Analysis of multiply substituted isotopologues of
molecules (‘clumped isotope geochemistry’) presents special
challenges to both precision and accuracy. Previous
discussions have focused on mass spectrometric precision for
these rare species and intralaboratory reference frames. This
discipline has spread, demanding interlaboratory
standardization. We present a four-laboratory study of the
calibration of mass-47 anomalies (Δ_(47) values) in CO_2
(especially extracted from carbonate). We consider:
instrument linearity, source fragmentation/recombination
reactions (which vary between mass spectrometers and with
time and instrument settings), and differences in methods,
materials and conditions for sample preparation. We address
these problems by developing a method for standardizing Δ_(47)
measurements to an absolute reference frame based on
theoretical predictions of the abundances of multiply-substituted
isotopologues of gaseous CO_2 that has reached a
thermodynamic equilibrium at a known temperature. By
analyzing CO_2 gases that have been subjected to established
laboratory procedures known to promote isotopic equilibration
(i.e., heated gases and water-equilibrated CO_2), and by
reference to the statistical thermodynamic predictions of
equilibrium isotopic distributions, it is possible to construct an
empirical transfer function that can then be applied to CO_2
samples with unknown Δ_(47) values. This reference frame may
be unique in that it is based on thermodynamic equilibrium,
rather than the isotopic composition of an arbitrary reference
material. We describe the protocol necessary to construct such
a reference frame, the method for converting gases with
unknown clumped isotope compositions to this frame, and
suggest a protocol for ensuring that reported Δ_(47) values can be
compared among different laboratories, independent of
laboratory-specific analytical or methodological artefacts.
Application of this approach to measurements of CO_2
extracted from several carbonate reference materials results in
interlaboratory agreement on their Δ_(47) values to within est.
±0.01 ‰, 1σ. Finally, we present a revised paleotemperature
scale that applies when using the absolute reference frame
described here, as opposed to the previous paleotemperature
equation based on data from a single laboratory. More
generally, this study presents a model for how interlaboratory
standardization might be approached for other ‘clumped
isotope’ measurements
