Many years ago, I performed an experiment with a FACScan that sup-
ports Matt's hypothesis that viscous fluids may not flow as readily through
the flow cytometer. My "viscous solution" was detergent-solubilized nuclei
from the Jurkat cell line, stained with propidium iodide. The stock solution
was fairly concentrated, as I recall -- perhaps 10 million cells/ml. I did
not have the means for measuring the viscosity of this solution. I tried
this at full strength and at various dilutions. I weighed the tubes on an
analytical balance, ran them on the FACScan for five minutes, and weighed
them again. I used the low flow rate, which pumps 12 microliters/minute
when the sample tube contains just plain water. Over the course of the
five minutes, I watched the acquisition event rate in order to be sure that
any drop that I might see in the *mass* flow rate (microliters/minute) was
not the result of temporary clogs in the sample injection tube. I think
that I did get such clogs at the higher cell concentrations. However, even at
the lower concentrations, there was clearly a drop in the mass of liquid
transferred, even when there were no hiccups detectable in the event rate.
Hope that helps!
- John Ladasky