Frank

I agree with your points about references gases in a general way.
However I think you will find that having a reference gas with isotope
ratios close to those of your typical sample gas does have advantages
for high precision, and perhaps especially for long-term comparisons. It
may depend on the instrument.

Some indication can come from a simple test running reference gas
against itself (sometimes called Zero Enrichment run), at different
pressures of, say, 2E-9 to 2E-8 amps <44> (with reference and sample
sides well balanced). Then repeat using a sample gas which has C and
O ratios quite different from those of the reference (e.g. CO2 produced
from carbonate vs normal cylinder CO2). Are the raw sample vs ref
ratios independent of pressure in the second test?

Best wishes

Tim