BSA Flow Software Installation and User's Guide - CSI

Thus, in a system with two photo-detectors, a compromise would be
necessary between, on the one hand, high sensitivity and small measurement
size range, and on the other hand, a larger measurement size range at the
expense of the sensitivity.
The solution to this problem as patented by Dantec is to use additional
detectors. In conventional PDA a third detector is introduced so that the
three are asymmetrically positioned. Two detectors, U1 and U2, form the
more distant pair giving the greater slope of the diameter–phase relationship
and hence a higher resolution and smaller working range. The detectors, U1
and U3, form another pair less separated and therefore giving a smaller slope
to the diameter–phase relationship (Figure 7-23). This corresponds to a
larger measurement size range, but also to a lower resolution. By comparing
the phase differences from the two detector pairs one can achieve at the same
time the high resolution and the large measurement range. In the DualPDA
(7.2.2 The concept of the DualPDA) the diameter–phase relationship with
the steeper slope is obtained from the detector pair U1, U2 while the less
steep slope is obtained from V1, V2.
A series of values for diameter D corresponding to a measured phase shift
value of Φ 12 is indicated by arrows in Figure 7-23. The phase shift value of
Φ 13 in the diagram (single arrow) is then used to select which one of the
values for D is the correct one.
Figure 7-23 The different slopes of the diameter-phase relation obtained in a
conventional PDA set-up with two pairs of photo-detectors at different separations.
Particle sphericity The arrangement with two detector pairs — either formed by three detectors
as in the convetional PDA or by four as in the DualPDA — has another
useful feature. The phase difference corresponding to each detector pair
gives us information about the curvature over a certain arc of the particle
surface. With two such pairs we can measure the curvature at two different
locations on the surface. If the particle is spherical, two such pairs of photodetectors
should measure identical curvatures.
7-28 BSA Flow Software: Reference guide