Jeff,
           Most of our work involves CF-EA-IRMS analysis of plant
material and we would not be able to manage without a SPEX
liquid-nitrogen cooled mill. I have ground around 20,000 samples using
this machine and can heartily recommend it.     These are marketed by
Glen Creston here in the UK.
   The sample is contained in a tube along with a magnetic impactor.
The tube is sealed at each end with a hardened steel plug. This then
goes into the middle of an electromagnetic coil immersed in liquid
nitrogen and its controller alternates the polarity from one end to
the other (at a frequency set by the operator) causing the impactor to
smash the frozen sample against the hardened end caps of the tube.
    Sounds like a bit of a performance I know, but it will reduce a
sample of wood or any other plant material (most other materials too,
for that matter - even rubber bands) to a finer than flour consistency
in less than two minutes (plus washing-up). It is quite possible to
grind 100 samples in a day.  Minimum sample size is about 100mg as
around 40-50mg will be spread around inside the tube and is hard to
salvage.

      Darren Sleep
     Stable Isotope Facility
     ITE Merlewood
     Grange-over-Sands
      Cumbria UK