Windowless Open Photoacoustic Helmholtz Cell

Abstract

A phenomenon which is usually considered as the main disadvantage of photoacoustic Helmholtz cells is photoacoustic background signal resulting from the light absorption by the cell windows. Although there are methods which reduce this background signal, e.g. light wavelength modulation or time domain filtering of the photoacoustic response from the sample, the methods are limited to particular applications, and none of them would completely remove the mentioned background signal. The paper presents design of a windowless open photoacoustic Helmholtz cell, and preliminary results of frequency response measurements of such a cell. Removal of the windows eliminated the main source of the photoacoustic background signal. Additionally, the proposed design allows for unrestricted or forced flow of the fluid filling the cell. As a result, the cell can be easily adapted for continuous, real-time photoacoustic measurements. Under proper selection of the cell dimensions, Q-factors of such open photoacoustic Helmholtz resonators are comparable to the Q-factors of corresponding closed Helmholtz cells.

Signal-to-Noise Analysis of the Improved Open Photoacoustic Helmholtz Cell

“…Another kind of a photoacoustic resonant cell is the Helmholtz resonator, which usually has a volume of just a few cubic centimeters. Open cells of this kind were proposed by Diószhegy et al [6] and by Starecki [15], but because of their sensitivity to the external acoustic noise, they are of no practical use in standard environments. To overcome this limitation, a new design of an open photoacoustic cell was proposed by Starecki and Geras [7], as shown in Fig.…”

“…The presented cell is an improved version of the open windowless photoacoustic Helmholtz cell [15]. Its main part is the Helmholtz resonator to which the two acoustic buffers are attached via coaxial ducts of relatively small diameter (denoted as 'internal ducts').…”

“…To amplify the photoacoustic signal, the modulation frequency of the light beam should be adjusted to an acoustic resonance of the cell [4,14,15,19]. Thus, if the resonance frequency is low, the modulation frequency would also be low.…”

“…Furthermore, due to separation of the cavities by a narrow duct, Helmholtz cells allow for good isolation of the microphone from the sample and the light beam. In single-cavity cells, where such isolation does not exist, the incident light beam can result in an increase of the background noise [15,18,20,21]. The mechanism of photoacoustic [7] signal triggering in the Helmholtz resonator is quite different from that in the other kinds of resonance cells (such as those with the standing wave resonance) [14,22]; the sample is illuminated by the light of the frequency in the sample's absorption spectrum.…”

“…The A. Geras (B) Institute of Electronic Systems, Warsaw University of Technology, Nowowiejska 15/19, 00-665 Warsaw, Poland e-mail: antonina@geras.pl problem arises when there is a demand for sensing low concentrations of a substance. In such a case, the photoacoustic signal may even be a few orders of magnitude weaker than the external noise, so its occurrence can seriously affect the measurements [4][5][6]. Definitely, the external acoustic noise should be attenuated or filtered out.…”