New photoacoustic cell design for solid samples

We have developed a new design for a photoacoustic (PA) cell particularly suited for quartz-enhanced photoacoustic spectroscopy (QEPAS), where a quartz tuning fork (QTF) is used as a sound detector for the PA signal. The cell is designed for the investigation of solid and semi-solid samples and represents a unilateral open cylinder. The antinode of the sound pressure of the fundamental longitudinal mode of the half-open cylinder occurs directly at the sample, where a measurement is difficult. Therefore, the first harmonic is used. A small hole in the resonator wall at the location of the pressure antinode allows signal detection outside the cylinder without (or only minimally) changing the resonance conditions. This design is particularly simple and easy to manufacture. A finite element (FE) simulation is applied to determine the optimal cell length for the given frequency and the location of the pressure maximum. One difficulty is that the open end dramatically changes the acoustic sound field. We answer the following research questions: where is the sound pressure maximum located and do simple analytical equations agree with the results of the FE simulation?