# Ammonia Gas Sensors: Comparison of Solid-State and Optical Methods
## 3 Optical Methods
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Sensing** & **Detection** & **Radiation** & **Wavelength** & **Other Parameter** & **Reference** \\
**Methods** & **Limit** & **Source** & & & & \\ \hline NDIR & 1 ppm & Deuterium & Filter 200–225 nm & \(\lambda_{\text{center}}\) = 205 nm & [44] \\  & & lamp & FWHM = 10 nm & [44] \\ MUPASS-WMS & 7 ppb & QCL & 1103.44 cm\({}^{-1}\) & Eff. path & [62] \\ CRDS-cw & 1.3 ppb & QCL-DFB & 10.33 μm & R = 0.9995, & [63] \\  & & & & L = 50 cm & \\ CRDS & 0.74 ppb & QCL & 6.2 μm & L = 50 cm, & [64] \\  & & & & p = 115 Torr & \\ CEAS & 15 ppb & QCL-DFB & L = 53 cm, & [65] \\  & & [pulsed] & 967.35 cm\({}^{-1}\) & L = 51.0 ns & \\ PAS & 0.7 ppb & EC-QCL & 10.36 μm & p = 220 Torr & [66] \\ QEPAS & \textless{}10 ppb & QCL & 10.6 μm & detection in & [67] \\  & & & & & \\ \end{tabular}
\end{table} 
Table 2: Parameters of some optical ammonia sensors. Figure 12: Idea of photoacoustic spectroscopy. diabetes, and oral cavity disease [71; 72]. The main problem in the construction of ammonia sensors for breath analysis does not consist in extreme sensitivity but in high selectivity. More than 3000 various constituents were already detected in exhaled air [73]. Their absorption spectra might overlap the spectral fingerprint of the ammonia and disturb the measurement. Water vapor and carbon dioxide are the main interferences since their concentrations can reach up to 5% in breath, and thus it exceeds the ammonia density by many orders of magnitude. Carbon monoxide and methane should also be taken into account. As it was mentioned above, the highest sensitivities of ammonia detection using laser absorption spectroscopy can be achieved in UV and in a range of 10-11 \(\upmu\)m, due to the largest values of the absorption cross-sections. However, the detection at the wavelengths used in telecommunication 1.4-1.6 \(\upmu\)m is more convenient because of a large variety of relatively cheap laser sources, optical elements, and photodetectors. The measurements of ammonia around 1.51 \(\upmu\)m with the detection limit of 4 ppm was already demonstrated using the CEAS technique [74]. We found that suitable wavelengths for ammonia detection are also the lines at 1.5270005 \(\upmu\)m and 1.5270409 \(\upmu\)m (Figure 13). The single-mode laser wavelength was controlled with the HighPrecision lambdameter (model WS6) ensuring precision in a short time (1 min) of 0.0005 nm. The NH\({}_{3}\) absorption coefficient (at 2 ppm) reaches here about 3.5-10-6 cm\({}^{-1}\), and experimental techniques like MUPASS-WMS can be effectively used. In this range, carbon dioxide and water vapor interference is about eight times weaker. Our ammonia sensor used the MUPASS-WMS approach (Figure 9). A single-mode diode laser (Toptica, model DL100, 20 mW) was applied as a light source. Output signals were integrated over 1 min. Results of sensor investigation are presented in Figure 14. It provides proper results for NH\({}_{3}\) concentrations of up to about 1 ppm. For lower values, the deviation from a linear characteristic is observed. This is probably a result of poor regulation of reference concentration inside the sensor due to NH\({}_{3}\) deposit on the walls of the system, which causes a systematic error of measured data. In order to avoid the concentration measurement disturbed by NH\({}_{3}\) molecules adsorbed on the walls (and then desorbed), the sensor was kept at a temperature of 50 \({}^{\circ}\)C.  
Nevertheless, due to the proper choice of spectral lines, we achieved a good immunity of the detection against H\({}_{2}\)O and CO\({}_{2}\) at the concentrations which might occur in breath (Figure 13). The detection limit of 1 ppm was better than in other experiments preformed at a spectral range of 1.5 \(\upmu\)m [74]. Our multi-pass sensor is suitable for rough monitoring of ammonia in the exhaled air and for detection of morbid states (>1.5 ppm). Figure 13: Absorption spectra of ammonia, H\({}_{2}\)O, and CO\({}_{2}\) around 1.527 \(\upmu\)m [75].