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On-Line Hydrogen-Isotope Measurements of Organic Samples Using Elemental Chromium: An Extension for High Temperature Elemental-Analyzer Techniques
journal contribution
posted on 2015-05-19, 00:00 authored by Matthias Gehre, Julian Renpenning, Tetyana Gilevska, Haiping Qi, Tyler B. Coplen, Harro A. J. Meijer, Willi A. Brand, Arndt SchimmelmannThe
high temperature conversion (HTC) technique using an elemental analyzer
with a glassy carbon tube and filling (temperature conversion/elemental
analysis, TC/EA) is a widely used method for hydrogen isotopic analysis
of water and many solid and liquid organic samples with analysis by
isotope-ratio mass spectrometry (IRMS). However, the TC/EA IRMS method
may produce inaccurate δ2H results, with values deviating
by more than 20 mUr (milliurey = 0.001 = 1‰) from the true
value for some materials. We show that a single-oven, chromium-filled
elemental analyzer coupled to an IRMS substantially improves the measurement
quality and reliability for hydrogen isotopic compositions of organic
substances (Cr-EA method). Hot chromium maximizes the yield of molecular
hydrogen in a helium carrier gas by irreversibly and quantitatively
scavenging all reactive elements except hydrogen. In contrast, under
TC/EA conditions, heteroelements like nitrogen or chlorine (and other
halogens) can form hydrogen cyanide (HCN) or hydrogen chloride (HCl)
and this can cause isotopic fractionation. The Cr-EA technique thus
expands the analytical possibilities for on-line hydrogen-isotope
measurements of organic samples significantly. This method yielded
reproducibility values (1-sigma) for δ2H measurements
on water and caffeine samples of better than 1.0 and 0.5 mUr, respectively.
To overcome handling problems with water as the principal calibration
anchor for hydrogen isotopic measurements, we have employed an effective
and simple strategy using reference waters or other liquids sealed
in silver-tube segments. These crimped silver tubes can be employed
in both the Cr-EA and TC/EA techniques. They simplify considerably
the normalization of hydrogen-isotope measurement data to the VSMOW-SLAP
(Vienna Standard Mean Ocean Water-Standard Light Antarctic Precipitation)
scale, and their use improves accuracy of the data by eliminating
evaporative loss and associated isotopic fractionation while handling
water as a bulk sample. The calibration of organic samples, commonly
having high δ2H values, will benefit from the availability
of suitably 2H-enriched reference waters, extending the
VSMOW-SLAP scale above zero.
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measurement qualityδ2 H resultshydrogen chloridetechniquecalibration anchor20 mUrhelium carrier gasreference watersδ2 H valuescaffeine samplescarbon tubemethodhandling problemsHCNreactive elementsHTCHot chromiumtemperature conversionVienna StandardIRMShandling waterbulk sample0.5 mUrδ2 H measurementshydrogen cyanideOrganic SamplesTCanalysissilver tubesElemental Chromium
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