A strange phenomenon when labeling microparticles with a fluorophore

[assa sittner]

Hi,
I've been labeling microparticles with an NHS-labeled fluorophore and at a
certain range of stoichiometries, as I increase the amount of fluorophore,
the fluorescence of a single microparticle (measured by FACS) decreases.....
when looking at the pelleted particles, the pellet looks more colored when I
use more dye.
to some up:
at a certain range of
MORE dye per microparticle-> More absorption (by eye, looking at the
particle pellet) LESS fluorescence (single microparticle, by FACS)
LESS dye per microparticle-> Less absorption (by eye, looking at the
particle pellet) MORE fluorescence (single microparticle, by FACS)
Is there a certain density effect that reduces the fluorescence at high
density?
If anyone has encountered such a thing, could you refer me to some
publications?
Cheers,
J