Potential improvements aimed at high precision δ¹³C isotopic ratio determinations in CO₂ mixtures using optical...

The manuscript explores some advantages and limitations of laser based optical spectroscopy, aimed at achieving robust, high-reproducibility ¹³C¹⁶O₂ and ¹²C¹⁶O₂ ratio determinations on the VPDB-CO₂ δ¹³C scale by measuring the absorbance of line pairs of ¹³C¹⁶O₂ and ¹²C¹⁶O₂. In particular, the sensitivities of spectroscopic lines to both pressure (P) and temperature (T) are discussed. Based on the considerations and estimations presented, a level of reproducibility of the ¹³C¹⁶O₂/¹²C¹⁶O₂ ratio determinations may be achieved of about 10^-6. Thus one may establish an optical spectroscopic measurement technique for robust, high-precision ¹³C¹⁶O₂ and ¹²C¹⁶O₂ ratio measurements aimed at very low uncertainty. (Notably, creating such an optical instrument and developing technical solutions is beyond the scope of this paper.) The total combined uncertainty will also include the uncertainty component(s) related to the accuracy of calibration on the VPDB-CO₂ δ¹³C scale. Addressing high-accuracy calibrations is presently not straightforward - absolute numerical values of ¹³C/¹²C for the VPDB-CO₂ scale are not well known. Traditional stable isotope mass-spectrometry uses calibrations vs CO₂ evolved from the primary carbonate reference materials; which can hardly be used for calibrating commercial optical stable isotope analysers. In contrast to mass-spectrometry, the major advantage of the laser-based spectrometric technique detailed in this paper is its high robustness. Therefore one can introduce a new spectrometric δ¹³C characterisation method which, being once well-calibrated on the VPDB-CO₂ scale, may not require any further (re-)calibrations. This can be used for characterisation of δ¹³C in CO₂-in-air mixtures with high precision and also with high accuracy. If this technique can be realised with the estimated long-term reproducibility (order of 10^-6), it could potentially serve as a more convenient Optical Transfer Standard (OTS), characterising large amounts of CO₂ gas mixtures on the VPDB-CO₂ δ¹³C scale without having to compare to carbonate-evolved CO₂. Furthermore, if the OTS method proves to be successful, it might be considered for re-defining the VPDB-CO₂ δ¹³C-scale as the ratio of selected CO₂ spectroscopic absorbance lines measured at pre-defined T & P conditions. The approach can also be expanded to δ¹⁸O characterisation (using ¹⁶O¹²C¹⁸O and ¹⁶O¹²C¹⁶O absorbance lines) of CO₂ gas mixtures and potentially to other isotope ratios of other gases.