Flow Cytometry :: Cell Sorting

How Cell Sorting Works

Cell sorters separate a complex cell mixture into fractions of a single cell type that then can be studied in isolation. In a cell sorter, the cells are carried by a thin jet of liquid emanating from a small nozzle orifice. Shortly after the cell leaves the nozzle, it passes through the waist of one or more tightly focused laser beams. The scattered and fluorescence light from these intereactions is collected and analyzed cytometrically to determine if there are events that bear closer inspection.

In order to sort cells that have been classified cytometrically, an acoustic vibration is coupled to the nozzle’s tip, leaving a trail of cyclical imprints onto the liquid’s surface. Surface tension pulls at the waists between the imprints, forcing the jet to separate into regularly spaced droplets. The cells, first carried by a cylinder of liquid, soon find themselves distributed among a string of discrete droplets.  If a cell meets one or more criteria set by the operator, an electrical charge may be applied to the droplets containing cells of interest as they separate from the main jet. In this manner, droplets with different cells types are directed towards separate collection vials by a static electrical field.

Modern electronics and detectors can easily analyze cells at speeds greater than 200,000 events per second. The physics of drop formation, however, and the statistics of distributing the cells among the droplets limits sort rates to about 50,000 cells per second. This combination of speed and reliable separtion allows scientists to seek out rare events (occuring less 0.0001% of the time) and isolate them for further study.