Fig. 2. Time responses of an array of ten gas sensors: (a) apple samples without pesticide, (b) apple sample with cypermethrin set to 0.2 ppm, (c) apple sample with chlorpyrifos set to 1 ppm, (d) the compound ratio (cypermethrin : chlorpyrifos = 1 : 9) on apples under the maximum residue of the national standard.