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 mm, 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

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... 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 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. † Present address: