--- You wrote:
How about analyzing two, known but very different isotopic composition of
carbonates (NBS-18 and NBS_19) determining the slope and additive
correction every day and forget about calibrating the reference gas.  That
is what we did Revesz-Landwehr 2002;  RCM, 16: 2102-2114.
--- end of quote ---

That is the procedure we use as well. Ideally the samples will fall between the two standard values.

Anthony Faiia
Dartmouth College