USGS44, a new high-purity calcium carbonate reference material for δ¹³ C measurements

The stable carbon isotopic (δ¹³ C) reference material (RM) LSVEC Li₂ CO₃ has been found to be unsuitable for δ¹³ C standardization work because its δ¹³ C value increases with exposure to atmospheric CO₂ . A new CaCO₃ RM, USGS44, has been prepared to alleviate this situation.

USGS44 was prepared from 8 kg of Merck high-purity CaCO₃ . Two sets of δ¹³ C 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 (δ¹³ C_VPDB  = +1.95 milliurey (mUr) exactly, where mUr = 0.001 = 1‰), and LSVEC Li₂ CO₃ (δ¹³ C_VPDB  = -46.6 mUr exactly), and normalized to the two-anchor δ¹³ C_VPDB-LSVEC isotope-delta scale. The second set of values was obtained by dual-inlet (DI)-IRMS of CO₂ evolved by reaction of H₃ PO₄ with carbonates, corrected for cross contamination, and normalized to the single-anchor δ¹³ C_VPDB scale.

USGS44 is stable and isotopically homogeneous to within 0.02 mUr in 100-μg amounts. It has a δ¹³ C_VPDB-LSVEC value of -42.21 ± 0.05 mUr. Single-anchor δ¹³ C_VPDB values of -42.08 ± 0.01 and -41.99 ± 0.02 mUr were determined by DI-IRMS with corrections for cross contamination.

The new high-purity, well-homogenized calcium carbonate isotopic reference material USGS44 is stable and has a δ¹³ C_VPDB-LSVEC value of -42.21 ± 0.05 mUr for both EA/IRMS and DI-IRMS measurements. As a carbonate relatively depleted in ¹³ C, it is intended for daily use as a secondary isotopic reference material to normalize stable carbon isotope delta measurements to the δ¹³ C_VPDB-LSVEC 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 δ¹⁸ O reference material. A δ¹³ C_VPDB-LSVEC value of -29.99 ± 0.05 mUr was determined for NBS 22 oil.