# Advancements and Prospects of Electronic Nose in Various Applications: A Comprehensive Review
## 3 Application Areas
### Environmental Monitoring
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Gas Target** & **Type of Sensor** & **Description and Results** & **Reference** \\ \hline \multirow{7}{*}{CO, NO\({}_{2}\), CH\({}_{4}\)} & The objective of the research was to establish the most suitable & \\  & & duration for substances and sensors to interact. To achieve this, & \\  & & MOX sensors underwent exposure to different concentrations of & \\  & & nitrogen oxide, methane, and carbon monoxide, spanning the range & \\  & & of 500 ppb to 2000 ppm. It was observed that after approximately & \\  & & half an hour, the responses of the sensors stabilized when exposed & \\  & & to air mixtures containing these compounds. This 30 min time & \\  & & frame was subsequently adopted for additional examinations & \\  & & stoke-on-Trent, UK) & \\  & & & into consideration variables such as humidity and temperature. \\  & & & The investigation underlined the impact of these variables on & \\  & & sensor reactions and their role in enhancing the electronic nose’s ability to discern and quantify substances. Under conditions of & \\  & & unchanging temperature and humidity, the system adepthy & \\  & & distinguished between compounds with a high level of precision. & \\ \hline \multirow{7}{*}{Acetone, chloroform, methanol} & \multirow{7}{*}{QCM sensors (NDK Ltd., Tokyo, Japan)} & Employing an e-nose fitted with QCM sensors, the investigation & \\  & & aimed to scrutinize gas mixtures encompassing acetone, & \\  & & chloroform, and methanol, exhibiting concentrations ranging from & \\  & & 4000 to 10,000 ppm. The primary objective was to evaluate the & \\  & & system’s capacity to differentiate among distinct compounds while & \\  & & ascertaining their respective concentrations through principal & \\  & & component analysis (PCA). The outcomes showcased the electronic & \\  & & nose’s prowess in discerning mixtures comprising a sole compound & \\  & & versus binary combinations, with marked fluctuations in sensor & responses as the levels of these chemical compounds fluctuated. \\  & & Nevertheless, the study shed light on the influence of humidity & \\  & & levels on the findings, emphasizing the necessity to consider & \\  & & humidity variations in outdoor settings to minimize interference & \\  & & and ensure dependable results. & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Overview of studies on electronic nose applications for gas analysis in the environment. [PERSON] et al. conducted a comprehensive study investigating ambient air composition in proximity to a gas and oil pretreatment plant located southwest of COVA (see Figure 4), utilizing a cutting-edge electronic nose equipped with 10 metal oxide semiconductor (MOX) sensors [122]. The research spanned three months, involving the continuous outdoor deployment of the electronic nose to oversee air quality [122]. As depicted in Figure 4, a map detailing the monitored site offers a visual depiction of the geographical context of the investigation. The red dot signifies the precise placement of the deployed electronic nose, while the adjacent yellow rectangle outlines the position of the gas and oil pretreatment plant. To provide a broader geographical context, an inset in the upper right corner, indicated by an arrow, signifies the site's location within Italy. This spatial insight provides a comprehensive overview of the monitoring setup's relation to the surrounding area. Employing advanced data analysis techniques, such as the self-organizing map (SOM) algorithm and k-means clustering, [PERSON] proficiently processed the extensive e-nose dataset, successfully uncovering anomalous data clusters that pointed to dynamic multisensory system responses ([PERSON], Year).