A sensitivity comparison of three photoacoustic cells containing a single microphone, a differential dual microphone or a cantilever pressure sensor

摘要

Three photoacoustic (PA) cells designed for trace-gas sensing were compared by measuring absorption by the P(15) rotational line of the v_1+v_3 vibrational combination band of acetylene using a distributed-feedback diode laser. Normalised sensitivities were determined for each cell by analysis of the signal to noise ratios of acquired spectra. The first cell was constructed in-house, and contained a single electret microphone held in a stainless steel tube. The second cell was a differential PA cell that contained two microphones housed in identical flow tubes, with one microphone to detect the PA signal and the other to determine background noise levels. The third cell contained a novel cantilever pressure sensor, movement of which was measured by a compact laser interferometer. Normalised sensitivities (2σ)of 3.1 x 10~(-7), 1.7 x 10~(-7) and 2.2 x 10~(-9) cm~(-1) WHz~(-1/2), respectively, were obtained. An erbium-doped fibre amplifier was used to amplify the laser power, and a detection limit of 9.8 x 10~(10) molecule cm~(-3) was obtained using the cantilever pressure sensor PA cell, with a laser power of 1.17 W. This detection limit corresponded to a mixing ratio of 14.5 parts per billion by volume at 277 mbar.