Solution: Flow CytometryFlow cytometry, illustrated in

Chapter 20, Problem 75GP

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Problem 75GP

Flow Cytometry

Flow cytometry, illustrated in Figure P20.71, 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.

Another way to sort the droplets would be to give each droplet the same charge, then vary the electric field between the deflection plates. For the apparatus as sketched, this technique will not work because

A. Several droplets are between the plates at one time, and they would all feel the same force.

B. The cells in the solution have net charges that would affect the droplet charge.

C. A droplet with a net charge would always experience a net force between the plates.

D. The droplets would all repel each other, and this force would dominate the deflecting force.