Unambiguous detection of the clumped carbon dioxide isotopologue 13 C 16 O 18 O with isotope ratio mass spectrometry is difficult due to isobaric interference on m/z = 47. We present an analytical technique based on direct absorption laser spectroscopy for precise, direct and simultaneous detection of all isotopologues involved in the isotope exchange reaction 12 C 16 O 2 + 13 C 16 O 18 O K ← − → 12 C 16 O 18 O + 13 C 16 O 2 and of 12 C 16 O 17 O. The achieved precision of 2 × 10 −5 for the 13 C 16 O 18 O/ 13 C 16 O 2 and 12 C 16 O 18 O/ 12 C 16 O 2 isotopologue ratios allows to determine the equilibrium constant K of the isotope exchange reaction with an external reproducibility of better than 5 × 10 −5 (1σ) after 9 reference-sample comparisons, each comparison requires 7 minutes. The isotopic composition of the pure gas can be simultaneously analysed with a precision of 0.05% (1σ) for δ 13 C and δ 18 O, and 0.15% (1σ) for δ 17 O. The instrument deploys two interband cascade lasers (ICL) with center wavelengths of 4.3 µm and 4.4 µm. A custom built 1 optical cell is designed for single pass and multi pass optical paths (pathlength ratio ≈ 1:100), it allows simultaneous detection of rare and abundant isotopologues. The set up is capable to analyse pure CO 2 samples of ∼100 µmol.

Simultaneous analysis of carbon dioxide isotopologues involved in the isotope exchange between the doubly substituted 13C16O18O molecule and 12C16O2 has become an exciting new tool for geochemical, atmospheric and paleoclimatic research with applications ranging from stratospheric chemistry to carbonate-based geothermometry studies. Full exploitation of this isotope proxy and thermometer is limited due to time consuming and costly analysis using mass spectrometric instrumentation. Here, we present an all optical clumped CO2 isotopologue thermometer with capability for rapid analysis and simplified sample preparation. The current development also provides the option for analysis of additional multiply-substituted isotopologues, such as 12C18O2. Since the instrument unambiguously measures all isotopologues of the 12C16O2 + 13C16O18O  13C16O2 + 12C16O18O exchange, its equilibrium constant and the corresponding temperature are measured directly. Being essentially independent of the isotope composition of the calibration gas, an uncalibrated working reference is sufficient and usage of international calibration standards is obsolete. Other isotopologues and molecules can be accessed using the methodology, opening up new avenues in isotope research. Here we demonstrate the high-precision performance of the instrument with first gas temperature measurements of carbon dioxide samples from geothermal sources.

Intramolecular or position-specific carbon isotope a n a l y s i s o f p r o p a n e ( 1 3 C H 3 − 1 2 C H 2 − 1 2 C H 3 a n d 12 CH 3 − 13 CH 2 − 12 CH 3 ) provides unique insights into its formation mechanism and temperature history. The unambiguous detection of such carbon isotopic distributions with currently established methods is challenging due to the complexity of the technique and the tedious sample preparation. We present a direct and nondestructive analytical technique to quantify the two singly substituted, terminal ( 13 C t ) and central ( 13 C c ), propane isotopomers, based on quantum cascade laser absorption spectroscopy. The required spectral information on the propane isotopomers was first obtained using a high-resolution Fourier-transform infrared (FTIR) spectrometer and then used to select suitable mid-infrared regions with minimal spectral interference to obtain the optimum sensitivity and selectivity. We then measured high-resolution spectra around 1384 cm −1 of both singly substituted isotopomers by mid-IR quantum cascade laser absorption spectroscopy using a Stirling-cooled segmented circular multipass cell (SC-MPC). The spectra of the pure propane isotopomers were acquired at both 300 and 155 K and served as spectral templates to quantify samples with different levels of 13 C at the central (c) and terminal (t) positions. A prerequisite for the precision using this reference template fitting method is a good match of amount fraction and pressure between the sample and templates. For samples at natural abundance, we achieved a precision of 0.33 ‰ for δ 13 C t and 0.73 ‰ for δ 13 C c values within 100 s integration time. This is the first demonstration of sitespecific high-precision measurements of isotopically substituted non-methane hydrocarbons using laser absorption spectroscopy. The versatility of this analytical approach may open up new opportunities for the study of isotopic distribution of other organic compounds.