The flow cytometer records the interactions of individual cells that pass by the focused laser beam in a single-file line with the help of a fluid stream. This fluid stream is created by a sheath fluid, which is pressurized as it enters the flow cell with the patient sample. The sheath fluid utilized is isotonic, which helps to preserve cellular integrity for accurate measurement. This process is called hydrodynamic focusing.
As the cells pass through the flow cell single-file to the lasers, various measurements are taken within the instrument. These measures are based on cellular:
- Light scatter – reflects intrinsic cell characteristics
- Emitted fluorescence – represents extrinsic cell characteristics
Scattered light is collected by photodetectors, also known as photodiodes, as cells pass through the laser beam one by one. Cells that have fluorescent molecules attached, due to the staining process explained earlier, will also emit light. The different wavelengths of emitted light are collected by a photomultiplier tube, which is a different type of photodetector.
The processes of collecting light scatter and emitted fluorescence are illustrated on the next two pages.