A pipette's error is a property of the instrument's full-scale range, not of the volume you asked it for, which is why using a large pipette for a small transfer is the most common accuracy problem in peptide work. Every other number in a protocol is usually read from a document or a balance. The transfer volumes are generated at the bench, by hand, and they are where the uncertainty actually lives.
Why the instrument matters more than the setting
An air-displacement pipette works by moving a column of air, and its mechanical tolerance is specified as a percentage of its maximum volume rather than of whatever it is currently set to. Set a 1,000 µL pipette to 20 µL and the absolute error that belongs to a 1,000 µL transfer travels with it — now expressed against a volume fifty times smaller. The setting changed; the mechanism did not.
| Transfer | Instrument used | Relative error |
|---|---|---|
| 20 µL | A pipette whose range ends near 20 µL | Small — the volume sits near full scale |
| 20 µL | A pipette whose range ends at 200 µL | Several times larger for the same transfer |
| 20 µL | A pipette whose range ends at 1,000 µL | Large enough to dominate the experiment |
The rule that follows is simple and worth being rigid about: use the smallest pipette whose range covers the volume, and keep transfers in the upper part of that range wherever the protocol allows. Where a choice exists between one 20 µL transfer and one 200 µL transfer that reaches the same concentration, the larger volume is the more accurate route to it — which is also the argument for designing dilution series around convenient volumes rather than convenient factors.
Why does a 20 microlitre transfer matter so much?
Because error at that step is not diluted away — it is carried forward intact. A stock prepared 8% below its nominal concentration produces every subsequent dilution 8% low, and the figure recorded in the notebook is the nominal one. Nothing downstream detects it. In a concentration–response experiment this shifts the whole curve along the concentration axis while leaving its shape intact, so the result looks clean and sits in the wrong place. A systematic volumetric error is the kind of mistake that produces confident, reproducible, wrong numbers.
The techniques that actually change the result
Pre-wet the tip. Aspirate and expel the solution two or three times before the transfer that counts. This equilibrates the air column's humidity, which otherwise causes the first draw to come up short. For peptide solutions it does a second job as well: it saturates the plastic's binding sites, so the transfer that matters is not also the one losing material to the tip wall.
Control the immersion depth. Two or three millimetres below the surface, held steady. Too shallow draws air; too deep leaves liquid clinging to the outside of the tip, which then contributes to the delivered volume. The latter error is in the opposite direction to most others and is easy to introduce by resting the tip on the bottom of a vial.
Aspirate and dispense slowly. The air column compresses, and it needs time to reach equilibrium. Releasing the plunger quickly delivers less than the setting states, consistently.
Reverse-pipette anything viscous. Overfill past the setting, then dispense only to the setting and discard the remainder in the tip. This removes the error caused by liquid that will not fully leave the tip — the dominant problem with DMSO and other viscous solvents, which an air-displacement instrument is not calibrated for.
Let everything reach room temperature. A pipette calibrated at ambient temperature delivering a solution straight from a freezer block is measuring a gas column at one temperature and a liquid at another — a real and avoidable discrepancy.
How do you know a pipette is accurate?
By weighing water, the only check that does not depend on the instrument being checked. Water's density is near enough to 1 mg per µL at room temperature that a balance reading gives the delivered volume directly. Pipette ten replicates of the nominal volume onto a balance: the mean against the setting is the accuracy, the spread between them the precision. The two fail independently, and an instrument can be reproducibly wrong — the more dangerous condition, because it looks reliable. The check takes a few minutes and is worth doing on any pipette a shared stock depends on.
Is a syringe better than a pipette for small volumes?
For the specific job of adding solvent to a vial, a syringe is the appropriate tool, because it delivers through a septum without opening the vial. For transfers out of a vial into tubes, a pipette is more accurate. The difference is dead space: a syringe retains a volume in the needle hub that is neither delivered nor easily measured, whereas a pipette tip is designed to empty. Use each for what it is shaped for — syringe in, pipette out.
Does the tip need changing between every transfer?
Between different solutions, always — a tip carries enough residue to matter at the concentrations peptide work runs at. Within a dilution series moving consistently from dilute to concentrated, a single tip is defensible; moving from concentrated to dilute it is not, because the carryover lands where it does the most damage. The habit worth keeping is to work up the series rather than down, which makes carryover negligible rather than requiring it to be remembered. Where a blend or a shared stock bottle is involved, a fresh tip each time is the only safe rule, since contamination there propagates to every future experiment rather than one.
Does the volume need to be exact, or just recorded?
Recorded, in most cases, and this is the more useful discipline. Concentration is mass over volume, so an unintended volume is not an error provided the actual volume is what gets written down and used in the arithmetic. If 1.1 mL went in instead of 1 mL, the stock is simply more dilute than planned, and a stock of known concentration is entirely usable. The failure mode is recording the intended figure instead of the delivered one. Reading the reconstitution calculator backwards — entering the volume you actually added — gives the concentration you actually have.
Where to spend the effort
Not every transfer deserves equal care, and treating them all as critical wastes attention the critical ones need. One transfer defines the stock concentration every later step inherits; the rest mostly move material between tubes. Pre-wetting for that one transfer and recording the delivered figure addresses most of the available error.
The case worth singling out is the first step of a dilution series, where an error is multiplied by every subsequent step rather than merely carried. A modest error at the top compounds into something substantial at the bottom — and the arithmetic of how that propagates is why intermediate stocks are prepared fresh rather than diluted from each other indefinitely.
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