Nozzle Liquid Cells stream Laser probe Charging collar Deflection plates Collection tubes FIGURE P20.75
Flow cytometry, illustrated as shown, is a technique used to sort cells by type. The cells are placed in a conducting saline solution which is then forced from a nozzle. The stream breaks up into small droplets, each containing one cell. A metal collar surrounds the stream right at the point where the droplets separate from the stream. Charging the collar polarizes the conducting liquid, causing the droplets to become charged as they break off from the stream. A laser beam probes the solution just upstream
from the charging collar, looking for the presence of certain types of cells. All droplets containing one particular type of cell are given the same charge by the charging collar. Droplets with other desired types of cells receive a different charge, and droplets with no desired cell receive no charge. The charged droplets then pass between two parallel charged electrodes where they receive a horizontal force that directs them into different collection tubes, depending on their charge.
If the charging collar has a positive charge, the net charge on
a droplet separating from the stream will be
A. Positive. B. Negative.
C. Neutral. D. The charge will depend on the type of cell.
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