low-volume transfer with a small tip behaves differently from a large-volume transfer with a larger tip. The liquid level and available working space also vary among microtubes, centrifuge tubes, reagent bottles, and microplates.
During low-volume pipetting, it is important to avoid excessive immersion while keeping the tip securely below the surface. This helps reduce liquid on the outside of the tip and limits the effects of hydrostatic pressure.
During larger-volume aspiration, the liquid level may fall more noticeably as the tip fills. The tip position may therefore need to follow the changing surface to avoid becoming too shallow.
Appropriate immersion depth should be considered in relation to:
- tip size,
- vessel shape,
- the amount of liquid in the vessel,
- and the movement of the liquid surface during aspiration.
Keep the Pipette as Vertical as Practical
Pipette angle should also be considered together with immersion depth.
When the pipette is strongly tilted, one side of the tip opening sits deeper in the liquid while the other side moves closer to the surface. Even when the tip appears sufficiently immersed, part of the opening may become more likely to leave the liquid.
A changing angle also changes the aspiration conditions, even when the tip is inserted to what appears to be the same depth.
Some vessel shapes make perfectly vertical handling difficult. Even so, the pipette should be held as close to vertical as practical, and the angle should be kept consistent between transfers.
Stable aspiration depends not only on plunger movement, but also on the position and orientation of the tip.
Do Not Remove the Tip Immediately After Aspiration
The moment the plunger returns to its starting position does not always mean that all of the liquid has finished moving into the tip.
Viscous liquids and larger aspiration volumes may require a little more time to complete the transfer into the tip.
If the tip is removed from the liquid immediately, the last portion may not enter fully, or the liquid column may break at the tip opening.
A brief pause with the tip still below the liquid surface can help. The tip can then be withdrawn slowly.
Longer waiting is not automatically better, however. With volatile liquids, extended pauses and temperature effects may introduce other problems.
The waiting time should therefore reflect the sample properties and should be kept consistent within the same procedure.
Small Handling Differences Matter More at Low Volumes
During low-volume pipetting, minor differences in aspiration speed or immersion depth can have a relatively large effect.
A small bubble, a trace of liquid on the outside of the tip, or a slight aspiration shortage may represent a meaningful percentage of the total transferred volume.
Low-volume work therefore requires particular consistency in:
- plunger return speed,
- immersion depth,
- waiting time after aspiration,
- pipette angle,
- and tip withdrawal speed.
It is also important to check whether the target volume is close to the lower limit of the pipette. Even careful technique may not provide the desired stability if the pipette’s working range is not appropriate for the intended volume.
Aspiration Speed and Immersion Depth Should Be Considered Together
Aspiration speed and tip immersion depth are not separate issues.
Even with an appropriate aspiration speed, a tip that is too shallow may draw in air. Conversely, even when immersion depth is suitable, releasing the plunger too quickly may cause bubbles or splashing.
With a viscous liquid, the user may aspirate slowly but still draw in air if the falling liquid surface is not followed during the process.
Stable aspiration therefore requires both:
returning the plunger at a speed appropriate for the liquid, and
keeping the tip opening at an appropriate position below the surface.
Both should be treated as parts of the same aspiration motion.
How WATSON Thinks About Stable Aspiration
At WATSON, stable pipetting is not viewed as something determined only by the pipette or the tip.
Selecting a tip suited to the sample matters.
Choosing between forward and reverse pipetting matters.
Pre-wetting may matter in certain applications.
And aspiration speed and immersion depth must also match the properties of the liquid.
These factors work together.
When aspiration is unstable, first check how quickly the plunger is returning. Then check whether the tip is positioned too deeply or too close to the surface. It is also worth reviewing whether enough time is being allowed for the liquid to enter the tip before withdrawal.
Pipetting is more than simply aspirating and dispensing. Stable routine liquid handling depends on matching the speed and position of each movement to the behavior of the sample—and repeating those conditions consistently.