Sample filters and clogged nozzles

A nozzle clog is a particle that was larger than the orifice, arriving after the filter you did not use. The decision this protocol overview supports is how to match a mesh class to the restriction in your cytometer, and when to stop a run rather than interpret the file. How a clean stream becomes one pulse per cell is in how a flow cytometer reads a cell. A microscope will not unclog the stream, though it can show you the clumps, as a picture, in the spirit of fluorescence microscopy without the myths.

Sorter and analyser classes are in the scientific instruments catalogue. Sample handling may be part of the molecular biology pathway. Orifice class and plate or tube format belong in a quote request.

Two different narrow places

On a cell sorter the narrow place is the nozzle orifice. Orifice classes come in a smaller band, often used for lymphocytes and compact lines, and a larger band, used when the cells themselves are large or when clumps are hard to avoid. Pressure and drop drive are part of that choice. Copying a pressure from a small-orifice setup onto a large orifice, or the reverse, is outside this page and inside the manual.

On many analysers there is no user-swappable nozzle. The restriction is a flow cell and a probe. You still filter, because a clump that fits in the tube does not fit in the flow cell. The manual names that limit. A mesh sold for tissue dissociation is not automatically the mesh for the cytometer. Compare the pore class with the restriction class. The pore should be comfortably smaller.

Sheath filters are a third object. They keep particles out of the sheath, not out of your sample. A dirty sheath filter, or sheath made from poor water, fills the plot with debris even when the sample is perfect. Replace or clean sheath filters on the instrument's schedule. Do not rinse them into the tank.

A workflow before the tube goes on

Dissociate or resuspend until the eye sees a cloudy suspension rather than flakes. Pass the sample through a strainer whose pore is below the orifice or flow-cell limit. If the strainer clogs instantly, the preparation is not ready: dilute, settle out the big debris, or revise the digest. Forcing a paste through a mesh with a syringe plunger can damage cells and still pass elongated clumps edge-on.

Stain after or before that filter according to the biology, then filter again if the tube waited. Aggregates form in warm tubes and in tubes that sat pelleted. Mix gently before acquisition. A rate that starts high and falls often means the cells settled or a partial clog is growing. Mix, or stop.

Choose the orifice class before you book the sort. Large cells on a small orifice class clog and also suffer. If you do not know the cell size, look at an aliquot under a microscope and then choose the larger class when the cells are obviously not lymphocytes. That look is morphology. It is not a cytometer count.

When the rate collapses

Stop acquiring. Note the time so the file can be cut if some events were good. Follow the instrument's clearing procedure: that may be a backflush, a purge, or a nozzle change, and it is written for that machine so that people do not improvise with force. If the procedure restores a clean stream on a bead or a plain buffer, filter the sample again and start a new file. If it does not, the instrument needs the person who maintains it. Continuing to run sample into a blocked head is how a clog becomes contamination of the fluidic path.

After a clear, watch pulse width or area versus height on the next file. A doublet fraction that is suddenly huge means the sample is still aggregated. Filtering again is the branch. Tightening the gate and publishing the remainder hides the preparation failure.

Failure modes

A mesh wider than the orifice lets through the exact particles that lodge there. An elongated clump can also pass a square mesh corner-first. If clogs continue despite a correct pore class, look at the dissociation and at DNA from dead cells, which makes a stringy suspension that filtration only partly cures. A DNase treatment is a reagent-class decision that belongs to the tissue protocol, not to a guess at the cytometer. Viability dyes will then show whether the remaining events are worth gating.

Humidity and heat make tubes sweat and make cells unhappy. Condensation drips into an open tube and can carry debris. Cap the tube, wipe it, and do not leave it in a sunlit queue. A power cut mid-sort is not a clog, but the restart can leave a half-pressurised fluidic path. Bring the instrument to ready, run a bead, then a new sample file.

Mesh pore and orifice class both belong in the run record. A note that says only that the sample was filtered cannot be audited. If you change nozzle class later in the day, the morning mesh may now be too coarse for the new restriction. Filter again rather than inheriting the morning strainer by habit. Write the pore class next to the nozzle class so the next clog has a cause you can see.

Safety and research limits

Clearing a nozzle can aerosolise the sample that was inside it. Sorters are the higher-risk case. Whether that sample may be sorted at all is an institutional decision, informed by the WHO Laboratory biosafety manual and the CDC BMBL. This overview does not approve unfixed human or infectious material, and it does not describe a workaround for a missing containment cabinet. A clean event rate is not a diagnosis. Practice around cytometer operation is part of the life of the International Society for Advancement of Cytometry.

What to send with an enquiry

State the cell type and an approximate size class, whether you must sort or only analyse, and whether the sample is clumpy tissue or a clean line. Ask which orifice classes are available and what filter pore the manual expects for each. Use the scientific instruments catalogue and the quote request. Ask whether a quotation is possible. A sorter name without an orifice class is not a fluidic specification.