Pipetting is the most frequent manual step in almost every QC laboratory, and small errors travel. A pipette that delivers two per cent too much shifts every result it touches; inconsistent technique between analysts widens the spread of results and hides real differences. For laboratories choosing and using Eppendorf pipettes in the UAE, the instrument is only part of the story. This article covers the technique, calibration and equipment choices that keep liquid handling accurate.
Accuracy, precision and where errors come from
Two numbers describe pipette performance. Accuracy, or systematic error, is the difference between the mean delivered volume and the set volume. Precision, or random error, is the spread of repeated deliveries, usually expressed as a coefficient of variation. A pipette can be precise but inaccurate, delivering the wrong volume consistently, or accurate on average but imprecise.
Errors come from several sources:
- The instrument: wear, damaged seals, or a calibration that has drifted.
- The tip: a poor fit leaks air and delivers less; low-quality tips retain liquid.
- The liquid: viscosity, volatility, density and temperature change what is delivered.
- The operator: angle, immersion depth, speed and rhythm.
- The environment: temperature differences between the pipette, the liquid and the room.
Technique that makes the difference
Most technique errors are easy to correct once they are understood.
- Pre-wet the tip. Aspirate and dispense the liquid two or three times before the measured aspiration. This saturates the air inside the tip and coats the inner wall, improving accuracy, particularly for volatile liquids.
- Hold the pipette near vertical when aspirating. Tilting changes the height of liquid pulled in and the volume aspirated.
- Immerse to the right depth. Too shallow draws in air; too deep coats the outside of the tip with liquid that is then carried over.
- Use a smooth, consistent rhythm. Release the plunger slowly when aspirating, and pause briefly before withdrawing the tip.
- Dispense against the vessel wall. Touch the tip to the wall and use the blow-out stroke to deliver the last drop.
- Let everything reach the same temperature. Pipetting a cold reagent with a warm pipette changes the air cushion and the volume delivered.
Choosing the right pipetting mode
Forward pipetting is the standard mode for aqueous solutions: press to the first stop, aspirate, and dispense to the second stop to blow out the remaining liquid.
Reverse pipetting suits viscous, foaming or volatile liquids and very small volumes. Press to the second stop before aspirating, then dispense only to the first stop, leaving a small volume in the tip that is discarded. This avoids bubbles and incomplete delivery.
For very viscous or volatile liquids, positive-displacement instruments, where a piston contacts the liquid directly, remove the air cushion altogether and are often the most reliable choice.
Calibration and ISO 8655
ISO 8655 specifies requirements and test methods for piston-operated volumetric apparatus, including single- and multichannel pipettes. The reference method is gravimetric: water is dispensed onto a sensitive balance, the mass is converted to volume using a correction factor for temperature and air pressure, and the results are compared with maximum permissible errors for accuracy and precision.
Calibration intervals depend on how often the pipette is used, how critical the work is, and the laboratory's quality system. Many laboratories combine periodic full calibration with quick routine checks in between, and check a pipette immediately if it is dropped or behaves unusually. Keep a record for each instrument: its identity, service history, calibration results and any adjustments.
Manual, electronic or automated?
| Option | Best for | Consider |
|---|---|---|
| Manual single-channel | General work, flexible volumes | Operator technique drives results |
| Manual multichannel | Plate work with moderate throughput | Even tip sealing across channels |
| Electronic pipettes | Repetitive dispensing, critical volumes, ergonomics | Programmable modes reduce variation between users |
| Dispensers | Repeated delivery of the same volume | Fast serial dispensing |
| Automated liquid handling | High throughput, reproducibility-critical assays | Method development and validation effort |
Electronic pipettes reduce variation caused by differences in thumb pressure and speed, and they reduce the physical strain of repetitive pipetting, which is a genuine occupational health concern in busy laboratories. Automated systems make sense when throughput is high, when the same protocol runs repeatedly, or when manual pipetting is a documented source of error.
Tips and consumables
The tip is part of the measuring system. A tip that does not seal properly on the cone leaks air and delivers too little; a tip with a rough inner surface retains liquid. Using tips designed for the pipette ensures a correct fit and the performance the pipette was calibrated with. For sensitive applications, tips certified free of specific contaminants and low-retention tips can reduce loss of sample and cross-contamination. Filter tips protect the pipette from aerosols when working with hazardous or contamination-sensitive samples.
Multichannel pipetting and plate work
Multichannel pipettes bring their own challenges. Every channel must seal its tip equally, so tips should be loaded with even pressure rather than by rocking the pipette. Check visually that the liquid level is the same in every tip after aspiration; a short tip is a sign of a leak. When filling plates, work in a consistent direction and keep the pipette vertical so that all channels immerse to the same depth. Reservoirs should be deep enough for every channel to stay below the surface throughout aspiration.
Common problems and quick fixes
| Symptom | Likely cause | Fix |
|---|---|---|
| Volume consistently low | Leaking seal or poorly fitting tip | Check tip fit; service the pipette |
| Droplets left in the tip | Viscous liquid or low-quality tip | Use reverse pipetting or low-retention tips |
| Drifting results during a session | Hand warming the pipette | Rest the pipette in a stand between series |
| Results vary between analysts | Inconsistent technique | Retrain; consider electronic pipettes |
| Bubbles in the tip | Aspirating too fast or too shallow | Slow release; correct immersion depth |
Building a liquid-handling routine
A practical routine combines these elements:
- Train every analyst on technique and assess it periodically, for example with a gravimetric check.
- Define which pipetting mode and tip type is used for each method.
- Label each pipette with its calibration status and next due date.
- Store pipettes upright in a stand to keep liquid out of the shaft.
- Clean and inspect pipettes regularly, and send them for service when performance drifts.
Eppendorf pipettes in the UAE from Arab Lab
Eppendorf designs its instruments, consumables and accessories around complete laboratory workflows, from manual and electronic pipettes to multichannel pipettes, dispensers and automated liquid-handling systems, and offers maintenance and service contracts at custom frequencies. Arab Lab supplies Eppendorf across the UAE, Saudi Arabia and Egypt. See liquid handling and bioprocess equipment for the full range, or send your application through the contact page.
