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 ...
