## 5 Conclusions  
The results obtained with the photoacoustic system, based on a 1532 nm semiconductor laser and a multipass cell, demonstrated an effective capacity for gas detection, particularly ammonia (NH\({}_{3}\)). The minimum detection limit of 770 ppb was achieved, with a standard deviation of noise of 5.36 \(\times\) 10\({}^{-7}\) V and a noise-normalized equivalent absorption coefficient (NNEA) of 1.07 \(\times\) 10\({}^{-8}\) W cm\({}^{-1}\) Hz\({}^{-1}\)/2. These values were determined under controlled conditions, using a cell with pure N\({}_{2}\) and statistical analysis of the noise recorded over 60 s. While this performance is competitive with certain optical detection methods, it falls short of more advanced techniques, such as those using mid-infrared (MIR) quantum cascade lasers, which can achieve sub-ppb detection limits. MPC-PAS offers several significant advantages over other gas detection devices. Firstly, the circular multipass geometry allows the effective optical path to be substantially increased without increasing the volume of the cell, which improves the sensitivity of the system by amplifying the amount of energy absorbed by the gas. This, when combined with photoacoustic detection, which directly converts optical absorption into a proportional acoustic signal, allows very low detection limits (even in the ppb range) to be achieved with high spectral specificity, in contrast to NIR or fiber optic sensors, which generally have lower spectral resolution and are more prone to background interference. In addition, in comparison with conventional sensors that rely on chemical reactions or electrical properties, photoacoustic spectroscopy is not dependent on the degradation of reactive materials. This results in enhanced long-term stability and a reduced requirement for frequent recalibration [28; 35]. Notwithstanding, the proposed system exhibits distinct advantages in terms of simplicity, cost, and adaptability for industrial and field monitoring applications. Whilst QCL- or QEPAS-based technologies offer superior sensitivity, they also involve a greater degree of complexity and cost, which limits their use in practical environments. The enhancement of the system through the implementation of a multipass cell has been demonstrated to be a successful strategy for achieving sensitivity enhancement without the necessity for highly sophisticated devices. Furthermore, the results suggest that a future transition to the mid-infrared region, combined with miniaturization and laser stabilization techniques, could significantly increase the performance of portable photoacoustic systems. This, in turn, could enable new applications in real-time environmental monitoring and industrial emissions control. Conceptualization, O.E.B.-M. and J.V.P. ; methodology, O.E.B.-M.; software, A.P.G.-B. and J.V.P. ; validation, O.E.B.-M. and J.V.P. ; data curation, J.V.P., A.P.G.-B., and G.D.; writing--original draft preparation, O.E.B.-M.; writing--review and editing, O.E.B.-M., J.V.P., and G.D.; visualization, A.P.G.-B. ; supervision, O.E.B.-M.; funding acquisition, O.E.B.-M. and G.D. All authors have read and agreed to the published version of the manuscript. Funding: The first and fourth authors are supported by the Grants for research activity of young PhD holders, part of the Universidad Carlos III de Madrid (UC3M) Own Research Program (Ayudas para la Actividad Investigadora de los Jovenes Doctores, del Programa Propio de Investigacion de la UC3M - Proyectos Jovenes PPIT2024). Furthermore, they disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work has been supported by the Madrid Government (Comunidad de Madrid--Spain) under the Multiannual Agreement with the UC3M (IA_aCTRI-CM-UC3M). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Dataset available on request from the authors.