An ultra-compact CO 2 isotope analyzer exclusively based on quantum cascade technology

Summary form only given. The CO 2 isotopes are of great interest for various applications ranging from medical diagnostics to climate research. Quantum cascade laser absorption spectroscopy has shown its potential to measure isotope ratios with very high precision and its capability for long-term remote measurements [1, 2].Here, we report for the first time on an ultra-compact spectrometer design exclusively employing III-V semiconductor technology. The light source is a cw quantum cascade laser (QCL), packaged in an HHL housing and emitting radiation at 2310 cm-1 [3]. For detection, we use a quantum cascade detector (QCD) whose absorption spectrum is optimized for this spectral range [4]. In this configuration, the concentrations of the four most abundant, stable CO2 isotopes can be simultaneously analyzed (see Fig. 1a). The analyzer sensitivity is enhanced by using a newly developed multipass cell consisting of a diamond turned, 80 mm diameter copper cylinder with a toroidal surface carved into the plane of the recirculating light beam. The toroidal surface refocalizes the light at each reflection and keeps the spherical aberration of the beam to a minimum. The cell geometry allows for a small detection volume of 40 ml, while the optical path in the cell can be adjusted from 16 cm to more than 4 m. Using this cell, we have demonstrated 813C and 818O measurements with a precision significantly below 0.1 [5]. The combination of QCL, QCD, and toroidal gas cell results in an optical setup with a remarkably small footprint of 30 x 13 cm2, shown in Fig. 1(b). To complement the setup, we have developed a fast data acquisition and processing system, allowing for 1 kHz real time data rates. We demonstrate the suitability of this compact CO2 isotope sensor for high precision and fast response measurements, as required for medical and environmental applications.