Pipetting accuracy basics

Accuracy is how close a measured volume is to the volume intended. Precision is how close repeated measurements are to each other. A pipette can be precise and inaccurate — delivering the same wrong volume every time — and that is the failure mode calibration exists to catch. Technique errors tend to hurt both.

An adjustable pipette has a rated range, and its error is specified as a percentage that grows sharply at the low end. A pipette rated 20–200 µL set to 20 µL is at the edge of what it can do; the same 20 µL from a 2–20 µL pipette is at the top of that instrument's range, where it is at its best. The rule is simple: choose the smallest pipette whose range comfortably contains the volume. This is also why serial dilutions beat single large dilutions — see dilution arithmetic.

For a calibrated pipette in the right range, instrument error is small. The errors that matter are in the hands:

• Tip not seated. A loose tip leaks air and delivers less than set. Press it on firmly and check.
• Wrong immersion depth. Too shallow draws air; too deep carries extra liquid on the outside of the tip.
• Angle. Hold the pipette vertical when aspirating. A tilt changes the hydrostatics and the volume.
• Speed. Aspirate and dispense smoothly. A snapped plunger aspirates air and aerosolises liquid.
• Pre-wetting. The first aspiration with a dry tip coats its inside and delivers short. Aspirate and expel once before the measured draw.
• Temperature. A pipette and a liquid at different temperatures deliver a different volume than the setting. Let them equilibrate.

The standard check needs a balance, water and a few minutes. Water at room temperature weighs very close to 1 g per mL, so a pipette set to 100 µL should deliver very close to 0.100 g. Dispense ten times onto a tared vessel, record each mass, and look at both the average (accuracy) and the spread (precision). Do it at the top, middle and bottom of the range. A pipette that fails gets serviced; a technique that fails gets practised. Write the results in the notebook with the pipette's identifier and the date.

The same hands that control volume control contamination. A tip touches one liquid and one vessel, then it is discarded. A container is open for as short a time as possible. Nothing that has touched a bench surface goes back into a container. These habits cost nothing and they are why multi-use supplies stay usable — see bacteriostatic water vs sterile water for why a preservative is a backstop, not a substitute for technique.

Everything above about pipettes applies, with small changes, to the balance a mass is weighed on. A balance is accurate in the middle of its range and poor at the bottom; a milligram-scale mass needs a balance rated for it, not a top-loading bench balance with a readability of a tenth of a gram. Draughts, vibration, a warm sample and static on a plastic weigh boat all introduce error that the display does not reveal. Tare, weigh, record the reading and the balance identifier, and check the balance against a reference mass on the schedule the laboratory sets. A mass and a volume together define a concentration, and an error in either carries straight through — see concentration units. The material itself arrives with its measured purity on the certificate, described on the quality page; the mass you weigh is your own measurement, and it is only as good as the balance.

This is laboratory technique for measuring liquid volumes and applies to any liquid. It is not a preparation, dilution or usage instruction for any material sold on this site; we publish none. All material is supplied for in-vitro laboratory research only, and handling is the responsibility of the qualified researcher under their own procedures — see research use only.