The electrokinetic chromatography (EKC) of a novel mixed surfactant system consisting of oppositely charged surfactants, sodium dodecyl sulfate (SDS) and n-dodecyltrimethylammonium bromide (DTAB), was investigated. The chromatographic characteristics of large liposome-like spontaneous vesicles and rodlike mixed micelles formed from the mixture were explored and compared with those of SDS micelles. Separations of a series of n-alkylphenones showed that the spontaneous vesicles provided about a 2 times wider elution window than SDS micelles. Both vesicle and mixed micelle systems were found to provide larger methylene selectivity than SDS. The different elution order of a group of nitrotoluene geometric isomers with DTAB/SDS spontaneous vesicles and SDS micelles pseudostationary phases suggested the possibility of different separation mechanisms with these two systems. Comparisons of polar group selectivity, retention, and efficiency were made between vesicles, mixed micelles, and SDS micelles. The correlation between the logarithms of the retention factors (log k') and octanol-water partition coefficients (log P(ow)) for a group of 20 neutral compounds was also studied with DTAB/SDS vesicles. Spontaneous vesicles have great potential as a pseudostationary phase in electrokinetic chromatography.

We have found that the Haarhoff-Van der Linde (HVL) peak function provides excellent fitting to the shapes of CZE peaks. Initially designed for overloaded peaks in gas chromatography, this function describes a Gaussian peak when there is no peak distortion, and a triangular peak when there is no diffusional peak broadening. As such, it is ideal for CZE peaks distorted by electromigration dispersion (EMD). Fitting peaks with this function gives four parameters: three of them can be related to the Gaussian peak that would have been obtained in case of no EMD; the last one is a measure of the peak distortion. Using moving boundary theory, this peak distortion parameter may readily be expressed in terms of analyte and background electrolyte mobilities and concentrations, electric field, and sample injection length. The variance of an HVL peak is shown to be described by a universal function, and a master equation is presented. The region where EMD adds less than 10% to the Gaussian variance is shown to be very narrowly spread around the mobility matching condition. Under typical CZE operating conditions with an analyte at 1% of the BGE concentration, significant peak distortion is always present. Because the total peak variance is not an addition of the Gaussian and triangular contributions, the HVL model and the methodology introduced here should always be used to correctly combine variances.

The effect of particle size and pore sue of the aminopropylated silica support for cellulose tris(pheny1carbamate) and tris(3,5-dimethylphenylcarbamate) chiral HPLC phases was investigated. It was necessary to reduce phase loading below 20% w/w as pore size and particle size were reduced, but high efficiency columns could be prepared at a 15% w/w loading on 5 and 2.5 pm supports with 12O-~-diameter pores. The 2.5 pm phase permits the use of relatively high flow rates and very efficient enantioselective separations of a range of chiral compounds could be achieved in less than 3 min. o 1994 Wiey-Liss, Inc.KEY WORDS: cellulose, phenylcarbamate, 3,5-dimethylphenylcarbamate, resolution 2.5 pm silicaSilica gel derivatives coated with tris(ary1carbamate)s of polysaccharides represent one of the most useful classes of chiral stationary phases currently available. Developed by Okamoto et al.'" and marketed by Diacel Chem. Ind. as Chiralcel and Chiralpak columns, they are particularly useful for polar racemates and show high enantiomeric resolution power for many drugs with complex structures. [4][5][6] In virtually all published work to date, aminopropylated spherical silica (APS) with a particle diameter of 7 or 10 pm and a pore diameter of 1000 or 4000 A has been used to support these coated polysaccharide carbamates. l4 However, no justification has been presented for the necessity of using such large pore particles of the derivatised silica support and there have been no systematic studies reported of the influence of pore size or particle size on the performance of this type of coated carbohydrate carbamate phase. Our recent work has indicated that reduction in pore size does not, in general, lead to loss of resolution or efficiency for many chiral analytes and may offer advantages in some cases.In this paper we report the results of a study using the aminopropylated form of Hypersil silica, with a mean pore diameter of 120 A and particle diameters of 10, 5, and 2.5 pm, to support two cellulose tris(ary1carbamate) phases. MATERIALS AND METHODSChemicals Aminopropylated Hypersil silica (Shandon Scientific Ltd., UK) which had the following properties was used particle sizes, 10, 5, and 2.5 pm; pore sue, 120 A; surface area, 170-180 m2/g. Racemic mixtures and enantiomers were purchased from Aldrich, UK. Isocyanates were purchased from Fluorochem UK. Solvents (HPLC grade) were obtained from Rathburn UK.0 1994 Wiey-Liss, Inc. EquipmentThe HPLC equipment consisted of a Beckman Gold system containing a 126 Binary Pump, 166 UV Detector and 506Autosampler. Data were acquired using a Waters 860 Expertease package. Dead times (to) were estimated using 1,3,5-tri-tert-butylbenzene.A high pressure slurry packer fitted with a Haskel 780-3 pump was used for column packing. Particle size distributions for column packings were determined on a Mavern Mastersizer X analyser. Preparation of CarbohMdrate CarbamateStationary Phases Cellulose tris(pheny1carbamate) (1) and cellulose tris(3,5-dimethylphenylcarbamate) (2) (Fig. 1) were ...

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