Flow cytometry instruments analyze suspensions of single cells by focusing them hydrodynamically in a columnar stream and causing them to flow past a point or points in the stream where lasers have been focused. As each cell passes through the point of excitation, it scatters the laser light, and if it is appropriately stained, it will also fluoresce. Sensors collect both the scattered light and fluorescence, and based on the timing of the pulses, the collected signals can be identified with an individual cell. When the stream is agitated, droplets form, some of which contain single cells. With some careful timing, droplets containing a desired cell can be either positively or negatively charged and then deflected away from the main stream and collected. Significant variations from this basic design include cells flowing in a quartz tube or across a microscope slide. Both of these configurations improve resolution but preclude physical...

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