Versatile multipass cell for laser spectroscopic trace gas analysis

The design, construction and characterization of a novel circular multipass cell for sensitive trace gas analysis are presented. This cell allows for easy modification of the optical path length without any changes of its physical parameters. Furthermore, it is suited for three different detection techniques: direct absorption, wavelength modulation and photoacoustics. To demonstrate its performance, mixing ratios of (CO2)-C-13 and N2O were measured from ambient air, using a quantum cascade laser. With the direct absorption method, noise equivalent 1-s precisions of 2.7 ppb and 0.2 ppb are achieved for (CO2)-C-13 and N2O, respectively. The wavelength modulation technique resulted in 4.3 ppb precision with 1-s averaging for the (CO2)-C-13 measurements. A Q-factor of 190 and a normalized noise equivalent minimum absorption of 1.3x10(-8) cm(-1) W Hz(-1/2) are achieved using the photoacoustic technique.