To obtain highest sensitivity for colorimetric detection of capsaicinoids
on the PAD, the ratios of FC and Na2CO3 were also optimized.
The results are shown in Fig. 4 for CS concentration of 200 mg L−1. At
the 1:1 ratio of FC and Na2CO3, yellow color slightly appeared, while
the lowest grayscale intensity value was obtained. Increasing the
volume of Na2CO3 to be higher than FC, the yellow color completely
disappeared. However, the blue color intensity also decreased. With the
volume of FC higher than Na2CO3, the colorimetric reaction was not
completed and the yellow color still appeared. Therefore, the ratio of
FC and Na2CO3 at 1:2 was used for further experiment as the standard
deviation of the grey intensities was also lowest.