Headspace solid phase microextraction – GC/C-IRMS for δ13CVPDB measurements of mono-aromatic hydrocarbons using EA-IRMS calibration
This work aims at comparing the δ13CVPDB of mono-aromatic hydrocarbons benzene, toluene, ethylbenzene and xylene isomers (BTEX) measured by elemental analyser (EA)-isotope ratio mass spectrometer (IRMS) with the δ13CVPDB measured on the same compounds by headspace solid phase microextraction – GC/C-IRMS (hSPME – GC/C-IRMS) with the final goal of using these compounds as internal standards on the latter system. The EA-IRMS measurements were done using calcium and lithium carbonate isotopic reference materials: NBS19 and L-SVEC for establishing the δ13CVPDB scale. The EA-IRMS measurements with helium dilution of a set of five reference materials (USGS40, USGS41, IAEA-CH-6, IAEA-CH-3 and IAEA-601) show systematic bias of 1‰ relative to their assigned values. This bias due to the dilution mechanism in the used ConfloII interface device could not be avoided. As the selected hydrocarbons: BTEX could not be analysed by EA-IRMS without helium dilution, their δ13CVPDB must be corrected from this observed bias using an external calibration. The CO2 gas calibrated using EA-IRMS without helium dilution, was used as an in-house reference for the δ13CVPDB measurements of the BTEX by the hSPME – GC/C-IRMS system. The comparison made between the δ13CVPDB measured on the same BTEX compounds by EA-IRMS (with external calibration) and by hSPME – GC/C-IRMS techniques showed good agreement.
New Guidelines for δ13C Measurements
Consistency of δ13C measurements can be improved 39−47% by anchoring the δ13C scale with two isotopic reference materials differing substantially in 13C/12C. It is recommended that δ13C values of both organic and inorganic materials be measured and expressed relative to VPDB (Vienna Peedee belemnite) on a scale normalized by assigning consensus values of −46.6‰ to L-SVEC lithium carbonate and +1.95‰ to NBS 19 calcium carbonate. Uncertainties of other reference material values on this scale are improved by factors up to two or more, and the values of some have been notably shifted:  the δ13C of NBS 22 oil is −30.03‰.
Value Assignment and Uncertainty Estimation of Selected Light Stable Isotope Reference Materials: RMs 8543-8545, RMs 8562-8564, and RM 8566 | NIST
We report the technical basis for value assignments of several carbon and oxygen stable isotope reference materials distributed by NIST and the International Atomic Energy Agency (IAEA), and make recommendations regarding laboratory practices and future intercomparison exercises. Assignments and estimates of uncertainty are based on measurement data using one method at NIST and another method at outside collaborating laboratories. All methods are based on isotope ratio mass spectrometry (IRMS) of carbon dioxide: outside collaborators measured three isobaric forms of CO2, whereas NIST measured four forms. This additional capability allowed NIST to assess measurement accuracy, and design an IRMS method that minimized bias from cross-contamination. The international comparison exercise was designed, involving standard chemical preparation and isotopic measurement ─ under strict requirements designed by the IAEA-Working Group for Reference Materials ─ of CO2 derived from carbonates, waters, and pure CO2 reference materials. Our goal was to determine precise δC and δO value assignments that were consistent across the materials and corrected for cross-contamination, and to relate variations in results with specific laboratory practices to guide future intercomparisons. Measurement data from all participants were compiled in a spreadsheet and inspected for consistency. Those data meeting the performance requirements were corrected for cross-contamination through direct measurement of this effect, then processed using internationally accepted procedures to determine standardized δC and δO values. For the carbonate and CO2 materials, the results of this intercomparison provide value assignments shifted slighted towards more depleted compositions with uncertainties improved by factors up to two over the previous assignments. For the water materials, results were more variable yet suggested that the δO value of SLAP on the VSMOW isotope ratio scale is between –55.7 ‰ and –56.2 ‰. When normalized, δO values were consistent with prior determinations. Statistical approaches were used to discern relationships between results and discretionary factors; these relationships are described. To investigate the effects of oxygen isotope compositional assumptions used in data processing, several sets of assumptions were applied. Results are compared and recommendations made regarding the use of these assumptions and the effect on intercomparability of data.