Isotope selective analysis of CO₂with 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 (30(0)1)(III) <-- (000) band of (12)CO(2) around 1.6 microm, the dominating isotope species (12)CO(2), (13)CO(2), and (12)C(18)O(16)O 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 (per thousand) and +/-6 (per thousand) for 3% and 0.7% of CO(2), respectively -- renders this experimental approach a promising analytical concept for isotope-ratio determination of carbon dioxide in soil and breath gas. For a moderate (12)CO(2) line, the pressure dependence of the line profile was characterized in detail, to account for pressure effects on sensitive measurements.

Isotopic resolution of carbon monoxide and carbon dioxide by NIR diode laser spectroscopy

“…9 Furthermore, the DFB diode laser has a larger output power (~ 20 mW) compared to the ECDL (~ 5 mW) and a larger mode-hopping free scan range. The use of the DFB diode laser realizes the simultaneously isotopic resolution of the main three isotopologues of carbon monoxide and carbon dioxide.…”

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

“…During a feasibility study with incubation of two forest and wheat field soils, overall CO 2 concentrations of around 5,400 and 2,600 ppmv were obtained after 24 h in a closed chamber system. 10 Limited resolution prevented the discussion of isotope effects at that time. With an improved experimental concept, time-resolved determination of the overall respired CO 2 is now combined with isotope selective measurements.…”

“…The TDL spectrometer set up has been described in detail elsewhere. 10 Only a brief summary of main features is given here. The spectrometer is based on a temperature stabilized external-cavity-diode-laser (ECDL; TEC 500 Sacher Lasertechnik, Marburg, Germany) with a maximum output power of 8 mW and an emission line width of < 1·10 -4 cm -1 .…”

“…Fig. 1 Schematic depiction of the soil gas measurement unit including a soil chamber with gas reservoir, the TDL spectrometer for isotopic ratio determination, 10 and optode with fibre-optical chemical sensors (FOCS) for permanent CO 2 monitoring in the headspace. 11 gas reservoir FOCS Detection Data processing ECDL Herriott cell soil Gas mixtures (from Messer-Griesheim, Darmstadt, Germany) were used as concentration standards without further preparation.…”

“…%. 10 In this paper we demonstrate the performance of a further improved tunable diode laser (TDL) spectrometer, working around 1.6 µm, concerning simultaneous multi-line detection of carbon dioxide. The TDL spectrometer has been evaluated for isotope-ratio determination of carbon dioxide in soil gas (typically 1.000 -20.000 ppmv CO 2 ).…”