62 BD FACSAria User’s Guide
Sorting
During sorting, drop drive energy is applied to the stream to break it into highly
uniform droplets. Droplets detach from the stream a few millimeters downstream
from the nozzle. The time between when a particle intercepts the laser and when
it reaches the droplet breakoff point is determined using BD FACS Accudrop
technology (see Drop Delay Overview on page 69).
When a particle is detected and meets the predefined sorting criteria, an electrical
charge is applied to the stream just as the droplet containing that particle breaks
off from the stream. Once broken off from the stream, the droplet—now
surrounded by air—still retains its charge. The charged droplet passes by two
strongly charged deflection plates. Electrostatic attraction and repulsion cause
each charged droplet to be deflected to the left or right, depending on the
droplet’s charge polarity. Uncharged droplets are not affected by the electric field
and pass down the center to the waste aspirator. See Figure 2-14 on page 63.
The following sections describe how the BD FACSAria cytometer creates and
charges drops, how the drops are deflected, and how sorting populations are
identified.
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