# Ammonia Gas Sensors: Comparison of Solid-State and Optical Methods
## 3 Optical Methods  
Most of the optical approaches to ammonia detection are based on the effects of absorption and luminescence, more rarely on the refraction or the light reflection. In an optical absorption technique, the measured gas is contained in the sensor chamber. The radiation passing through the chamber can be absorbed by gas molecules. Measuring the light absorption at the specific wavelength (\(\lambda\)), with possibly no other gas species absorbing in this spectral range, the concentration (\(N\)) of ammonia can be determined. The concentration estimation follows the Beer-Lambert absorption law  
\[\alpha(\lambda)L=\sigma(\lambda)NL=ln\left(\frac{I_{0}}{I}\right) \tag{1}\]  
where \(\alpha(\lambda)\) is the absorption coefficient defined as the logarithmic ratio of the incident (\(I_{0}\)) and the transmitted (\(I\)) light intensities, \(L\) denotes the light path length in the chamber, and \(\sigma(\lambda)\) denotes the absorption cross section. Ammonia exhibits UV and IR absorption bands around 135 nm, 155 nm, 195 nm, 1.5 \(\upmu\)m, 2 \(\upmu\)m, 3 \(\upmu\)m, 4 \(\upmu\)m, 6 \(\upmu\)m, around 11 \(\upmu\)m, and 16 \(\upmu\)m (Figure 3). The cross-section (\(\sigma\)), defining the system sensitivity, is the highest in the UV region. It is about 10-times larger than at the 11 \(\upmu\)m region (e.g., \(\sim\)10\({}^{-18}\) cm\({}^{2}\)) and even 10\({}^{5}\)-times larger than in other NIR ranges [44]. Figure 3: Absorption cross-section of ammonia for different temperatures in UV [45; 46] (**a**). Absorption cross-section of selected molecules existing in standard conditions (1 atm, 288.2 K) calculated on the basis of the HITRAN database in IR range (**b**). Figure 2: The schematic of the colorimetric optoelectronic ammonia sensor [43]. Unfortunately, in the UV and NIR range, there is a problem with interferences by water vapor and N\({}_{2}\)O or NO\({}_{2}\) molecules, which occur at high concentrations in air. Their absorption bands might overlap with NH\({}_{3}\) spectra disturbing the measurements. Therefore, the selection of proper spectral range is crucial for successful optical detection. Nondispersive infrared (NDIR) sensing belongs to simplest approaches of NH\({}_{3}\) detection. Figure 4 shows a schematic diagram of such a gas sensor. It usually consists of a broadband source (cheaper and smaller black body emitters or IR LEDs), absorption cell, optical filters, and detectors. Radiation from the broadband source passes through the chamber and two filters. The first filter covers the absorption band of the target gas (named active channel), while the other covers a non-absorbed spectral range (the reference channel). Transmission bands of the filters should not overlap with absorption bands of the other gases present in the chamber. Thus, absorption in the active channel is proportional to NH\({}_{3}\) concentration. The light transmitted through the reference channel is not attenuated. Therefore, it is used to compensate instabilities of the light source. The sensitivity of NDIR is influenced by the intensity of the source, the optical waveguide and detector parameters. The disadvantage of this technique is the low precision of the detecting small signal changes at an eventual large background, which results in low selectivity and a high detection limit. There are various methods to improve the performance of NDIR sensors. For example, Max-IR Labs utilizes the NDIR technique together with fiber-optic evanescent wave spectroscopy (FEWS) for ammonia detection [47]. The IR radiation is transmitted through a silver-halide (AgClClxBr1-x) optical waveguide without cladding and the detection performed by means of the evanescent field (Figure 5). The maximum of the peak due to ammonia absorption was observed at 1450 cm\({}^{-1}\). Such a sensor allows NH\({}_{3}\) detection beyond a 1 ppm limit. Laser spectroscopy is the best choice for trace gas analysis among optical approaches. It is characterized by high sensitivity and selectivity.