FQs existed as cations in acidic pH, as zwitterions in neutral pH,
or as anions in basic pH (Chen & Li, 2013). The influence of different
pH values in sample matrix on signals intensities had also been
investigated. The subject preliminary research showed that an
alkaline sample matrix was beneficial to the FASS. Therefore, several
sample matrixes differing in pH values (in the range from
9.1 to 11.0) were tested. As shown in Fig. 3B, the most significant
changes in signals intensities of the analytes occurred when the
pH value of sample matrix was 10.0. The reason was as follows:
NaOH was added to increase pH and the ionic strength of the
sample resulting in improving matching the conductivity ratio
between the BGS and the sample with pH increasing from 9.1 to
10.0. The efficiency of FASS increasing was observed. However,
with further pH increasing from 10.0 to 11.0, the mismatch
between the local electroosmotic velocities in the sample and in
the BGE generated laminar flows and inducing the broadening of
the stacked zone (Burgi & Chien, 1991).
For FASS, the concentrating effect basically relied on the change
in electrophoretic velocity when the analyte molecules reach the
interface between the high-resistance sample solution or water
zone and low-resistance background solution zone. The higher difference
in resistance or conductance, the greater concentrating
effect was obtained (Dziomba et al., 2012). Tris solution with low
conductance was chosen as the sample matrix. Our further study
showed that the best result was obtained by using 300 mM Tris,
which could maintain the pH of sample matrix at 10.0 without
other pH regulators.