Near-infrared (NIR) absorption spectroscopy with tunable diode lasers allows the simultaneous detection of the three
most important isotopologues of carbon dioxide (<sup>12</sup>CO<sub>2</sub>, <sup>13</sup>CO<sub>2</sub>, <sup>12</sup>C<sup>18</sup>O<sup>16</sup>O) and carbon monoxide (<sup>12</sup>CO,<sup>13</sup>CO, <sup>12</sup>C<sup>18</sup>O).
The flexible and compact fiber-optic tunable diode laser absorption spectrometer (TDLAS) allows selective
measurements of CO<sub>2</sub> and CO with high isotopic resolution without sample preparation since there is no interference
with water vapour. For each species, linear calibration plots with a dynamic range of four orders of magnitude and
detection limits (LOD) in the range of a few ppm were obtained utilizing wavelength modulation spectroscopy (WMS)
with balanced detection in a Herriott-type multipass cell. The high performance of the apparatus is illustrated by fill-evacuation-
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 (30<sup>0</sup>1)<sub>III</sub>;←(000) band of <sup>12</sup>CO<sub>2</sub> around 1.6 μm, the dominating isotope species <sup>12</sup>CO<sub>2</sub>, <sup>13</sup>CO<sub>2</sub>, and<sup>12</sup>C<sup>18</sup>O<sup>16</sup>O were detected simultaneously. In contrast to most established techniques, selective measurements are performed without any sample preparation. This is possible since the CO<sub>2</sub> 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 CO<sub>2</sub> 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 CO<sub>2</sub>, 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.