## 3 Results and Discussion  
### Resistance Change as a Sensor Response  
Figure 4 shows the dynamic resistance response of all sensors to single gases of citronellal (No. 12) and _d_-limonene (No. 6) and double gases of _a_-terpinene (No. 2) and _d_-limonene (No. 6), as examples. All the sensors in sensor array C were n-type semiconductor gas sensors that decreased in resistance in response to the target gases, whereas the two sensors in sensor array L were p-type semiconductor gas sensors that increased in resistance in response to the target gases. Figure 4a-d show the measurements acquired while decreasing the single gas concentration from levels 4 to 1. Because all responses were for the same target gas, the change in the resistance of each gas sensor decreased as the target gas concentration decreased.  
In sensor array C, for TGS2602 and TGS2603, which have particularly large resistance changes as sensor responses, the value of resistance change decreased as the target gas concentration decreased. The other sensors in sensor array C exhibited minimal responses. The sensors included in sensor array C were manufactured for various purposes, as listed in Table 1. Sensors used to monitor fuel systems, such as liqu  
Figure 4: Dynamic resistance responses: sensor array and target gas are (**a**) C and single gas No. 12, (**b**) L and single gas No. 12, (**c**) C and single gas No. 6, (**d**) L and single gas No. 6, (**e**) C and double gases Nos. 2 + 6, and (**f**) L and double gases Nos. 2 + 6, respectively. Concentration levels of the target gases are, in order, (**a–d**) Lvs. 4, 3, 2, and 1; (**e,f**) Lvs. 3 + 3, 3 + 1, 1 + 1, and 1 + 3.  
methane, showed particularly low responses. Furthermore, TGS2602 and TGS2603 showed greater responses to number 6 than to number 12. The concentration ranges of the target gas numbers 1, 4, 10, 11, 12, and 13 were approximately one order of magnitude lower than those of the other target gases, as shown in Table 3. Because they are all polar molecules with alcohol, ester, or aldehyde functional groups, their volatility is reduced by intermolecular forces. Empirically, semiconductor sensors show a high response to gas species that have oxidizable functional groups, but this response is offset by the difference in concentration.  
In sensor array L, because the two LaFeO\({}_{3}\)-based sensors were p-type semiconductors, the resistance changed as the sensor response increased when the target gas was introduced. Responses to number 12 from all sensors in sensor array L were better than those from TGS2602 and TS2603. These sensors were designed by considering conditions such as film thickness, which were optimized in past research. For the ZnO-based sensors, the response to number 12 was better than that of number 6. The other sensors appeared to have approximately the same response to numbers 6 and 12. Therefore, in sensor array L, the influence of the functional groups was greater than that of their concentration.  
Figure 4e,f show the change in the resistance of each sensor against double gases, including numbers 2 and 6. The concentration combinations were levels \(3+3,3+1,1+1\), and \(1+3\), in the order shown in Figure 4e,f. Levels \(3+1\) and \(1+3\) had approximately the same level of resistance change, but the resistance change varied depending on the sensor. These results indicate that the relationship between the magnitude of the response of each sensor to the gas type and concentration was different.