## 4 Discussion  
The system, which uses near-infrared photoacoustic spectroscopy with a 1532 nm semiconductor laser and a multipass cell, has proven to be efficient for gas detection, especially with regard to the detection of ammonia (NH\({}_{3}\)). The achieved detection limit is approximately 770 ppb, with a normalized noise equivalent absorption coefficient (NNEA) for NH\({}_{3}\) of 1.07 \(\times\) 10\({}^{-8}\) W cm\({}^{-1}\) Hz\({}^{-1}\)/\({}^{2}\). This performance is competitive but also reveals areas for improvement compared to other more advanced approaches. A comparison with other studies reveals that the multipass cell photoacoustic system demonstrates lower performance in terms of sensitivity and detection limits when compared with the results obtained in studies by [PERSON] [2] and [PERSON] et al. [3]. This difference stems from the fact that these systems utilize different technologies and spectral regions, such as quantum cascade lasers (QCL) in the mid-infrared (MIR), and, consequently, achieve lower detection limits and better spectral resolution. For instance, in the study by [PERSON], a spectroscopy system was employed for the detection of H\({}_{2}\)S with high-precision MIR lasers, resulting in detection limits of less than 1 ppb. However, this  
Figure 11: Noise signal for the _2f_-WMS photoacoustic signal acquired for pure N\({}_{2}\) in a range from 84.9 to 103 mA. Figure 10: Photoacoustic signal measured inside the circular multipass cell for different NH\({}_{3}\) concentrations. (**a**) _2f_-WMS signal for the absorption line under study. (**b**) Linear fitting. The pressure is set to 1 atm and temperature to 296 K.  
system is limited by the need to use more complex and expensive devices. The use of the multipass cell in this study has the advantage of increasing the optical path, which improves the absorption efficiency and, therefore, the photoacoustic signal. This approach aligns with the research of [PERSON] and [PERSON] [26], who demonstrated that the MPC significantly enhances the sensitivity in the detection of trace gases by facilitating a greater interaction between the gas and the light. However, a critical point is that, although improvements in the signal are achieved, the system remains sensitive to thermal noise and fluctuations in the laser, which can compromise the stability of long-term measurements. This is a crucial consideration, particularly in the context of continuous monitoring applications. Additionally, a significant challenge posed by such systems is the calibration and stability of the laser, a problem that has been highlighted by [PERSON] et al. [27]. Despite the increase in optical path provided by the cell, ensuring wavelength accuracy and signal linearity remains paramount for the accuracy of measurements. A comparative assessment of techniques such as QEPAS and TDLAS, which use tunable diode lasers, also shows notable differences. In the case of QEPAS, the use of acoustic resonators allows for greater sensitivity and resolution, overcoming the limitations of traditional multipass cells in terms of volume and gas pressure. [PERSON] et al. [23] demonstrated that QEPAS can achieve detection limits in the order of ppb for gases such as CO\({}_{2}\) or NO\({}_{2}\), indicating the potential for further enhancement of photoacoustic based systems with MPCs to achieve comparable detection limits. However, QEPAS systems are more complex, which can limit their practicality for mobile or low-cost applications. The detection limits in parts per billion (ppb) were obtained through primary calibration in the range of 200 to 5000 ppm. This decision was based on several technical factors that allow valid extrapolation to lower concentrations under controlled conditions. Firstly, the spectroscopic techniques employed delineate a linear relationship between the degree of absorption and the target gas concentration, provided that the signal-to-noise ratio is satisfactory.