We report on spectroscopic real-time detection of <sup>13</sup>CH<sub>4</sub> in ambient air. Our measurements were carried out by means of cavity leak-out absorption spectroscopy employing a tunable cw laser in the mid-infrared spectral region near <i>λ</i> = 3 <i>μ</i>m. A CO laser in combination with tunable microwave sideband generation was used as the light source. Using a 50-cm-long ringdown cell with <i>R</i> = 99.98% mirrors, we achieved a detection limit of 290 parts in 10<sup>12</sup> (ppt) <sup>13</sup>CH<sub>4</sub> in ambient air (integration time, 100 s). The corresponding noise-equivalent absorption coefficient was 5 × 10<sup>-9</sup>/cm.
© 2000 Optical Society of America
(010.1120) Atmospheric and oceanic optics : Air pollution monitoring
(120.6200) Instrumentation, measurement, and metrology : Spectrometers and spectroscopic instrumentation
(300.6340) Spectroscopy : Spectroscopy, infrared
D. Kleine, H. Dahnke, W. Urban, P. Hering, and M. Mürtz, "Real-time detection of 13CH4 in ambient air by use of mid-infrared cavity leak-out spectroscopy," Opt. Lett. 25, 1606-1608 (2000)