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

## TL;DR

A new dual comb spectrometer can measure CO2 isotopic ratios with high precision at atmospheric pressure.

## Contribution

A mid-infrared dual comb spectrometer achieves sub-permille precision for CO2 isotopic ratios at atmospheric pressure.

## Key findings

- The spectrometer achieves <0.1‰ precision on δ13C isotopic ratios after 172 seconds of integration.
- Measurements show linearity with R² values of 0.999 for CO2 concentrations and 0.998 for δ13C values.
- The system offers a high signal-to-noise ratio and noise equivalent absorption coefficient for precise measurements.

## Abstract

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 (R
2) 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.