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.
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