Microemulsion electrokinetic chromatography (MEEKC) has been developed for fingerprint analysis of resina draconis, a substitute for sanguis draconis in the Chinese market. The microemulsion as the running buffer was made up of 3.3% (w/v) sodium dodecyl sulfate (SDS), 6.6% (w/v) n-butanol, 0.8% (w/v) n-octane, and 10 mmol/L sodium tetraborate buffer (pH 9.2), which was also used as the solvent for ultrasonic extraction of both water- and fat-soluble compounds in the traditional Chinese medicine samples. Four batches of resina draconis obtained from different pharmaceutical factories located in different geographic regions were used to establish the electrophoretic fingerprint. MEEKC was performed using a Beckman PACE/MDQ system equipped with a diode-array detector and with monitoring at 280 nm. The fingerprint of resina draconis comprised 27 common peaks within 100 min. The relative standard deviations of the relative migration time of these common peaks were less than 2.1%. Through repetitive injection of the sample solution six times in 24 h, all relative standard deviations of the migration time and peak area of loureirin A and loureirin B were less than 2.5 and 3.8%, which demonstrated that the method had good stability and reproducibility. The relative peak areas of these common peaks in the electropherograms of four batches of resina draconis were processed with two mathematical methods, the correlation coefficient and the interangle cosine, to valuate the similarity. The values of the similarity degree of all samples were more than 0.91, which showed resina draconis samples from different origins were consistent. On the other hand, high-performance liquid chromatography (HPLC) coupled with photodiode-array detection was also applied to establish the fingerprint of resina draconis. The samples were separated with a LiChrospher C(18) column using acetonitrile (solvent A) and water containing 0.1% H(3)PO(4) (solvent B) as the mobile phase in linear gradient elution mode at a flow rate of 0.6 mL/min and detection was at 280 nm. There were only 20 common peaks in the HPLC fingerprint, and the values of the similarity degree of all samples were also more than 0.91. Though the similarity results of fingerprint analysis seemed to be the same, MEEKC resulted in more common peaks and higher separation efficiency for a variety of polarities of the components than HPLC. So, MEEKC was more suitable for development of the fingerprint of resina draconis.
The influences of the composition of microemulsion on the microstructure including dimensions and zeta potentials of microdroplets were measured in details. The average dynamic dimension of microdroplets was measured by dynamic laser light scattering, and zeta potential was determined to characterize average surface charge density of microdroplets. The experiment results showed that increase of the amount of surfactant resulted in decrease of microdroplet size but almost invariant zeta potential, which would enlarge migration time of the microdroplet in MEEKC. With increment of cosurfactant concentration, the microdroplet size had an increasing trend, whereas the zeta potential decreased. Thus, observed migration velocity of microdroplets increased, which made the separation window in MEEKC shortened. Neither dimension nor zeta potential of microdroplets changed by varying both the type and the amount of the oil phase. Adding organic solvent as modifier to microemulsion did not change the microdroplet size, but lowered zeta potential. The migration time of microdroplet still became larger, since EOF slowed down owing to organic solvent in capillary. So, besides increment of surfactant concentration, organic additive could also enlarge the separation window. Increase of cosurfactant concentration was beneficial for separation efficiency thanks to the looser structure of swollen microdroplet, and the peak sharpening might compensate for the resolution and peak capacity owing to a narrow separation window. Except the oil phase, tuning the composition of microemulsion would change the microstructure, eventually could be exploited to optimize the resolution and save analysis time in MEEKC.
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