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These are highly linear devices over ranges which they are valid. We have looked
at the Hamamatsu R928s over a very broad range of dynode voltages using plastic
microspheres and an external broad-range linear amp (Tektronix AM504) and found
that these PMTs were fine from about 200v to 1000v. We saw non-linearity below
200v; we did not investigate the upper end saturation. We also verified that the
gain characteristic is a power function of the dynode voltage with an exponent
of approximately 7.4 for the R928s.
As long as one remains within the working range of the PMT, adjustment of the
dynode voltage to set standards at a known signal level can be an intregral part
of a valid method for standardizing flow instruments. Maintaining consistancy of
CVs of standards (directly related to amount of fluorescence reaching the PMT)
and low levels of background noise are two other important standardization
considerations.
Such procedures has been used at the Facility here for over 10 years on several
different flow instruments. We have found no inconsitancies in measurements done
on different days as long as proper standardization has been done.
-Marty Bigos
Stanford Shared FACS Facility
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