# Sensory Perception Systems and Machine Learning Methods for Pesticide Detection in Fruits
## 2 Materials and Methods
### E-Nose  
A multisensory system was developed to detect pesticide residues in fruits. This system consists of a concentration chamber and a measurement chamber. The concentration chamber is constructed from durable methacrylate and has dimensions of 12 cm in length, 12 cm in width, and 12 cm in height, resulting in a total internal volume of 1728 cm\({}^{3}\). The measurement chamber, also made from methacrylate, contains 16 metal oxide gas sensors, specifically Taguchi sensors from the Figaro Company, located in Arlington Heights, IL, USA. This chamber measures 8.7 cm in length, 8.7 cm in width, and 5.3 cm in height, with an internal volume of 401.157 cm\({}^{3}\). The sensors are capable of detecting a wide range of compounds, including ammonia, amines, monochloroodifluoromethane, hydrogen, methane, volatile gases, water vapor in food, hydrogen sulfide, hydrocarbons, carbon monoxide, iso-butane, ethanol, alcohol, organic solvents, and tetrafluoromethane (as detailed in Table 2).  
Figure 2 illustrates two simple diagrams created to explain the measurement process of the E-nose.  
The sampling protocol for the E-nose consists of 3 stages: (a) concentration, (b) measurement, and (c) cleaning. A quantity of organic fruit was taken at first, and with the help of a spray bottle, the sample was contaminated with the pesticide of interest. Subsequently, the sample was placed in the concentration chamber (see Figure 2A) for 5 min to generate a headspace and concentrate the volatile organic compounds (VOCs) emitted by the contaminated fruit. During 5 min of concentration, valves 1 and 2 were closed; however, an electric pump transfers an airflow that passes through valve 3 in parallel to the chamber where the sensors are located. At the end of the first stage, the sample acquisition stage continued for 7 min (see Figure 2B). In this stage, valve 3 was closed, and valves 1 and 2 were opened, keeping the electric pump on, which allows for airflow to pass to the concentration chamber and thus drag the VOCs from the sample to the sensor  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Channel** & **Sensor Type** & **Gas Target** & **Detection Limit** \\ \hline S1 & TGS 826 & Ammonia and amines & 30 ppm \\ \hline S2 & TGS 831 & R-22 (monochloroodifluoromethane) & 10 ppm \\ \hline S3 & TGS 821 & Hydrogen & 50 ppm \\ \hline S4 & TGS 826 & Ammonia and amines & 30 ppm \\ \hline S5 & TGS 842 & Methane and natural gas & 350 ppm \\ \hline S6 & TGS 880 & Volatile gases, water vapor in food & 500 ppm \\ \hline S7 & TGS 825 & Hydrogen sulfide & 1 ppm \\ \hline S8 & TGS 830 & R-22 (monochloroodifluoromethane) & 10 ppm \\ \hline S9 & TGS 800 &
\begin{tabular}{c} Carbon monoxide, methane, \\ iso-butane, hydrogen, ethanol. \\ \end{tabular} & 50 ppm \\ \hline S10 & TGS 880 & Volatile gases, water vapor in food & 10 ppm \\ \hline S11 & TGS 822 & Alcohol and organic solvents & 50 ppm \\ \hline S12 & TGS 821 & Hydrogen & 50 ppm \\ \hline S13 & TGS 832 & R-134  (tetrafluoroethane) & 10 ppm \\ \hline S14 & TGS 842 & Methane and natural gas & 500 ppm \\ \hline S15 & TGS 831 & R-22 (monochloroodifluoromethane) & 10 ppm \\ \hline S16 & TGS 813 & Hydrocarbons in general & 500 ppm \\ \hline \hline \end{tabular}
\end{table}
Table 2: Sensor description of E-nose.  
chamber. For the cleaning stage of the sensor chamber, valves 1 and 2 were closed, and valve 3 was opened again for 5 min to circulate the air through the sensor chamber. This way, the volatile compounds are extracted from the previously acquired sample, avoiding the memory effect in the measurement chamber due to poor cleaning. The total duration of the measurement was 17 min for each sample.