Forward scatter side scatter and a gate

A scatter plot is the first argument a cytometer file makes, and it is easy to over-read. Forward scatter and side scatter are optical measurements collected at different angles. A gate is a region you draw and then apply to later plots. The decision this page supports is whether a cloud can be named, and what you must store so a second person can apply the same region. The physical path from sheath stream to pulse is set out in how a flow cytometer reads a cell. This page stays with the two scatter axes and the gate you put on them.

Instrument classes for acquisition sit in the scientific instruments catalogue. The experiment often belongs to a wider molecular biology pathway. A specification of lasers, detectors and tube or plate format goes with a quote request.

What each axis is responding to

Forward scatter is light deflected by small angles and collected in front of the laser, usually after a blocker that hides the unscattered beam. Larger particles tend to deflect more light into that detector, which is why the axis is loosely related to size. Refractive index moves it as well. A bead, a lymphocyte and a vesicle of similar physical diameter do not owe you the same forward-scatter value, because they bend light differently. The collection angle is part of the instrument design. Two cytometers with the same laser wavelength can still place the same cells in different places if their forward-scatter optics differ. Treat the axis as a discrimination parameter you learn with the cells in front of you.

Side scatter is collected nearer a right angle. It is more sensitive to internal structure: granules, a rough nucleus, membrane folds, and other boundaries that send light sideways. In lysed whole blood, many laboratories see a low side-scatter lymphocyte cloud, a higher side-scatter monocyte cloud, and a high side-scatter granulocyte cloud. Those names are an interpretation. They become defensible when lineage markers agree. They are not labels printed by the axes.

Both measurements are pulses. Height is the peak as the cell crosses the beam. Area integrates the pulse. Width describes how long the pulse lasted. A gate drawn on height alone, while the file was acquired with a drifting flow rate, can mix single cells and pairs. Plot the shape parameter your instrument actually stores before you trust a rare fluorescent population.

Gates are arguments, stored with the file

A gate is a polygon, a rectangle, an interval or a more elaborate region. Events inside it are passed to the next plot. The cytometer did not discover the region. An analyst drew it, usually after looking at controls. A different analyst, or the same analyst on a different day, can draw a different outline on the same cloud. That is why the gate hierarchy belongs in the record: the parent population, the order of regions, and the file the regions were drawn on.

The usual research order starts with time, so a clog or a pressure change is visible as a shift. It then uses scatter to set aside debris, which often sits at low forward scatter. It removes doublets. Only then does a fluorescence question make sense. Gating on the marker you hope to prove, and only afterwards glancing at scatter, hides debris that happened to be bright.

Fluorescence gates have their own controls. Compensation, built from single-stain tubes, corrects overlap between fluorophores. It does not repair a scatter gate that swallowed granulocytes into a lymphocyte region. Unstained cells still show where autofluorescence sits inside the scatter region you kept.

Equipment that changes the picture

The flow cell or nozzle, the sheath fluid, and the threshold are part of the scatter measurement. A threshold on forward scatter that is set high enough to hide debris will also hide genuinely small cells. A threshold set on a fluorescence channel will ignore particles that do not glow in that channel, which is correct for some bead assays and disastrous for a scatter survey of mixed cells.

Sheath made with poor water adds particles that raise the debris cloud until the cells are a minority. Sample concentration changes coincidence: too many cells per second and two particles cross as one event, with summed scatter and summed fluorescence. The instrument manual states an event-rate range. Staying inside it is part of the gate's honesty.

Optical filters do not define scatter the way they define a fluorescence channel, but the forward-scatter detector still has a wavelength and a mask. Copying a voltage from a cytometer with a different scatter geometry is how clouds fall off scale. Put the negative debris and the brightest granulocytes on scale before you draw.

A workflow with branch points

Start with a tube you trust, often unstained cells of the same type as the experiment, and watch the event rate and the time plot. If the rate sputters or the cloud walks upward through the acquisition, stop. Filtering, diluting, or clearing a partial clog comes before any polygon. A gate drawn on a drifting file will not match the next tube.

When the stream is stable, plot forward scatter against side scatter with both axes on a scale that shows the whole distribution. If the main cloud is piled on the top or the right edge, lower the voltage or gain until it sits inside the plot. Events on the axis maximum are unmeasured. If you see only a thin diagonal streak and almost no cloud, the threshold may be too high, the sample may be dilute, or the probe may be sipping air. Those are acquisition problems, not gating problems.

Draw a region around the cells you intend to keep, wide enough that a small shift between tubes does not throw away the edge of the cloud. Then plot a doublet parameter on that region. If the off-diagonal tail is large, tighten the doublet gate or slow the sample. Do not "solve" doublets by shrinking the scatter gate until the tail is merely harder to see.

If two biological clouds overlap on scatter, stop naming them from scatter alone. Add the marker that separates them, and record that the scatter region was only a parent gate. If a fluorescence-minus-one control, built inside that parent, shows that your positive gate is sitting on spread from other dyes, move the fluorescence boundary. The scatter gate cannot absorb that error.

When the clouds mislead

A population that appears only in the last third of the tube is often a clog releasing, or cells settling in a tube that was not mixed. Check time before you call it a rare subset. A cloud that grows a high side-scatter shoulder after the sample waits on the bench is often death and clumping, not a new lineage. A viability dye, read inside the same scatter parent, tells you whether the shoulder should be excluded.

Beads used to set compensation can sit far from your cells on scatter. That is expected. Do not force the bead gate to look like the cell gate. Use beads for the spectral matrix when your workflow says beads are valid for that fluorophore, and use cells for the scatter region and for fluorescence-minus-one boundaries.

If forward scatter of a culture line collapses toward debris between passages, the optical voltages may be unchanged while the biology moved. Compare an unstained aliquot acquired with the saved voltages. A global drop in forward scatter with a rise in the viability-dye positive fraction is a sample problem. A drop that appears only when you reopen the file on another computer is a display scale problem.

Safety and research limits

Scatter gating of unfixed material is still handling of whatever the tube contains. Whether that tube may be acquired on a shared instrument is an institutional decision, informed by references such as the WHO Laboratory biosafety manual and the CDC BMBL. This page does not assign a containment level. A research cloud is not a diagnosis. Clinical cytometry has its own quality system and is outside the claim of a scatter lesson. The International Society for Advancement of Cytometry is a professional home for standards and reporting practice. Membership of a society is not a validation of your gate.

Heat, waiting, and copied voltages

In a hot room, a blood or culture tube that waits uncapped will shift. Dead cells lose forward scatter and can gain side scatter, and the gate you drew on a fresh control no longer matches. Acquire a viability dye on the same day, and do not splice a file that stalled when the power dipped. Two instruments in the same building can need different scatter voltages because their collection optics differ. Write the voltage next to the gate. Copying a sheet from the cooler room is how the cloud sits on the axis edge.

What to send with an enquiry

State whether you are analysing or sorting, which scatter parameters you need recorded (height, area and width), the sample type, and the event-rate range you expect. Ask for the optical layout in writing, including how forward scatter is collected. Use the scientific instruments catalogue and the quote request. Ask whether a quotation is possible. A family name is not a scatter configuration and does not mean an instrument has been reserved.