Extremely sensitive trace gas analysis with modern photoacoustic spectroscopy V. Koskinen a, * , J. Fonsen a , J. Kauppinen a , I. Kauppinen b a Department of Physics, University of Turku, FIN-20014 Turku, Finland b Gasera Ltd., Tykisto ¨katu 4, FIN-20520 Turku, Finland Available online 19 June 2006 Abstract 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. # 2006 Elsevier B.V. All rights reserved. PACS: 07.07.Df; 82.80.Kq; 82.80.Gk; 07.60.-j Keywords: Photoacoustic spectroscopy; Infrared; Trace gas; Black body radiation 1. Introduction Trace gas detection has widespread applications, for example, in biology, medicine, controlling industrial pro- cesses and monitoring pollutants or toxic gases [1]. The most critical characteristics of the sensing device are: long-term stability, high signal-to-noise ratio (S/N), i.e., high sensitiv- ity, and good selectivity. Low resolution FTIR spectrometers with a broadband IR source are excellent instruments for multi-component gas analysis although in several applications they are not sensitive enough. It is also very difficult to further improve, e.g., the radiation source, the throughput of the optics or the semiconductor IR detectors. Thus, in order to gain significant improvement in the sensitivity, some part(s) of the spectrometer must be changed dramatically. 2. Photoacoustic detection 2.1. Basic photoacoustic spectroscopy Photoacoustic spectroscopy (PAS) is recognized as a sensitive, zero background method for trace gas analysis. A conventional gas measurement setup is shown in Fig. 1. As the IR beam penetrates the sample cell, the absorption of the radiation heats up the gas increasing both the temperature and the pressure. Thus, the IR radiation modulated by a chopper with a certain frequency will create temperature and pressure variations in the sample gas with the same frequency. These variations form a sound wave, which is usually measured with a capacitive microphone. The selectivity is achieved by using an optical filter or a laser with a certain wavelength. Even though almost ppt-level concentrations can be detected with efficient and complicated laser setups, PAS is known to be less sensitive than FTIR spectrometers with semiconductor detectors if a broadband radiation source is used. Fortunately, the sensitivity of the whole photoacoustic system is limited by the pressure sensor, i.e., the microphone. Thus, major enhancements could be achieved by replacing the capacitive microphone with a better pressure sensor. www.elsevier.com/locate/vibspec Vibrational Spectroscopy 42 (2006) 239–242 * Corresponding author. Tel.: +358 2 333 5754; fax: +358 2 333 5070. E-mail address: vesa.koskinen@utu.fi (V. Koskinen). 0924-2031/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2006.05.018