Selectivity in capillary electrochromatography using native and single isomer anionic cyclodextrin reagents.

Abstract

Separations of naphthalene compounds that differ in position of substitution and type of substituent were accomplished using cyclodextrin distribution capillary electrochromatography. Separation systems composed of running buffers containing mixtures of native neutral and single isomer anionic cyclodextrins (CDs) were employed yielding efficiencies of approximately 200,000 plates/meter. Solute migration rates and relative orders can be readily modified by changing CD types and concentrations. Experiments were performed to determine distribution coefficients between each of the CDs used in these studies and an aqueous running buffer. For this work, naphthalene-CD cavity inclusion is assumed to be the principal mode of interaction. The distribution coefficients for carboxymethyl-beta-cyclodextrin (CM-beta-CD), degree of substitution 1, were 10-70% larger than those for native beta-CD and 75-1800% large than those for gamma-CD. The CM-beta-CD was singly charged and yielded a narrow elution window. Nevertheless, baseline resolution was achieved for several substituted naphthalene compounds using CM-beta-CD in conjunction with beta-CD or gamma-CD. Under certain conditions, the gamma-CD system yielded an elution order that differed from that of the beta-CD system. Heptakis-(2,3-dimethyl-6-sulfato)-beta-CD with its -7 charge produced a much larger elution window. The extensive substitution with sulfonic groups at the truncated bottom of the CD seemed to inhibit inclusion as the distribution coefficients for the naphthalene compounds were generally more than an order of magnitude smaller than those for CM-beta-CD. Moreover, there was evidence that this sulfato-CD interacted with both the capillary wall and neutral beta-CD. This work differs from prior uses of CDs in that relatively complicated mixtures of neutral, achiral compounds are separated using combinations of recently developed single-isomer CDs as running-buffer additives. The single-isomer CDs, as opposed to most highly complex derivatized CD products, facilitate predictions of separation performance for multicomponent samples. In this manner, the ability to use knowledge of distribution coefficients to predict elution characteristics for a ternary CD system is demonstrated.