Heiko Moossen Harro A.J. Meijer Tyler B. Coplen Anita T Aerts-Bijma Lauren T Reid Heiko Geilmann Jurgen Richter Michael Rothe Willi A. Brand Blaza Toman Jacqueline Benefield Jean-Francois Helie Haiping Qi 2021 <h3 id="rcm9006-sec-0001-title" class="article-section__sub-title section1">Rationale</h3><p>The stable carbon isotopic (<i>δ</i>¹³C) reference material (RM) LSVEC Li₂CO₃<span>&nbsp;</span>has been found to be unsuitable for<span>&nbsp;</span><i>δ</i>¹³C standardization work because its<span>&nbsp;</span><i>δ</i>¹³C value increases with exposure to atmospheric CO₂. A new CaCO₃<span>&nbsp;</span>RM, USGS44, has been prepared to alleviate this situation.</p><h3 id="rcm9006-sec-0002-title" class="article-section__sub-title section1">Methods</h3><p>USGS44 was prepared from 8 kg of Merck high-purity CaCO₃. Two sets of<span>&nbsp;</span><i>δ</i>¹³C values of USGS44 were determined. The first set of values was determined by online combustion, continuous-flow (CF) isotope-ratio mass spectrometry (IRMS) of NBS 19 CaCO₃<span>&nbsp;</span>(<i>δ</i>¹³C_VPDB = +1.95 milliurey (mUr) exactly, where mUr = 0.001 = 1‰), and LSVEC Li₂CO₃<span>&nbsp;</span>(<i>δ</i>¹³C_VPDB = −46.6 mUr exactly), and normalized to the two-anchor<span>&nbsp;</span><i>δ</i>¹³C_VPDB-LSVEC<span>&nbsp;</span>isotope-delta scale. The second set of values was obtained by dual-inlet (DI)-IRMS of CO₂<span>&nbsp;</span>evolved by reaction of H₃PO₄<span>&nbsp;</span>with carbonates, corrected for cross contamination, and normalized to the single-anchor<span>&nbsp;</span><i>δ</i>¹³C_VPDB<span>&nbsp;</span>scale.</p><h3 id="rcm9006-sec-0003-title" class="article-section__sub-title section1">Results</h3><p>USGS44 is stable and isotopically homogeneous to within 0.02 mUr in 100-μg amounts. It has a<span>&nbsp;</span><i>δ</i>¹³C_VPDB-LSVEC<span>&nbsp;</span>value of −42.21 ± 0.05 mUr. Single-anchor<span>&nbsp;</span><i>δ</i>¹³C_VPDB<span>&nbsp;</span>values of −42.08 ± 0.01 and −41.99 ± 0.02 mUr were determined by DI-IRMS with corrections for cross contamination.</p><h3 id="rcm9006-sec-0004-title" class="article-section__sub-title section1">Conclusions</h3><p>The new high-purity, well-homogenized calcium carbonate isotopic reference material USGS44 is stable and has a<span>&nbsp;</span><i>δ</i>¹³C_VPDB-LSVEC<span>&nbsp;</span>value of −42.21 ± 0.05 mUr for both EA/IRMS and DI-IRMS measurements. As a carbonate relatively depleted in<span>&nbsp;</span>¹³C, it is intended for daily use as a secondary isotopic reference material to normalize stable carbon isotope delta measurements to the<span>&nbsp;</span><i>δ</i>¹³C_VPDB-LSVEC<span>&nbsp;</span>scale. It is useful in quantifying drift with time, determining mass-dependent isotopic fractionation (linearity correction), and adjusting isotope-ratio-scale contraction. Due to its fine grain size (smaller than 63 μm), it is not suitable as a<span>&nbsp;</span><i>δ</i>¹⁸O reference material. A<span>&nbsp;</span><i>δ</i>¹³C_VPDB-LSVEC<span>&nbsp;</span>value of −29.99 ± 0.05 mUr was determined for NBS 22 oil.</p> application/pdf 10.1002/rcm.9006 en Wiley USGS44, a new high-purity calcium carbonate reference material for δ13C measurements article