ABSTRACT
Cytometry is the modern day term to describe instruments that are used to identify cells. The light microscope is a cytometer and when it is equipped with advanced optics, video camera and associated computers it is called an image cytometer. When cells are dispersed into a suspension which can flow in a moving stream for electronic interrogation, the instrument is called a flow cytometer. Whether a simple instrument that merely counts cells, such as an electronic (Coulter) particle counter or a more sophisticated laser-based instrument that also measures cellular fluorescence and can sort cells, often called a fluorescent activated cell sorter (FACS), a flow cytometer provides rapid high speed analysis of cells. Figure 1 shows a schematic representation of a flow cytometer. There are two kinds of flow cells. In both types, the cell stream merges with a sheath fluid at higher pressure which constricts the cell stream into a laminar flow. In the first type, known as stream in air, the cell stream and sheath fluid exit an orifice that is usually 70}4 in diameter whereupon it intersects with the center of a laser beam. In the second type of flow cell the interrogation point is inside the flow chamber itself. The latter type has the advantage of providing better resolution because there is no refractive index change at the interrogation point. Because of its greater mass, droplet gen~ration for sorting is more difficult.
