Pipetting Accuracy and Volumetric Error in Peptide Work
Divide mass by volume and you have a concentration. Nearly all the care goes into the numerator. The denominator gets taken on faith, assumed equal to whatever number is showing in the pipette’s window, and that unexamined assumption is the origin of a fair share of the variability nobody can account for in peptide work.
Ten minutes and a balance settle the question
Verification by weight requires no service contract. Dispense distilled water onto a tared balance and read the mass: at room temperature, a microliter of water weighs a milligram closely enough for this purpose. Run ten replicates at the nominal setting and both numbers fall out together. How the mean compares with the setting is accuracy. How tightly the ten cluster is precision.
Repeat at the top and the bottom of the range, write the result in a book, and an assumption has become a measurement. It is also the least expensive route to discovering that the instrument behind every standard on the bench has been reading low ever since somebody dropped it.
Scatter is honest, bias is not
Those two numbers describe different failures. Accuracy asks whether delivered volume matches the setting; precision asks whether successive deliveries match each other.
When precision goes, replicates spread out, and the spread announces the problem. When accuracy goes, nothing announces anything. Every solution made on that pipette leans the same way by roughly the same fraction, so the whole data set stays internally consistent. Three percent low yields results that are coherent, repeatable, mutually agreeable, and three percent wrong. Silence is what makes the systematic failure the worse of the two.
The small-volume trap
Quoted accuracy is a percentage of nominal volume, and that figure describes behavior near the top of the range. Slide down toward the bottom and an unchanged absolute error becomes a much bigger proportion of the delivery. Set a 200 µL instrument to 20 µL and you are operating at the ragged edge, where real tolerance may run to several times the number on the label.
Two habits follow, both widely disregarded: reach for the smallest pipette that spans the volume, and stay above roughly ten percent of any pipette’s maximum.
A column of air doing all the work
Nothing inside a conventional pipette touches the sample. A column of air moves, and liquid follows it. Accuracy therefore depends entirely on that air behaving predictably, which it does with water sitting at equilibrium and does not with several things routine here. Acetonitrile and methanol evaporate into the column, pressure rises, liquid gets pushed out of the tip; delivery runs high and the tip drips. Concentrated glycerol or a heavy buffer moves too sluggishly to fill a tip drawn at normal speed, so the tip leaves the liquid partly empty. Calibration assumes the density of water, meaning a noticeably denser solution puts a different mass into the tube for the same indicated volume. Temperature mismatch does its own damage: cold liquid handled in a warm room, or the reverse, alters the air column’s volume during the operation, which is one reason, alongside condensation, that anything out of a freezer should equilibrate before volumes come off it.
All four problems disappear with a positive-displacement instrument, where a piston meets the liquid directly. Volatile and viscous work belongs on one of those.
Habits that move the number more than expected
| Pre-wetting | A dry tip’s first aspiration saturates its internal air with vapor and wets the walls. Throw that draw away and use the next one; it is worth a percent or better, and it doubles as the mitigation for adsorptive loss to surfaces. |
| Immersion depth | Shallow, and air gets drawn in. Deep, and liquid clinging to the tip’s outside rides along into the delivery. Aim for two to three millimeters on small volumes. |
| Plunger speed | Let it snap back and sample aerosolizes into the tip and up the shaft. Slow, even movement is not fastidiousness; it is the condition the calibration assumed. |
| Angle | Vertical is how the instrument was calibrated. Tilting it alters the hydrostatic head bearing on the air column. |
| Hand warmth | Gripping a pipette through a long session measurably warms the air inside it, which argues for working in short runs. |
Two ways to push the plunger
Default practice is forward: draw to the first stop, expel to the second. Aqueous solutions want exactly that. The reverse approach draws beyond the first stop, expels only as far as the first stop, and the leftover leaves with the discarded tip.
Consistency improves in reverse mode for anything viscous or prone to foaming, and the cost is a small volume sacrificed on every transfer. Where the solution represents real material that took effort to prepare, that exchange deserves a deliberate decision rather than a reflex.
Why this sits next to a certificate
What a certificate documents is a measurement made by one laboratory on a solution that laboratory put together. Everything after that point, a dilution, a working standard, a head-to-head between two lots, depends on volumes measured where you are, and volumetric bias looks exactly like a genuine difference in the material.
So before declaring one lot weaker than its predecessor, examine the arithmetic under both figures, together with the other reasons for apparent disagreement collected in why certificates disagree on purity.
