Dual Comb Spectrometer for the Determination of Stable Isotopic Ratios of Atmospheric CO₂ with Sub-Permille Precision...

We present a mid-infrared dual comb spectrometer for the precise determination of the isotopic ratio of the stable CO₂ isotoplogues ¹²C¹⁶O₂ and ¹³C¹⁶O₂ under atmospheric pressure. The spectrometer is based on electro-optic intensity modulation at 1550 nm wavelength and subsequent wavelength flexible conversion to the mid-infrared. Here, the fundamental absorptions of CO₂ in the ν₃ band around 4.3 μm wavelength (2300 cm^-1) were accessed to achieve the needed sensitivity to investigate the isotopic composition at atmospheric concentrations. The high spectral resolution of 0.004 cm^-1 and spectral coverage of 8 cm^-1 enable the measurements of the three most abundant CO₂ isotopologues ¹²C¹⁶O₂, ¹³C¹⁶O₂ and ¹⁶O¹²C¹⁸O at ambient pressure. The high average signal-to-noise ratio per comb mode of 51 dB and a noise equivalent absorption coefficient of 5.4(9)·10^-6 cm^-1 Hz^-1/2 ensures high precision. After an integration time of 172 s a precision on the stable isotopic ratio (δ¹³C) of <0.1‰ is achieved according to Allan deviation analysis. A linearity analysis on the measured concentrations of the single isotopologues results in coefficients of determination (R²) of 0.999 for CO₂ concentrations ranging from 300 to 450 ppm, whereas for the measurement of δ¹³C-values ranging from -36.5‰ and -5.4‰ a coefficient of determination of 0.998 was achieved. This linear behavior and the high precision on the measurements demonstrate the great potential of the presented dual comb spectrometer for atmospheric research. Especially, where measurements under low pressure must be avoided, the here presented system is a promising alternative to established quantum cascade laser-based systems.