Determination of three curcuminoids in Curcuma longa by microemulsion electrokinetic chromatography with protective effects on the analytes

Alkaline degradation of curcuminoids was observed when they were exposed to alkaline solution, which usually occurs during capillary electrophoresis (CE) analysis with alkaline buffers. In the present study, the stability of curcuminoids (curcumin, demethoxycurcumin, bis-demethoxycurcumin) in different solutions (a boric acid buffer, a microemulsion with n-octane as the oil phase and a boric acid buffer, and a microemulsion with an ionic liquid [BMIM]PF6 as the oil phase and a boric acid buffer) was investigated. The results indicated that the microemulsion systems have a significant protective effect on the analytes, even under alkaline conditions. Compared to the boric acid buffer only, the microemulsion with an ionic liquid showed the best protective effect. Therefore, a stable and rapid microemulsion electrokinetic chromatography (MEEKC) method, with an ionic liquid as the oil phase, was developed for the simultaneous determination of the three curcuminoids in the rhizome (Jianghuang) and tuberous root (Yujin) of Curcuma longa. The experimental parameters, including the microemulsion compositions (surfactant, co-surfactant and oil phase), pH, concentration of borate buffer and capillary temperature, were intensively investigated. Finally, the investigated curcuminoids were well separated within 9 min, using a running buffer containing 100 mM SDS, 1.6 M n-butanol and 20 mM [BMIM]PF6 in 10 mM borate buffer at pH 10.2. The developed method was fully validated (using LOD, LOQ, intra- and inter-day precision and recovery) and successfully applied to determine the content of the investigated curcuminoids in four real samples (Yujin and Jianghuang from Chengdu and Leshan). All of the results showed that MEEKC with an ionic liquid as the oil phase should be an ideal choice for the analysis of curcuminoids by CE.