Filter tips and aerosol risk

Filter tips are a control for the inside of an air-displacement pipette. The workflow is to use them when the risk assessment wants less aerosol in the shaft, and to keep every larger control, especially a biosafety cabinet, exactly where that assessment put it. The pipette stroke around the filter is still the one in accurate micropipetting technique: seat the tip, pre-wet if the method says so, respect the stops.

What the barrier is

During aspiration, air moves up the tip as liquid moves in. Aerosols and small droplets can ride that air. A filter, a porous plug partway up the tip, catches much of that material before it reaches the cone and the piston. Some designs swell and seal if liquid hits them. That is a class of barrier, not a promise about a particular brand's pore size. This overview does not invent retention percentages.

The sample below the filter is unchanged by the filter's presence. You have not sterilised a culture by using a sterile filter tip. Sterile, as supplied, describes the tip before you touch it and before you aspirate. It does not describe the liquid. It does not survive a contaminated glove fishing in the rack.

The shaft stays cleaner when the filter works. That matters because a contaminated piston sheds into later, unrelated samples and is miserable to decontaminate. The filter is therefore a cross-contamination control and an instrument-protection control. It is not personal protective equipment and not a room control.

Where it sits among other controls

The institutional risk assessment decides the containment. Advisory references that laboratories use when they build that assessment include the WHO laboratory biosafety manual, fourth edition, and CDC BMBL. Both are advisory. Neither is a law this article can apply to your organism, and neither says that a filter tip is sufficient. If the assessment says the work is done in a cabinet, the cabinet stays. If it says eye protection and a closed tube, the filter tip does not delete those lines.

Inside the cabinet, ordinary aerosol mistakes still matter. A blow-out aimed at the grille, a tip ejected with a snap, and a wet glove on a rack are airflow and behaviour problems. The filter does not collect the droplet that left sideways onto the well rim.

ISO 8655-1:2022 remains the terminology and user-recommendation standard for the piston apparatus. ISO 8655-2:2022 treats the pipette as complete with its selected tip, which is why a filter tip used in quantitative work belongs in the calibration story. Maximum permissible errors are in that part and in the manufacturer specification. Compare a check with the specification you claim. Do not assume the filter is metrologically silent.

The ordered workflow and its branches

Confirm the assessment and the waste path before the rack is opened. Choose a tip volume that can hold the aliquot with margin so liquid does not reach the filter. Seat every tip. A multichannel with one unseated filter tip is an open shaft plus a false sense of protection.

Pre-wet when the liquid and the method require it, using the filter tip you will measure with. Forward pipetting for aqueous liquids: first stop to aspirate, second stop only to blow out, withdraw before the plunger rises. If the blow-out of an infectious liquid is the step your assessment worries about, the assessment may prefer a method that does not blow out into open air, or a cabinet, or both. Do not invent that change silently. Write it.

Watch the filter. If it stains or wets, stop the series. Treat the pipette as possibly contaminated internally. Follow the manufacturer. Surface ethanol is not automatically permitted, and ultraviolet light on the outside does not clean the piston. Do not autoclave unless the insert says that model can be autoclaved. Detach the tip ejector if the insert says to detach it for cleaning.

Branch if a quantitative assay was calibrated with plain tips and you are now on filters. Run a gravimetric check with the filter family before you trust small volumes. The pattern in checking a pipette between calibrations is the right shape. Branch if the tip will not seal. A filter does not compensate for a gap at the collar. Air leaks around the cone and the barrier never sees the stream you think it sees.

Failure modes

The common failure is moral licensing: filtered tips, open bench, infectious liquid. The second is a wet filter ignored because the volume "looked about right". The third is a calibration that never saw the filter, then a small-volume assay that shifted. The fourth is reusing a tip to save a rack, which defeats both the barrier and the sterile presentation.

A filter tip does not fix a dripping piston. If every filtered tip drips water, the seal or the shaft is the cause. Remove the pipette from service. Do not keep pipetting because the filter feels like protection.

Safety and research-use limits

This is an overview of a consumable inside a research risk assessment. It is not an approval to handle a particular agent, not a diagnostic method, and not a substitute for training on the cabinet you actually have. Waste that held hazardous liquid follows the stream your institution named. Do not spray a used rack with a random disinfectant and call it decontaminated.

What to put in an enquiry

State the pipette cone, the volumes, whether the tip must be sterile as supplied, filter or self-sealing filter if that class is what the assessment asks, and whether a low-retention surface is also required. Say that quantitative work needs the tip family included in any calibration document. Racks are in the laboratory consumables catalogue. The pipette is in the scientific instruments catalogue. Send the containment context in ordinary words with the quotation request.