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1. The vertical waist of the focused laser beam from a 15 mW argon laser is
about 15 microns. This works well for 10 micron particles but is probably
too wide for analyzing particles of < 1 micron. It is difficult to ensure
that only one particle at a time is in the beam.
2. The flow channels and sample insertion needles are also sized to be
optimum for the analysis of 10 micron particles. With < 1 micron particles
it is difficult to obtain "single file" alignment of the particles in the
core stream so again it is difficult to present one particle at a time to
the sensing volume. Downsizing these parts will help if you can keep from
plugging them up.
3. The angle of forward light scatter collected using the standard ELite
detector is probably too great for < 1 micron particles. I have been able to
resolve 0.5 micron partilces from 0.8 micron particles using a 100 mw,
488nm laser beam focused to an 8 micron beam waist with with an 8mm diameter
"mask" inserted in front of the FALS detector on an EPICS Elite.
4. I wouldn't suggest using a UV laser with the Elite FALS detector for
small particles. The responsivity of the photodiode they use falls off
dramatically in the UV. You could always try a PMT instead.
I guess what I'm suggesting is that it's difficult to use an instrument
designed for analysis of 10 micron particles to analyze sub micron particles
without some modification.
Gary Durack
University of Illinois Biotechnology Center
Voice (217) 244-0559
FAX (217) 244-0466
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