Even experienced lab workers make pipetting mistakes that quietly throw off dilution and concentration results without producing any obvious sign of error. Because most dilution calculations assume the volumes pipetted are exactly what was intended, small technique errors can accumulate into meaningfully wrong concentrations.
Not Pre-Wetting the Tip
A new, dry pipette tip can retain a slightly different volume than one that has already been wetted with the liquid being transferred, particularly for viscous or foaming liquids. Drawing up and expelling the liquid once before taking the actual measured volume, sometimes called pre-wetting or pre-rinsing the tip, helps equalize this effect and is standard practice for precise work.
Holding the Pipette at an Angle
Pipettes are calibrated assuming a roughly vertical orientation. Pipetting at a steep angle changes the geometry of the air gap inside the tip and can alter the volume actually drawn, especially at small volumes where the effect is proportionally larger.
Submerging the Tip Too Deep
Inserting the tip too far below the liquid surface increases the amount of liquid that clings to the outside of the tip when it is withdrawn, which adds extra, unmeasured volume to the next container. Submerging only a few millimeters below the surface is generally sufficient and minimizes this carryover.
Pressing Through Both Stops at Once
Rushing the plunger straight through the first and second stop in one motion, rather than pausing at the first stop to draw up liquid and the second stop to expel it fully, is one of the most common pipetting mistakes. This single error can cause both under-aspiration when drawing liquid and under-delivery when dispensing it.
Releasing the Plunger Too Quickly
Releasing the plunger abruptly while drawing up liquid can create turbulence and introduce small air bubbles into the tip, which makes the drawn volume inaccurate even if the dial is set correctly. A slow, controlled release gives a cleaner, more reliable draw.
Not Changing Tips Between Transfers
Reusing a tip between different solutions, or between different tubes in a dilution series, risks carrying over trace amounts of concentrated solution, which introduces cross-contamination that is especially problematic in a serial dilution where even a tiny carryover shifts every later concentration.
Using a Pipette Outside Its Rated Range
Every pipette has a specified accurate volume range, and accuracy degrades noticeably near the very bottom of that range. Using a large-volume pipette to measure a volume near its lower limit, instead of switching to a smaller pipette, is a common and avoidable source of error.
Skipping Calibration Checks
Pipettes drift out of calibration over time with normal use, and this drift is invisible without periodic verification, typically done by weighing dispensed water on an analytical balance or through a formal calibration service. Skipping these checks allows a miscalibrated pipette to quietly affect every result produced with it.
When volumes are measured correctly, a Dilution Calculator can be used to confirm that the planned dilution produces the intended concentration, which is a useful check before committing an entire series of tubes.
Ignoring Temperature Effects on Viscous Liquids
Cold or highly viscous liquids move more slowly through a pipette tip, and drawing or dispensing them at the same speed used for water often leaves some liquid behind or results in an incomplete draw. Allowing a viscous or chilled reagent to reach a more workable temperature, and pausing slightly longer at each stop when pipetting it, reduces this source of volume error considerably.
Forcing a Pipette Past Its Maximum Setting
Turning the volume dial past a pipette’s rated maximum, even slightly, can jam or damage the internal piston mechanism, and the damage is not always obvious right away. A pipette that has been forced past its limit may continue to appear to function normally while quietly delivering volumes that no longer match its dial setting, which can go unnoticed for some time unless the pipette is checked against a known reference volume.
Frequently Asked Questions
How often should a pipette be calibrated?
Most labs follow a regular schedule, often quarterly or annually depending on usage and institutional requirements, in addition to checking after any drop or suspected damage.
Does tip brand matter if it physically fits the pipette?
Yes, tips from a different manufacturer than the pipette was calibrated with can seal slightly differently, which can introduce small but measurable volume errors even if they appear to fit.
Why does pipetting a viscous liquid need extra care?
Viscous liquids move more slowly into and out of the tip, so rushing the draw or dispense step is more likely to leave residual liquid behind or draw an incomplete volume.
