Potential improvements aimed at high precision δC isotopic ratio determinations in CO mixtures using optical absorption spectrometry.

The manuscript explores some advantages and limitations of laser based optical spectroscopy, aimed at achieving robust, high-reproducibility CO and CO ratio determinations on the VPDB-CO δC scale by measuring the absorbance of line pairs of CO and CO. 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 CO/CO ratio determinations may be achieved of about 10. Thus one may establish an optical spectroscopic measurement technique for robust, high-precision CO and CO 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-CO δC scale. Addressing high-accuracy calibrations is presently not straightforward - absolute numerical values of C/C for the VPDB-CO scale are not well known. Traditional stable isotope mass-spectrometry uses calibrations vs CO 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 δC characterisation method which, being once well-calibrated on the VPDB-CO scale, may not require any further (re-)calibrations. This can be used for characterisation of δC in CO-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), it could potentially serve as a more convenient Optical Transfer Standard (OTS), characterising large amounts of CO gas mixtures on the VPDB-CO δC scale without having to compare to carbonate-evolved CO. Furthermore, if the OTS method proves to be successful, it might be considered for re-defining the VPDB-CO δC-scale as the ratio of selected CO spectroscopic absorbance lines measured at pre-defined T & P conditions. The approach can also be expanded to δO characterisation (using OCO and OCO absorbance lines) of CO gas mixtures and potentially to other isotope ratios of other gases.