Recently (arXiv:1101.0973), it has been pointed out by us that the possible variation in any source (S) specific elemental isotopic (viz. 2H/1H) abundance ratio SR can more accurately be assessed by its absolute estimate Sr [viz. as (Sr - DR), with D as a standard-source] than by either corresponding measured-relative (S/W-DELTA) estimate ([Sr/Wr] - 1) or DELTA-scale-converted-relative (S/D-DELTA) estimate ([Sr/DR] - 1). Here, we present the fundamentals behind scale-conversion, thereby enabling to understand why at all Sr should be the source- and/ or variation-characterizing key, i.e. why different lab-specific results should be more closely comparable as absolute estimates (SrLab1, SrLab2) than as desired-relative (S/D-DELTALab1, S/D-DELTALab2) estimates. Further, the study clarifies that: (i) the DELTA-scale-conversion (S/W-DELTA into S/D-DELTA, even with the aid of calibrated auxiliary-reference-standard(s) Ai(s)) cannot make the estimates (as S/D-DELTA, and thus Sr) free of the measurement-reference W; (ii) the employing of (increasing number of) Ai-standards should cause the estimates to be rather (increasingly) inaccurate and, additionally, Ai(s)-specific; and (iii) the S/D-DELTA-estimate may, specifically if S happens to be very close to D in isotopic composition (IC), even misrepresent S; but the corresponding Sr should be very accurate. However, for S and W to be increasingly closer in IC, the S/D-DELTA-estimate and also Sr are shown to be increasingly accurate, irrespective of whether the S/W-DELTA-measurement accuracy could thus be improved or not. Clearly, improvement in measurement-accuracy should ensure additional accuracy in results.
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