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As pointed out by George, it is important to check the calibration of
the slit mechanism on the Leica SP-2.  I've used silicon wafers in a
number of situations on the SP-2, and I've done experiments to evaluate
the impact of scattering samples on spectral readings with this
instrument.  If the electro-mechanical slit mechanisms for the
individual PMT's are in working order and calibrated, the system is
remarkably resistant to reflected light appearing in regions of the
recorded spectrum where it should not.  A high-resolution spectral scan
taken over the laser lines, with the laser power and PMT gain adjusted
such that you are within the linear detection range of the PMT, should
yield a plot where the only peaks are coincident with the laser emission
range, and the width of the peaks should not surpass 10nm at half of the
maximum intensity.  If you are registering a signal between the laser
lines (and the room is dark), then there is almost certainly something
wrong with the system.
        Moving parts can and will fail or go out of calibration.  This
phenomenon can be readily observed in automobiles, helicopters, CNC
machines and confocal microscopes. Even brand new instruments which
haven't been used once can exhibit anomolous behavior.  When such events
occur, the software doesn't necessarily recognize the error and so the
configuration settings on the computer screen don't necessarily reflect
reality.  This is not at all uncommon--I would expect it to happen
periodically.
        For this reason, it is imperative to confirm that the detection system
is operating correctly; a spectral scan across the visible range using
the smallest possible bandwidth and many steps using reflected light
from the lasers is a quick and enlightening exercise.  I highly
recommend that this be done for every detector on the SP-2; the slit
mechanism for one detector can throw off the proper operation of the
others.


--
Karl Garsha
Applications Specialist
Photometrics and Optical Insights
3440 E. Brittania Drive
Tucson, AZ 85706
Office: 520-547-2704
Mobile: 217-390-1874
www.photomet.com
www.optical-insights.com