Tip fit is part of the measurement

The tip is not packaging around a finished measurement. On an air-displacement pipette it closes the air cushion. If it leaks, the piston moves and the liquid does not follow by the amount the dial named. The decision this selection guide supports is what to write down before a laboratory changes tip family, and which words on a box are not yet a specification. Seating, immersion, and the forward stroke are the handling side in accurate micropipetting technique.

What a seal has to do

The cone of the pipette and the collar of the tip have to match closely enough that air does not enter or leave except through the orifice. You feel a press. You may hear a click. Neither sensation measures the gap. A tip that sits slightly proud can hold for a water aspirate and leak when the liquid is warm, volatile, or held while you walk across the room. A tip forced on with a pounding motion can seal today and oval the shaft over months, so later tips never seal.

ISO 8655-2:2022 defines pipette requirements for the apparatus complete with its selected tip or tips. The selected tip is part of the instrument in that document, not an afterthought. Maximum permissible errors live there and in the manufacturer specification. This guide does not invent a table of them. ISO 8655-1:2022 is terminology and user recommendations for piston-operated volumetric apparatus. It does not say that a tip marketed as widely fitting is accurate on your cone today. You compare a gravimetric check of the pair you use with the specification you claim.

Criteria a specification should name

Name the pipette first: volume range, air displacement or positive displacement, single channel or multichannel. A positive-displacement system uses a capillary piston, not a cone tip. Writing "universal tips" into that enquiry sends the wrong object.

Name the cone family as the pipette manufacturer describes it. Then name the tip family you want considered, and ask which pipette models that family was tested on. A test on a different cone is a different seal. If you will calibrate with one family and run assays with another, say so, because you have planned two measurements.

Name filter or plain. Filter tips reduce aerosol moving up into the shaft. They are not a biosafety cabinet and not a sterilisation method. They also change the air path a little. If the calibration used plain tips, a later switch to filters is a change you either check or you accept in writing.

Name surface claims carefully. Low retention means a surface treatment or a resin chosen so less aqueous film stays behind. It is most visible at small volumes and with proteins that stick to ordinary polypropylene. The honest specification asks for the pipette, the volume, and the liquid used in that claim. It does not accept the adjective alone. Wide-orifice tips are a different criterion, used for viscous liquids and for samples you do not want to shear. They deliver differently. Do not mix them into a series that was checked on fine tips.

Name sterile presentation only if the work needs it. Sterile, as supplied, is a manufacturing and packaging claim. It does not survive an open box handled with bare hands, and it does not sterilise the sample you aspirate.

Name the liquids. Aqueous buffers, viscous mixes, and solvents stress the seal differently. Solvents can loosen a marginal fit that water forgives. A specification that says only "tips for pipettes" has not chosen.

Workflow for changing family, and the branch when the new box leaks

Keep the current family in service while you test the candidate. Seat a set the way a tired person seats them, not only the way a careful demonstration seats them. Heights should match. Hold a filled tip vertical. A drip before the dispense is a failed fit, a damaged shaft, or a volatile liquid. Repeat with water so you can separate those causes. If water drips on the new family and holds on the old, the new family does not seal on this cone. Reject it for this pipette. If both families drip, the pipette is out of service until it is checked. Do not blame the newest box for a bent shaft.

Then weigh forward dispenses at the low end of what you use and at a mid-range volume. Convert mass with the density of water at the measured temperature, about 0.998 grams per millilitre near room temperature and different as soon as the water is warmer or cooler. Use a density table for the actual temperature. Do not assume 1.000. Compare the result with the specification you claim. The working pattern for that check is checking a pipette between calibrations. If the new tips pass and you adopt them, update the method and the calibration record so the next person does not "save time" with the old box.

Failure modes after a "compatible" substitution

The quiet failure is a small systematic shortfall at the bottom of the range. Replicates look tight because every tip in the new lot leaks the same way, and the assay shifts. The loud failure is a drip, which at least stops the work. Viscous liquids make a marginal seal worse because the thumb stays on the plunger longer and any leak has time to matter. Forward pipetting of water can look acceptable while a slow glycerol reverse stroke on the same tip family does not.

A lot change inside the same family can also move. If the collar mould changes, the catalogue name may look continuous. Your check is the continuity that counts. Record the lot you actually seated when the volume is a result.

Safety

Tips that touched hazardous material are waste under your institutional rules. Fit testing with water is the clean experiment. Fit testing with solvent or culture belongs in the containment already assessed. Filter tips reduce aerosol into the shaft. They do not replace a cabinet. This page is a sourcing and metrology note for research use, not a diagnostic claim.

What to send

Send the pipette range and cone description, the liquids, filter and retention needs, and the calibration volumes that must still be true after the change. Use the laboratory consumables catalogue for tips and the scientific instruments catalogue for the pipette side. Put the test-pipette question in the quotation request. Ask which instrument the fit claim refers to.