following the recent questions and answers, let's also reiterate two main points
regarding NBS-18 and NBS-19, i.e.:
(1) the materials are sized, which means that the fine fractions and dust
(easily affected by poor storage conditions, and often the cause of
inhomogeneities and contamination) are absent. So far the lack of ultimate birth
certificate for NBS-19 seems not to hide any flaws.
(2) NBS-18 (prepared by Bruce Taylor and Hans Friedrichsen) is a carbonatite
from Fen, Norway. Together, NBS-18 and -19 were intended as a pair which
potentially allows normalization to the internationally adopted permille
difference between V-SMOW and SLAP.
This concept of "normalization" cannot be looked up in older manuals or
dictionaries, but arises from analytical experience in the (mostly oxygen)
isotope community. It concerns the apparent elasticity of the delta 18-O mass
spectrometer scale, in that the permille difference between given samples seems
to vary proportionally between laboratories. Scale normalization is therefore
different from the relatively simple zero setting to a particular reference,
like NBS-19 by itself, or NBS-28.
Some original discussions of the system of isotopic reference scales can be
found in Gonfiantini (1978; Nature 271, 534-536), and in full IAEA Reports on
meetings on stable isotope reference samples in Vienna in 1984 (R. Gonfiantini)
and 1987 (G. Hut).
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