Troubleshooting a cytometer with no events

No events is a phrase with several meanings, and each meaning has its own next check. This glossary names those meanings so a blank plot is not written up as a negative sample. The normal path from stream to pulse is in how a flow cytometer reads a cell. A dim microscope field is a different blank, discussed in fluorescence microscopy without the myths.

Analyser classes are in the scientific instruments catalogue. The experiment may be part of the molecular biology pathway. Fluidic and laser questions belong in a quote request.

Event

An event is one recorded particle: a row in the FCS file with a value for each parameter. No events means that row was never written. The display gate, the plot scale and the compensation matrix are all irrelevant until at least scatter events exist. Look at the acquisition counter and an ungated plot before you edit regions.

Threshold

The threshold is the pulse height a channel must cross before the electronics store an event. It exists so electronic noise does not fill the file. Set too low, the file fills with noise. Set too high, real cells never cross it and the counter stays at zero. The usual research threshold for intact cells sits on forward scatter, just above noise. A threshold left over from a bright bead run will hide lymphocytes. A threshold left on a fluorescence channel from a rare-event protocol will hide an unstained test tube. Read the threshold channel and the number before you blame the stain.

Trigger channel

The trigger channel is the parameter that threshold watches. It is often forward scatter and sometimes a fluorescence detector when the particles of interest are small and the scatter is mostly debris. If the trigger channel's detector is off, unplugged in software, or saturated in a way the instrument rejects, you can have a tube full of cells and a file with nothing. Confirm the trigger is a channel the sample actually produces.

Sample injection port

The sample injection port, often called the SIP, is where the tube meets the probe. No events follow when the probe never entered the liquid: the tube is too low in the holder, the volume is below the probe, a bubble sits at the bottom, or the arm did not seal. You are sipping air or sheath, not cells. Look at the tube. A wet probe and a falling liquid level mean fluid is moving. A dry acquisition with a full tube means it is not.

Sheath

Sheath is the clean fluid that focuses the sample core. If the sheath tank is empty, the pressure is off, or a valve was left in standby, there is no stable stream and often no events. Some instruments also stop when the waste tank is full. These are status-panel checks, not staining checks. Refilling sheath with unfiltered water replaces a zero event rate with a file of debris. Use the fluid the manual names.

Clog

A clog is a particle or aggregate lodged where the stream is narrow, in a probe, a flow cell or a nozzle. The signature is a rate that was healthy and then collapsed, often with a fluidic alarm. The next action is the instrument's documented backflush or nozzle procedure, then a filtered, diluted sample, then a new file. A glossary entry cannot include a force that would damage a flow cell. If the manual's steps do not restore flow, stop and call the person who maintains the instrument.

Laser interlock

An interlock is a switch that keeps a laser off when a cover or sort chamber is open. No laser means no fluorescence and, if that laser is part of the trigger path, sometimes no events at all. The check is the interlock message and the laser status, not a new antibody. A warm-up timer can look similar: the run button is available and the laser is not ready. Wait for ready.

Display gate

A display gate hides events from the plot without removing them from the file. If the counter increases and the plot stays empty, the plot is gated on a region from another sample. Switch to ungated. This is the failure mode most often mistaken for a fluidic zero, and it is the cheapest to fix.

Order of checks

Read the counter. If it is rising, clear the display gate and only then judge the stain. If it is zero, confirm run mode, sheath, waste, and laser interlock. Then look at the tube on the SIP. Then read the threshold and the trigger channel. Then suspect a clog if the previous tube ran. A bead or a cell line you trust, acquired after those checks, separates "the instrument is down" from "this sample is dilute or absent". Do not compensate a file of zeros.

After a power cut, instruments come back in standby with lasers cold and pressures off. A file started in that state is a zero for electrical reasons. Wait for the ready state and record the restart as a new acquisition. Do not append it to the tube that was running when the lights failed.

Safety and research limits

Clearing a clog can splash or aerosolise whatever was in the line. Follow the instrument procedure and the institutional rules for that sample. The WHO Laboratory biosafety manual and the CDC BMBL are references for the assessment, not a permit written by this glossary. A restored event rate is not a diagnosis. File and reporting practice sits with the community around the International Society for Advancement of Cytometry.

What to send with an enquiry

If you are specifying an instrument, ask how clogs are reported, whether threshold and trigger are stored in the file, and what the interlocks do. If you are asking for help with a named configuration, send those three facts from the blank run: counter behaviour, threshold channel, and any fluidic alarm. Use the scientific instruments catalogue and the quote request. Ask whether a quotation is possible. A blank plot without those facts is not yet an instrument fault.