Fluorescent-activated cell sorting

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Fluorescence-activated cell-sorting (FACS) is a type of flow cytometry (FCM), a method for sorting a suspension of biological cells into two or more containers, one cell at a time, based upon specific light scattering and fluorescent characteristics of each cell. However, FACS is a Trademarked term that is owned by Becton-Dickinson and is not a general term. The first cell sorter was invented by Mack Fulwyler in 1965 using the principle of Coulter volume, a relatively difficult technique to use for sorting. The technique was expanded by Len Herzenberg who was responsible for the term FACS. Herzenberg recently won the Kyoto Prize for his work in flow cytometry.

The cell suspension is entrained in the center of a narrow, rapidly flowing stream of liquid. The flow is arranged so that there is a large separation between cells relative to their diameter. A vibrating mechanism causes the stream of cells to break into individual droplets. The system is adjusted so that there is a low probability of more than one cell being in a droplet. Just before the stream breaks into droplets the flow passes through a fluorescence measuring station where the fluorescent character of interest of each cell is measured. An electrical charging ring is placed just at the point where the stream breaks into droplets. A charge is placed on the ring based on the immediately prior fluorescence intensity measurement and the opposite charge is trapped on the droplet as it breaks from the stream. The charged droplets then fall through an electrostatic deflection system that diverts droplets into containers based upon their charge. In some systems the charge is applied directly to the stream and the droplet breaking off retains charge of the same sign as the stream. The stream is then returned to neutral after the droplet breaks off.