Isotope selective analysis of CO2 with tunable diode laser (TDL) spectroscopy in the NIR

Abstract

The performance of a home-built tunable diode laser (TDL) spectrometer, aimed at multi-line detection of carbon dioxide, has been evaluated and optimized. In the regime of the (30⁰1)_III ← (000) band of ¹²CO₂ around 1.6 µm, the dominating isotope species ¹²CO₂, ¹³CO₂, and ¹²C¹⁸O¹⁶O were detected simultaneously without interference by water vapor. Detection limits in the range of few ppmv were obtained for each species utilizing wavelength modulation (WM) spectroscopy with balanced detection in a long-path absorption cell set-up. High sensitivity in conjunction with high precision —typically ±1‰ and ±6‰ for 3% and 0.7% of CO₂, respectively— renders this experimental approach a promising analytical concept for isotope-ratio determination of carbon dioxide in soil and breath gas. For a moderate ¹²CO₂ line, the pressure dependence of the line profile was characterized in detail, to account for pressure effects on sensitive measurements.