An on-line preconcentration capillary electrophoresis (CE) methodology, which combines a normal stacking
mode (NSM) with a dynamic pH junction technique, is introduced in this paper. A systematic study of the parameters
(concentration and pH value of background electrolyte, composition of sample matrix, injection pressure and time, separation
voltage and temperature) affecting on-line concentration of seven related phenolic compounds was investigated and
optimized. Under the optimum focusing conditions, about 54-fold improvement in the detection sensitivity was obtained
compared with usual hydrodynamic sample injection (0.5 psi, 5 s) without detriment in separation efficiency.
In particular, the concentration limits of detection (LOD) (S/N = 3) for the phenolic compounds obtained after preconcentration
were from 6.0 to 11.0 ng mL-1 with UV detection without any pretreatment procedure. The proposed method has
been validated with RSD values between 1.13% and 1.98% for migration times and between 1.28% and 4.65% for peak
areas. Developed NSM-dynamic pH junction method was applied for determination of related phenolic compounds in labeled
M. officinalis medicinal herbal products commercialized in our country.