Photoacoustic detection of plant emitted isoprene using a quantum cascade laser with external cavity, resolving the 𝜈27 and 𝜈28 bands

Isoprene plays an important role as an agent to protect plants from abiotic stress. It is emitted from leaves at ppb levels of concentration, requiring high sensitivity. A photoacoustic (PA) spectrometer using a pulsed external-cavity quantum cascade laser (QCL) was developed for isoprene detection. The QCL provides a tuning range of 150 cm-1 covering the and modes, close to 900 cm-1. The system achieved a sensitivity of 348 V/ppb, a noise-equivalent concentration of 0.83 ppb, and a detection limit of 2.5 ppb. The normalized noise-equivalent absorption coefficient was W cm. Allan deviation analysis showed white-noise behavior without significant drift. The measured spectrum aligned well with absorption cross-sections from the HITRAN database. Validation was performed by measuring isoprene emitted by Quercus robur leaves and comparing it with gas chromatography (GC). The results demonstrate compact, selective, high-sensitivity performance of the QCL-based PA spectrometer and can be applied to various research fields, such as environmental research, atmospheric chemistry research and breath-gas analysis.