Photoacoustic detection of NH3 by means of a differential double resonator cell

A. VALLESPI; SLEZAK V.; A. PEURIOT; F. GONZÁLEZ; A PEREYRA

Varsovia

Congreso; Conference on Photothermal and photoacoustic theory and applications; 2013

Warsaw University of Technology, Faculty of Electronics and Information Technology.

Ammonia is both a natural and man-made compound. It takes part on countless biological processes and has an increasing relevance in environmental studies. The average global NH3 concentration in the atmosphere ranges from 0.3 to 6 ppb with sometimes higher values in the environs of agricultural and industrial areas, ranging from 10 ppb to 90 ppb or many orders higher near livestock areas [1]. It is related to acidulation of water sources and eutrophication. Detecting ammonia traces is relevant in health, manufacturing, and security areas, among others. Since ammonia presents a strong absorption band (the mode) around 10, we propose its detection by means of photoacoustic spectroscopy (PAS) and study some properties with both a pulsed CO2 laser (TEA) and a cw CO2 laser. The laser beam is aimed to an innovative dual resonator differential cell (Fig. 1) designed by means of finite elements method, which lowest resonant frequency is the first longitudinal mode at 1204 Hz. A differential microphone (Knowles NR 23160) is coupled to both resonators [2][3] at the midpoint of the cell. The characteristics of the differential microphones would minimize noise from distant sources, such as ambient noise and local heating of the cell windows. The chosen cell?s material is polypropylene, suitable for reducing the effects of adsorption due to the polarity of the ammonia molecule [4]. In order to take into account error sources during the measurements of low concentrations, physical adsorption-desorption at the cell?s walls is studied by means of the record of the PA signal decay using the TEA CO2 laser. A theoretical model, based on Langmuir?s isotherms, fits well the experimental results. As a result, a 5 % PA signal decay from an enclosed sample of 248 ppmV of NH3 in N2 was registered within 1 hour. Based on this result, minimum errors of the concentrations values are expected from measurements carried out on flowing gas mixtures passing through both resonators. The setup for CW CO2 laser excitation takes advantage of the differential microphone by picking up out of phase signals. For this purpose, a polished chopper wheel was prepared to allow generating the direct and the reflected beam, alternatively aimed to one resonator and the other. The measurements show that for the double resonator configuration a signal increase is achieved as expected from the study of the sensitivity of both resonators separately, which had been previously characterized. The first measurements with this system indicate a limit of detection of around 90 ppbV at 1W, defined as the ratio of one standard deviation of PA signal of N2 to NH3-N2. Figure 1: schematics of the double resonator cell. [1] Agency for Toxic Substances and Disease Registry (ATSDR) [2] R. Bernhardt, G. Santiago, V. Slezak, A. Peuriot, M. González. Sensors and Actuators B 150 (2010) 513?516. [3] A. Schmohl, A. Miklós, and P. Hess. APPLIED OPTICS. Vol. 41, No. 9. 2002. [4] N. Melander and J. Henningsen. AIP Conf. Proc. 463, pp. 78-80; Proceedings of the 10th international conference on photoacoustic and photothermal phenomena, Date: 23-27 Aug 1998.