Investigation and optimisation of a multipass resonant photoacoustic cell at high absorption levels

Abstract

A theoretical and experimental investigation of photoacoustic (PA) signals in a resonant multipass PA cell with high background absorption (up to 29 m −1 ) is presented. An analogous electric transmission line model including discontinuity inductances at cross section changes was used to model the first longitudinal acoustic mode of the multipass PA cell equipped with two buffer volumes. This model was validated with experimentally obtained results and used to predict the behaviour of the PA cell for different multipass arrangements and different buffer volume diameters. The highest PA signal is obtained for high pass number and large buffer radius. Increasing the absorption coefficient of the medium enhances the PA signal until a maximum is reached, leading to a minimum for the PA signal sensitivity. For a given background absorption, the number of passes required to maximise the sensitivity depends on the absorption coefficient. The model allows the determination of the best-suited number of passes for a given absorption coefficient and cell geometry.PACS 82.80.Kd; 42.62.Fi; 82.80.Gk IntroductionPhotoacoustic (PA) spectroscopy is a powerful technique for detecting traces of gases due to its intrinsically high sensitivity, large dynamic range, and comparatively simple experimental set-up [1-3]. The detection limit of this technique is mainly determined by the light source used and the PA cell design. The sensitivity of PA cells has been improved by acoustic amplification of the PA signal thanks to the use of resonant PA cells [4][5][6][7][8][9][10][11]. Resonant cells have also been designed for multipass or intracavity operation [12][13][14][15][16]. Cell windows generate synchronous noise which has been minimized by introducing acoustic baffles [17], by the development of a "windowless" [18] or differential cell [19,20].Owing to its high sensitivity, PA spectroscopy is usually applied to the detection of weak absorption signals. In the case of low optical absorption, a uniform heat deposition along the light beam is observed and the generated PA signal depends only on the quantity of absorbed power. Nevertheless, even u Fax: +41-1-633-1230, E-mail: sigrist@iqe.phys.ethz.ch if the low absorption case is frequent, strong background absorption is sometimes present. This is particularly the case for in-situ monitoring where carbon dioxide, ammonia, alcohol or water absorptions can interfere with the species under investigation. At high absorption, the incident light power decreases exponentially in the PA cell and the course of the absorption along the beam path has to be taken into account for modelling the PA signal. Due to the longer absorption path in multipass cells, the exponential radiation decay affects the PA response more strongly than the one of single-pass cells. High-absorption phenomena have been observed previously in pulsed PA spectroscopy [21,22]. These effects are important since they influence both the PA response in presence of a highly absorbing background and the dynamic range of a ...

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