Small volumes near the bottom of the range

The symptom is a small volume that will not repeat: primers, enzyme, a standard at the bottom of a larger pipette. The likely cause is not mystery. Relative error grows as the selected volume approaches the bottom of the range, and the hand's ordinary millimetre of variation becomes the data. The next check is the instrument range, the first stop, and a tip that seals. The full forward habit is in accurate micropipetting technique.

Why the bottom of the range is a different measurement

Nominal volume names the top of the pipette. The selected volume is what you set. Manufacturers and ISO 8655-2:2022 specify errors across that span, and the relative form of those errors is typically least kind at the low end. This troubleshooting note does not reprint the table. Look up the specification you claim. If the decision in the assay is tighter than that relative tolerance, no amount of care on this pipette will make the specification smaller. Change range.

Absolute effects that were minor at mid-range dominate here. Liquid on the outside of a deeply immersed tip can equal the aliquot. A film left after a timid blow-out can equal it too. An air bubble that you would have shrugged at in a 200 microlitre tip is a disaster in a few microlitres. Pre-wetting removes the dry-cushion bias that would otherwise hit the first tube hardest. It does not shrink the specification.

ISO 8655-1:2022 is the terminology and user-recommendation part. It helps you say nominal volume, selected volume, systematic error, and random error without mixing them. It does not certify the pipette you are holding at its lowest mark today.

Symptom, likely cause, next check

If only the smallest volume in a dilution is noisy, and larger volumes from the same pipette are calm, the cause class is the bottom of the range plus handling. Next check: move that volume to a lower-range pipette, pre-wet, and watch the first stop. Confirm with a balance that can resolve it. If you have only a coarse balance, do not invent a pass.

If every volume is noisy, including mid-range, the cause class is the tip, the seal, or the stroke, not the special sadness of small numbers. Next check: a trusted tip and the interim water series in checking a pipette between calibrations. A drip retires the pipette. A seal that holds and a mid-range that passes sends you back to the small setting and the range question.

If the liquid is viscous, the cause class is drag in a tiny orifice. A slow reverse stroke may be enough. A bead that remains means positive displacement. Do not snap a small viscous aliquot and then spin the tube to pretend it all arrived.

If the first tube is the outlier and the rest cluster, the cause class is a dry tip. Next check: pre-wet and discard those preparatory strokes. If the last tubes walk away, think hand heat and a short stroke that feels the warming cushion more. Equilibrate.

If the outside of the tip is wet, immersion was too deep. Next check: enter only far enough to avoid sucking air. At small volumes that depth is easy to get wrong in a nearly empty tube. Use a vessel you can see into.

Failure modes of the diagnosis

Blaming a primer aliquot for a pipette that was set to a few percent of its nominal volume is the usual story. The primer may also be degraded. You only earn that conclusion after the volume is delivered by a pipette that can actually own it. Adjusting the large pipette's calibration screw to force a tiny setting is how you destroy the mid-range everyone else uses. Do not do that. The screw is not a small-volume adapter.

Another failure is reverse pipetting by accident, because the short stroke made you press to the second stop. The tip then holds extra liquid, you dispense only part of it or you blow it all out inconsistently, and the replicates fan. Watch the stop. Say which mode you are in.

Specify the replacement in the method, not only in the moment of frustration. Write the nominal range of the pipette that passed, the tip family, and the selected volume. A later reader who sees only a bare microlitre instruction will reach for the largest instrument on the rack and recreate the scatter. If two people share the assay, both should use that lower-range pipette, or the between-person difference returns disguised as biology. Note the balance readability, the water temperature, and whether the bottom setting was actually weighed. A sentence that the pipette is calibrated, with no volume attached, does not answer this failure.

Watch the receiving tube as carefully as the dial. At these volumes a droplet that climbs the wall and never joins the reagent is the entire aliquot. Touch the wall where the liquid can run down, or deliver into liquid already present if the method has standardised that contact. Do not dispense into the air and hope a flick will capture a bead you can barely see. If the liquid is volatile, a small volume also disappears into the cushion faster than a mid-range aliquot would. Pre-wet, work promptly, and if a drip still forms, change to positive displacement rather than repeating the same short stroke.

Safety

Small volumes of hazardous liquid are still hazardous. Aerosols from a blow-out of a culture do not become acceptable because the dial was low. Use the containment and the filter tip the assessment names. This is research troubleshooting, not a clinical micropipette dose.

What to ask for

Ask for a pipette whose usable middle covers the small volume, the tip family matched to that cone, and the systematic and random error at that volume, not only at nominal volume. If the liquid is viscous or volatile, say so, because the better class may be positive displacement. Browse the scientific instruments catalogue and tips in the laboratory consumables catalogue. Send the volume that is failing with the quotation request.