Metrology for stable isotope reference materials: 13C/12C and 18O/16O isotope ratio value assignment of pure carbon dioxide gas samples on the Vienna PeeDee Belemnite-CO2 scale using dual-inlet mass spectrometry

Abstract

Isotope ratio measurements have been conducted on a series of isotopically distinct pure CO2 gas samples using the technique of dual-inlet isotope ratio mass spectrometry (DI-IRMS). The influence of instrumental parameters, data normalization schemes on the metrological traceability and uncertainty of the sample isotope composition have been characterized. Traceability to the Vienna PeeDee Belemnite(VPDB)-CO2 scale was realized using the pure CO2 isotope reference materials(IRMs) 8562, 8563, and 8564. The uncertainty analyses include contributions associated with the values of iRMs and the repeatability and reproducibility of our measurements. Our DI-IRMS measurement system is demonstrated to have high long-term stability, approaching a precision of 0.001 parts-per-thousand for the 45/44 and 46/44 ion signal ratios. The single- and two-point normalization bias for the iRMs were found to be within their published standard uncertainty values. The values of 13C/12C and 18O/16O isotope ratios are expressed relative to VPDB-CO2 using the δ13CVPDB−CO2 and δ18OVPDB−CO2 notation, respectively, in parts-per-thousand (‰ or per mil). For the samples, value assignments between (−25 to +2) ‰ and (−33 to −1) ‰ with nominal combined standard uncertainties of (0.05, 0.3) ‰ for δ13CVPDB−CO2 and δ18OVPDB−CO2, respectively were obtained. These samples are used as laboratory reference to provide anchor points for value assignment of isotope ratios (with VPDB traceability) to pure CO2 samples. Additionally, they serve as potential parent isotopic source material required for the development of gravimetric based iRMs of CO2 in CO2-free dry air in high pressure gas cylinder packages at desired abundance levels and isotopic composition values.