Peter,
               We have been doing something similar for a while now - the
method you propose works well for liquid samples and fractionation (at least
for N) is minimal if you make sure that the sample is slightly acid and you do
not dry down at more than about 75 degrees C. Too much acid corrodes
the tin cups! And, more seriously, would cause loss of some Carbon.
  My biggest reservation is that the method may not be practical at ambient
concentrations of N - unless your mass-spec is unusually sensitive.  It
would pay to concentrate your sample first - possibly by very slow rotary
evaporation?
                                  Darren.