# Competitive Detection of Volatile Compounds from Food Degradation by a Zinc Oxide Sensor
## 3 Results
### Sensing Experiments
\begin{table}
\begin{tabular}{c c c c c} \hline \multirow{2}{*}{**Vapor Mixture**} & \multicolumn{4}{c}{**Response (R/R)**} \\ \cline{2-5}  & **Composition (w\%)** & **C = 5 ppm** & **C = 10 ppm** & **C = 50 ppm** \\ \hline Hexanal/1-pentanol & 98/2 & 1.10 & 1.69 & 3.90 \\  & 75/25 & 1.20 & 1.53 & 2.74 \\ Hexanal/1-ocen-3-ol & 98/2 & 1.55 & 2.25 & 5.50 \\  & 75/25 & 1.11 & 1.71 & 4.50 \\ Hexanal /1-pentanol/1-ocen-3-ol & 96/2/2 & 1.10 & 1.50 & 2.85 \\ \hline \end{tabular}
\end{table} 
Table 4: Sensor’s response to binary and ternary mixtures. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Chemical** & **Tb** & **P\({}^{\circ}\) (293 K)** & **P\({}^{\circ}\) (258 K)** \\
**Compound** & **[K]** & **[kPa]** & **[kPa]** \\ \hline Hexanal & 402 & 2 & \(1.1\times 10^{-1}\) \\
1-pentanol & 411 & \(5\times 10^{-1}\) & \(3.7\times 10^{-2}\) \\
1-ocen-3-ol & 447 & \(3\times 10^{-2}\) & \(3.1\times 10^{-2}\) \\ \hline \end{tabular}
\end{table} 
Table 3: Boiling temperatures and vapor pressures of pure compounds. of alcohol, as also evident in the comparison shown in Figure 3a. When exposed to the mixture 98/2, the sensor was more sensitive to 1-penthanol at low concentrations (5 and 10 ppm) and only when the compounds' concentration increased to 50 ppm did hexanal detection become predominant (Figure 3a). In the presence of a higher concentration of 1-pentanol, mixture 75/25, the dominant effect of alcohol was confirmed as the detection of the hexanal was almost completely inhibited and the trend of the signal was similar to the case of the pure compound (Figure 3a). For a more complete characterization of the mixture 98/2, Figure 3b shows the sensor's resistance as a function of time at C = 5 and 50 ppm. At low concentrations, the transient response follows the 1-pentanol trend while at 50 ppm, the resistance's variation is similar to that of pure hexanal, in agreement with the results shown in Figure 3a. The sensor's response to hexanal was only evident at the highest concentration. The effect of 1-octen-3-ol in combination with hexanal is shown in Figure 4 together with the response to the pure components. The ZnO sensor was significantly influenced by the presence of secondary unsaturated alcohol in both mixtures, 98/2 and 75/25, as evident from the data shown in Table 4 plotted in Figure 4a. The sensor signal, R\({}_{0}\)/R, is affected by the alcohol even in the mixture 98/2 at all concentrations, from 5 to 50 ppm (Figure 4a). At the highest concentration, the response was even more intense than the sum of the two signals. The transient response shown in Figure 4b confirms that the sensor is mainly influenced by the 1-octen-3-ol traces at all concentrations, although after increasing the concentration of the mixture 98/2 to C = 50 ppm, the sensor's resistance showed a decreasing rate regarding the detection, which is similar to the response to pure hexanal, which was much faster than in the presence of pure 1-octen-3-ol. Figure 3: (**a**) Sensor’s response to hexanal/1-pentanol mixtures (data from Table 4) and (**b**) transient response to mixture 98/2 (hexanal/1-pentanol) at 5 (**top**) and 50 ppm (**bottom**). Finally, the ternary mixture with the composition 96/2/2 was used to study the ZnO sensor in an environment similar to the condition that develops when monitoring food degradation. Figure 5 shows the ZnO response to the different concentrations in dry air compared to the response recorded in the detection of hexanal vapors only. Figure 4: (**a**) Sensor’s response to hexanal/1-octen-3-ol mixtures (data from Table 4) and (**b**) transient response to mixture 98/2 (hexanal/1-octen-3-ol mixtures) at 50 (**top**) and 50 ppm (**bottom**). Figure 5: (**a**) Sensor’s response to the ternary mixture 96/2/2 (data from Table 4) and (**b**) the transient response of the ZnO sensor to the ternary mixture 96/2/2 for 5 (**top**) and 50 ppm (**bottom**).