Just found the trypan blue reference:


Van Amersfoort and Van Strijp (1994). Evaluation of a flow cytometric
fluorescence quenching assay of phagocytosis of sensitized sheep erythrocytes by
polymorphonuclear leukocytes.  Cytometry 17: 294-301.


They compared trypan blue from Merck, Sigma, Fluka, and BDH. The trypan blue
from Merck quenched external fluorescence well (94%) at 31 micrograms per ml.
The trypan blue from the other vendors only quenched to about 50% even at 500
micrograms per ml.


So,  that was why we had such trouble with trypan blue  (ours was from Sigma).


Alice