Extremely sensitive trace gas analysis with modern photoacoustic spectroscopy

An extremely sensitive approach to detect weak pressure variations has been applied to photoacoustic spectroscopy. A capacitive microphone is replaced with a miniature silicon cantilever, whose displacement is measured with a compact Michelson-type laser interferometer. Major improvements to the sensitivity of photoacoustic detection have been achieved. For example, a sub-ppb detection limit for methane gas has been obtained with a conventional photoacoustic setup in a nonresonant operation mode, using a broadband black body radiator as a source. The new sensing method has also been applied to the detection of carbon dioxide with a distributed feedback diode laser. A noise equivalent sensitivity of 4.6 × 10−9 cm−1 WHz−1/2 was demonstrated. A novel selective differential method, which combines the photoacoustic detection with long path absorption spectroscopy, is described.