Reversed-phase micellar chromatography, efficiency

Hall et al. (127) compared free solution capillary electrophoresis (FSCE) and micellar elec-trokinetic capillary chromatography (MEKC) techniques with HPLC analysis. Four major food-grade antioxidants, propyl gallate (PG), BHA, BHT, and TBHQ, were separated. Resolution of the 4 antioxidants was not successful with FSCE, but was with MEKC. Separation was completed with excellent resolution and efficiency within 6 min and picomole amounts of the antioxidants were detectable using UV absorption. In contrast, reversed-phase HPLC separation was not as efficient and required larger sample amounts and longer separation time. 

A major drawback in the early reports of micellar chromatography was a serious loss of efficiency when compared to traditional hydroorganic mobile phases. If micellar mobile phases are ever to be widely accepted as a viable chromatographic technique, the efficiency achieved must at least approach that of conventional reversed-phase LC. 

There are several appealing factors for the use of micellar supercritical phases in chromatography. The peak efficiencies obtained in SFC are higher than in LC because the solute diffusion coefficients are higher in supercritical fluids than in liquids. In SFC, mass transfers are enhanced by the combination of high diffiision coefficients and low viscosities. This could compensate for the low efficiency induced by micelles. The polar aqueous core of the reverse micelles should allow the separation of hydrophilic or even ionic solutes with supercritical fluids. These polar compounds are difficult to analyze in SFC [10]. 

One major drawback to soap chromatography is the slow rate of mass transfer across the micellar interface. Poor wetting of the reversed-phase support by an aqueous mobile phase also slows mass transfer at the mobile phase/sta-tionary phase interface and, hence, reduces column efficiency. It has been recently shown that adding an appropriate organic modifier - 1-propanol, for example - to the mobile aqueous phase in as low a concentration as 3% greatly enhances the mass transfer and increases column efficiency to a level comparable to that attained with adsorption chromatography (86). 

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