Pipettes & tips: the small-volume discipline behind a clean result
Hereditary-cancer panels live or die on microlitres. We explain air-displacement accuracy, why filter and low-retention tips matter for contamination control, and how calibration keeps the numbers honest — from extracted DNA to the sequencer.
Why a few microlitres decide a genetic result
A multi-gene hereditary-cancer panel — genes such as BRCA1, BRCA2, PALB2, TP53 and the Lynch-syndrome set (MLH1, MSH2, MSH6) — passes through dozens of pipetting steps before any base is read: extraction elution, normalisation, library prep, indexing and pooling. Each transfer is typically a handful of microlitres. At those volumes a small percentage error compounds, and a single carry-over event can plant a false variant call. The bench tools that contain that risk are unglamorous: an air-displacement pipette kept in calibration, a filter (aerosol-barrier) tip to stop cross-contamination, and a low-retention tip so the precious eluate actually leaves the tip.
This is an educational explainer about laboratory technique, not medical advice. For what a test result means for you, speak with a clinician or genetic counsellor.
Matching the pipette to the volume
Air-displacement pipettes are most accurate at or near their nominal capacity; below ~50% of nominal, error climbs. Choose the smallest pipette whose nominal volume covers your target, and pair it with the right tip.
| Pipette class | Working range | Tip / barrier | Indicative max error* | Typical use |
|---|---|---|---|---|
P2 (0.1–2 µL) | 0.2–2 µL | Filter, low-retention | ±0.05 µL at 1 µL | Indexing primers, small normalisations |
P10 (0.5–10 µL) | 1–10 µL | Filter, low-retention | ±0.12 µL at 10 µL | Adapter ligation, qPCR setup |
P20 (2–20 µL) | 2–20 µL | Filter | ±0.2 µL at 20 µL | Library prep master-mix aliquots |
P200 (20–200 µL) | 20–200 µL | Filter (≥±0.8% applies) | ±1.6 µL at 200 µL | Bead clean-ups, elution buffer |
P1000 (100–1000 µL) | 100–1000 µL | Filter | ±8 µL at 1000 µL | Reagent make-up, sample dilution |
Table 1. Representative micropipette classes, ISO 8655-aligned tolerances and typical genetic-lab uses. Tolerance figures are nominal-volume maxima and widen at the low end of each range.
What good technique actually buys you
Approximate, literature-reported accuracy gains from individual technique fixes at low microlitre volumes. These are illustrative ranges, not guaranteed figures — magnitude depends on liquid, tip and operator.
Neutralises capillary & thermal effects in fresh plastic
Inaccuracy can double if immersed too deep
Avoids hydrostatic-pressure shift in the tip
Improves sample recovery vs standard tip
Good pipetting technique, step by step
The order and rhythm matter as much as the instrument. ISO 8655 expects at least five conditioning cycles per fresh tip before relying on a transfer.
- 01
Set the volume from above
Dial down to the target so the mechanism settles against gravity; never overshoot then climb back up, which leaves backlash in the plunger.
- 02
Pre-rinse the fresh tip
Aspirate and fully dispense the same liquid 3–5 times. This wets the new plastic, equalises the internal air cushion, and is most valuable in the low-µL range.
- 03
Immerse only a few millimetres
Tip just below the surface, pipette near-vertical (≤20°). Too deep aspirates extra liquid and risks bubbles; too shallow draws air.
- 04
Aspirate slowly and pause
A smooth draw with a one-second pause lets the liquid level equilibrate before withdrawal — critical for volatile or viscous samples.
- 05
Choose forward or reverse mode
Forward pipetting suits water and most buffers; switch to reverse mode for viscous, foaming or volatile liquids to avoid under-delivery.
- 06
Dispense touch-off, eject the tip
Dispense to the first stop, then blow-out to the second; touch the tip to the vessel wall. Use a fresh filter tip for every contamination-sensitive transfer.
Tip taxonomy, decoded
Three tip properties solve three different problems. They are not interchangeable, and they are often combined in one tip.
- Filter / aerosol-barrier tip
- A porous plug (often sintered polyethylene) seated in the tip stops aerosols and over-aspirated liquid from reaching the pipette shaft — the core defence against PCR/qPCR carry-over and sample-to-sample cross-contamination.
- Low-retention tip
- A hydrophobic surface treatment minimises liquid clinging to the inner wall, maximising recovery of small or precious volumes such as DNA eluate, enzymes and surfactant-containing buffers.
- Certified-clean tip
- Clean-room manufactured and certified free of detectable human DNA, DNase, RNase, PCR inhibitors and pyrogens — the baseline expected for clinical genetic workflows.
- Nominal capacity
- The volume at which a pipette is most accurate. Working far below it (e.g. 2 µL on a
P200) degrades both accuracy and precision.
Calibration is what makes the spec real
A tolerance on a datasheet only holds if the instrument is verified against it. ISO 8655 makes gravimetric testing — weighing dispensed water on a calibrated balance and converting mass to volume — the primary method. The 2022 revision tightened the balance requirement: a six-place balance for nominal volumes under 20 µL, and a five-place balance from 20 µL to 199 µL. Regulated labs verify at least annually, and many run interim in-house checks at multiple volumes (typically nominal, ~50% and ~10% of range) to catch drift before it reaches a patient sample.
Questions from the bench
Do I really need a filter tip for every step?
For any contamination-sensitive step — PCR/qPCR setup, library prep, handling amplified product — yes. The aerosol barrier protects both the sample from the pipette and the pipette from the sample. For making up bulk reagent stocks far from amplicons, a certified-clean non-filter tip can be acceptable per your lab's SOP.
What's the difference between accuracy and precision here?
Accuracy is how close the delivered volume is to the target (systematic error); precision is how repeatable successive deliveries are (random error). ISO 8655 sets maximum permissible limits for both, and a pipette can fail one while passing the other — for example, consistently dispensing 9.5 µL when set to 10 µL is precise but inaccurate.
Why does my small-volume transfer read low?
Common causes at low microlitres: an un-rinsed fresh tip, a standard tip retaining viscous eluate (use low-retention), immersing too deep or holding the pipette at an angle, or simply using a pipette whose nominal volume is far above your target. Work through the technique steps above before suspecting the instrument.
How often should a pipette be calibrated?
At minimum annually for regulated work, per ISO 8655 — but frequency should scale with use, criticality and any drop or repair. Daily or weekly in-house verification at a couple of volumes is good practice for instruments touching clinical genetic samples.
Does this affect my test result as a patient?
Indirectly — these controls are part of why an accredited lab's result is trustworthy. They don't change what a finding means for you. Interpretation, including any variant of uncertain significance, belongs with a clinician or genetic counsellor.
- [1]ISO 8655-2 / -7:2022. Piston-operated volumetric apparatus — accuracy and gravimetric calibration requirements; max permissible systematic and random error limits; balance resolution by nominal volume.↗
- [2]METTLER TOLEDO — Good Pipetting Practice. Pre-rinsing, immersion depth, ≤20° pipetting angle, and forward vs reverse technique and their effect on accuracy and precision.↗
- [3]Thermo Fisher — 10 Steps to Improve Pipetting Accuracy. Practical bench guidance on volume setting, pre-wetting and technique-driven error at low volumes.↗
- [4]Rainin / METTLER TOLEDO — Filtered & Low-Retention Tips. Aerosol-barrier filter construction, low-retention hydrophobic surface, and clean-room certification (free of human DNA, DNase, RNase, PCR inhibitors, pyrogens).↗
Follow a sample from swab to sequencer
Pipetting is one link in the chain. See how extraction, plating and flow cells fit together in our lab-bench explainers.