The result showed no obvious change
in retention time at a pH range of 4.1e7.8 (Figure 1A); however,
the peak areas for DCD, CYR, and MEL increased with
decreasing pH from 4.1 to 4.7, and when pH > 4.7, the peak
areas decreased (Figure 1B). The best response for the three
analytes was achieved at pH 4.7, which was used in subsequent
work.