A rapid high‐precision analytical method for triple oxygen isotope analysis of CO2 gas using tunable infrared laser direct absorption spectroscopy

Abstract

RationaleThe simultaneous analysis of the three stable isotopes of oxygen—triple oxygen isotope analysis—has become an important analytical technique in natural sciences. Determination of the abundance of the rare 17O isotope in CO2 gas using magnetic sector isotope ratio mass spectrometry is complicated by the isobaric interference of 17O by 13C (13C16O16O and 12C16O17O, both have mass 45 amu). A number of analytical techniques have been used to measure the 17O/16O ratio of CO2 gas. They either are time consuming and technically challenging or have limited precision. A rapid and precise alternative to the available analytical methods is desirable.MethodsWe present the results of triple oxygen isotope analyses using an Aerodyne tunable infrared laser direct absorption spectroscopy (TILDAS) CO2 analyzer configured for 16O, 17O, and 18O combined with a custom gas inlet system. We evaluate the sensitivity of our results to a number of parameters. CO2 samples with a wide range of δ18O values (from −9.28‰ to 39.56‰) were measured and compared to results using the well‐established fluorination‐gas source mass spectrometry method.ResultsThe TILDAS system has a precision (standard error, 2σ) of better than ±0.03‰ for δ18O and ±10 per meg for Δ′17O values, equivalent to the precision of previous analytical methods. Samples as small as 3 μmol CO2 (equivalent to 300 μg CaCO3) can be analyzed with a total analysis time of ~30 min.ConclusionsWe have successfully developed an analytical technique for the simultaneous determination of the δ17O and δ18O values of CO2 gas. The precision is equal to or better than that of existing techniques, with no additional chemical treatments required. Analysis time is rapid, and the system is easily automated so that large numbers of samples can be analyzed with minimal effort.