Most analytical flow cytometers are enclosed in that cells are aspirated from
a reservoir and hydrodynamically focused so that they pass one by one through
a light source, generally from one or more lasers. At this point, scattered light
and fluorescence signals are generated, detected, and measured. After this, cells
are removed under vacuum to a waste reservoir. In general, flow sorters use a
principle involving the electrostatic deflection of charged droplets similar to
that used in ink-jet printers. To sort particles by this method, the process has
to be performed in a more open system where cells are ejected into air in a
stream of sheath fluid