Photoacoustic spectroscopy (PAS) is a measuring principle that uses the interaction of matter and electromagnetic radiation. We developed a PAS sensor particularly suited for the investigation of solid and semi-solid samples. A quantum cascade laser (QCL) excites molecules in a pulsed way, thus generating an acoustic signal. The specific concentration can be derived from the level of the signal. Often acoustic resonances of the sample cell are exploited to improve the SNR of the PAS measurements.

In this investigation, a new cell is designed in the form of a half-open cylinder. One end is tightly sealed by the sample and the opening on the other side is intended for the entry of the laser beam. This design is particularly useful for solid and semi-solid samples, e.g. in blood glucose detection [1].

If the development of a standing wave in the cylinder is considered, the maximum of the sound pressure of the fundamental vibration will occur directly at the sample where a measurement is difficult. Therefore, the first harmonic is to be used and a small hole is placed in the wall of the resonator at the location of the pressure maximum. A sound detector can then be placed close to the hole outside the cylinder and is therefore not (or only minimally) changing the resonance conditions. A finite element simulation confirms the pressure distribution.

[1] KAYSIR, Md Rejvi, et al. Progress and Perspectives of Mid-Infrared Photoacoustic Spectroscopy for Non-Invasive Glucose Detection. Biosensors, 2023, 13. Jg., Nr. 7, S. 716.