NIR-diode laser spectroscopy for isotope-selective sensing of soil-respired carbon dioxide

Abstract

The performance of a home-built tunable diode laser (TDL) spectrometer has been optimized regarding multi-line detection of carbon dioxide in natural gases. In the regime of the (30⁰1)_III;←(000) band of ¹²CO₂ around 1.6 μm, the dominating isotope species ¹²CO₂, ¹³CO₂, and¹²C¹⁸O¹⁶O were detected simultaneously. In contrast to most established techniques, selective measurements are performed without any sample preparation. This is possible since the CO₂ detection is free of interference from water, ubiquitous in natural gases. Detection limits in the range of a few ppmv were obtained for each species utilizing wavelength modulation (WM) spectroscopy with balanced detection in a long-path absorption cell set-up. Linear calibration plots cover a dynamic range of four orders of magnitude, allowing for quantitative CO₂ detection in various samples, like soil and breath gas. High isotopic resolution enables the excellent selectivity, sensitivity, and stability of the chosen analytical concept. The obtained isotopic resolution of typically ± 1.0 % and ± 1.5 % (for 3 vol. % and 0.7 vol. % of CO₂, respectively) offers a promising analytical tool for isotope-ratio determination of carbon dioxide in soil gas. Preliminary experiments on soil respiration for the first time combine the on-line quantification of the overall carbon dioxide content with an optode sensor and isotopic determination (TDL system) of natural gas species.