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

Separations of naphthalene compounds that differ in position of substitutio n and type of substituent were accomplished using cyclodextrin distribution capillary electrochromatography. Separation systems composed of running bu ffers containing mixtures of native neutral and single isomer anionic cyclo dextrins (CDs) were employed yielding efficiencies of approximately 200 000 plates/meter. Solute migration rates and relative orders can be readily mo dified by changing CD types and concentrations, Experiments were performed to determine distribution coefficients between each of the CDs used in thes e studies and an aqueous running buffer. For this work, naphthalene-CD cavi ty inclusion is assumed to be the principal mode of interaction. The distri bution coefficients for carboxymethyl-beta-cyclodextrin (CM-beta-CD), degre e of substitution 1, were 10-70% larger than those for native beta-CD and 7 5-1800% larger than those for gamma-CD. The CM-beta-CD was singly charged a nd yielded a narrow elution window, Nevertheless, baseline resolution was a chieved for several substituted naphthalene compounds using CM-beta-CD in c onjunction 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 syst em. Heptakis-(2,3-dimethyl-6-sulfato)-beta-CD with its -7 charge produced a much larger elution window. The extensive substitution with sulfonic group s at the truncated bottom of the CD seemed to inhibit inclusion as the dist ribution coefficients for the naphthalene compounds were generally more tha n 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 a nd neutral beta-CD. This work differs from prior uses of CDs in that relati vely complicated mixtures of neutral, achiral compounds are separated using combinations of recently developed single-isomer CDs as running-buffer add itives. The single-isomer CDs, as opposed to most highly complex derivatize d CD products, facilitate predictions of separation performance for multico mponent samples. In this manner, the ability to use knowledge of distributi on coefficients to predict elution characteristics for a ternary CD system is demonstrated.