Pipetting small volumes accurately, especially below 10 microliters, requires deliberate technique adjustments that go beyond standard pipetting practice. The smaller the volume, the more every variable matters: plunger speed, tip immersion depth, ambient humidity, and even whether you pre-wet your tip can shift your delivered volume by a meaningful percentage. Here’s how to get it right.
Use Reverse Pipetting for Small Volumes
The single most impactful change you can make when pipetting small volumes is switching from forward to reverse pipetting. In forward pipetting, you press the plunger to the first stop, aspirate, then dispense by pressing back to the first stop and blowing out. It works well for most routine volumes of aqueous solutions. But for very small volumes, reverse pipetting gives you better precision.
The difference is simple. In reverse mode, you press the plunger all the way to the second stop before aspirating. This draws up slightly more liquid than your set volume. When you dispense, you press only to the first stop, delivering the exact target volume while a small surplus stays behind in the tip. That surplus gets discarded with the tip. This approach eliminates the inconsistency that comes from blowing out tiny droplets, where surface tension and small air pressure differences can cause the final bit of liquid to cling to the tip wall or spray unpredictably.
Slow Down Every Step
Speed is the enemy of precision at low volumes. When you release the plunger to aspirate, let it rise smoothly and slowly to its resting position. Jerky or fast release creates turbulence inside the tip and can introduce tiny air bubbles that displace liquid volume. After the plunger reaches the top, wait at least one second before withdrawing the tip from your source liquid. This pause gives the liquid time to fully travel up into the tip, which matters more at small volumes where the liquid column is short and surface tension effects are proportionally larger.
The same principle applies when dispensing. Press the plunger down gently and steadily. A fast push can cause splashing or incomplete delivery, particularly if you’re working with anything other than plain water.
Pre-Wet Your Tips
Before you pipette your actual sample, aspirate and dispense the same liquid two or three times using the same tip. This pre-wetting step neutralizes capillary effects inside the tip and equilibrates the air temperature in the tip’s air cushion with the temperature of your sample liquid. Without pre-wetting, the first aspiration tends to pick up slightly less volume than you set, because some liquid coats the inner tip wall and never makes it to your destination.
Pre-wetting dramatically improves accuracy for very small microvolumes. It’s one of the easiest habits to adopt and one of the most commonly skipped.
Mind Your Immersion Depth
When aspirating, dip the tip into your source liquid to a depth of about 1 centimeter. Going deeper creates excess liquid clinging to the outside of the tip, which adds uncontrolled volume to your delivery. Going too shallow risks aspirating air. After aspirating, touch the tip gently against the inside wall of the source vessel as you withdraw it. This wipes off the small amount of liquid that adheres to the outside of the tip, which at volumes below 5 microliters can represent a significant fraction of your total delivery.
Choose the Right Pipette for the Range
Always use the smallest pipette that covers your target volume. If you need to pipette 2 microliters, use a 0.5 to 10 microliter pipette, not a 20 or 200 microliter model set to its lowest range. Pipettes are least accurate at the bottom of their volume range, where the plunger barely moves and the air cushion becomes disproportionately large relative to the liquid volume.
For most aqueous solutions, a standard air displacement pipette works fine as long as you apply proper technique. But if you’re working with viscous liquids like glycerol, volatile solvents like acetone or methanol, or liquids at temperatures significantly different from ambient, consider a positive displacement pipette. These use a disposable piston built into the tip that makes direct contact with the liquid, eliminating the air cushion entirely. Since there’s no air gap between the piston and the sample, the physical properties of the liquid (viscosity, vapor pressure, surface tension) don’t affect volume accuracy the way they do with air displacement systems.
How Environment Affects Small Volume Accuracy
The air cushion inside an air displacement pipette is sensitive to temperature and humidity, and those effects become significant at small volumes. In dry lab environments, water evaporates from inside the pipette tip between aspiration and dispensing, leading to consistent underdelivery of aqueous solutions. Labs with low relative humidity see statistically worse pipetting accuracy than humid ones, and pipettes calibrated in a controlled facility may underdeliver when used in a drier working lab.
Temperature mismatches create a separate problem. Aspirating warm liquids into a cooler pipette causes the air cushion to expand as it warms, pushing out some liquid and resulting in underdelivery. Cold liquids do the opposite, contracting the air cushion and causing overdelivery. These errors scale with the temperature difference and become most noticeable at volumes below 5 microliters. If your samples have been in a water bath or on ice, let them equilibrate closer to room temperature before pipetting, or use a positive displacement pipette to bypass the issue entirely.
Handling Viscous Liquids at Small Volumes
Viscous solutions like concentrated glycerol are notoriously difficult to pipette at any volume, but small volumes amplify every problem. The liquid moves slowly through the narrow tip orifice, so you need to wait two to three seconds after aspiration before withdrawing the tip from the source. The same delay applies after dispensing: hold the tip against the vessel wall for two to three seconds to let the liquid fully drain.
Use reverse pipetting for viscous samples. Forward pipetting relies on a blowout step to empty the tip completely, but viscous liquids cling to the interior walls and resist that final push. Reverse mode sidesteps this by leaving the residual liquid in the tip from the start, so your delivered volume stays accurate. For highly viscous solutions like 80% glycerol, wide bore tips allow the liquid to enter and exit the tip more easily. If you’re using an electronic pipette, reduce the aspiration and dispensing speed to give the liquid time to flow.
Calibration Matters More at Low Volumes
Small volume pipettes in the 0.5 to 20 microliter range are more sensitive to mechanical drift than larger models. A tiny change in seal integrity or piston condition that would be negligible on a 1000 microliter pipette can throw off a 2 microliter pipette by a meaningful percentage. Most quality standards recommend calibrating pipettes every 6 to 12 months, but smaller volume pipettes often need shorter intervals. In regulated industries, critical application pipettes may be calibrated monthly or bimonthly to meet ISO 8655 requirements.
Between formal calibrations, you can check your pipette’s performance by dispensing water onto an analytical balance. At 20°C, 1 microliter of water weighs approximately 1 milligram. Repeat the measurement ten times and check both the average (accuracy) and the spread between measurements (precision). If either drifts beyond the pipette manufacturer’s specifications, it’s time for service.
Quick Reference: Small Volume Best Practices
- Pre-wet tips two to three times with your sample liquid before pipetting
- Use reverse pipetting for volumes below 5 microliters or for viscous and foaming samples
- Release the plunger slowly and wait one second before withdrawing from the source
- Immerse the tip about 1 centimeter into the source liquid
- Touch off against the vessel wall when withdrawing to remove external liquid
- Dispense slowly along the vessel wall at a consistent speed
- Use the smallest pipette whose range covers your target volume
- Switch to positive displacement for volatile, viscous, or temperature-sensitive liquids
- Calibrate frequently since small volume pipettes drift faster than large ones