  In isotope ratio mass spectrometry (IRMS), any sample (S) measurement is
performed as a relative-difference ((S/W)di) from a working-lab-reference (W),
but the result is evaluated relative to a recommended-standard (D): (S/D)di. It
is thus assumed that different source specific results ((S1/D)di, (S2/D)di)
would represent their sources (S1, S2), and be accurately intercomparable.
However, the assumption has never been checked. In this manuscript we carry out
this task by considering a system as CO2+-IRMS. We present a model for a priori
predicting output-uncertainty. Our study shows that scale-conversion, even with
the aid of auxiliary-reference-standard(s) Ai(s), cannot make (S/D)di free from
W; and the ((S/W)di,(A1/W)di,(A2/W)di) To (S/D)di conversion-formula normally
used in the literature is invalid. Besides, the latter-relation has been worked
out, which leads to e.g.,
fJ([(S/W)dJCO2pmp%],[(A1/W)dJCO2pmp%],[(A2/W)dJCO2pmp%]) = ((S/D)dJCO2pm4.5p%);
whereas FJ([(S/W)dJCO2pmp%],[(A1/W)dJCO2pmp%]) = ((S/D)dJCO2pm1.2p%). That is,
contrary to the general belief (Nature 1978, 271, 534), the scale-conversion by
employing one than two Ai-standards should ensure (S/D)di to be more accurate.
However, a more valuable finding is that the transformation of any d-estimate
into its absolute value helps improve accuracy, or any reverse-process enhances
uncertainty. Thus, equally accurate though the absolute-estimates of
isotopic-CO2 and constituent-elemental-isotopic abundance-ratios could be, in
contradistinction any differential-estimate is shown to be less accurate.
Further, for S and D to be similar, any absolute estimate is shown to turn out
nearly absolute accurate but any (S/D)d value as really absurd. That is,
estimated source specific absolute values, rather than corresponding
differential results, should really represent their sources, and/ or be closely
intercomparable.

Data Analysis, Statistics and Probability;Computational Physics