Influence of organic-modifier concentration on plate number in micellar electrokinetic chromatography. Acetonitrile

Summary

A quantitative theory of plate numberN in micellar electrokinetic chromatography shows that analytes are dispersed by longitudinal diffusion and instrumental contributions in buffers containing sodium dodecylsulfate (SDS) surfactant and 0–10%v/v acetonitrile. TheN's of weakly retained neutral and negatively charged analytes decreased slightly as acetonitrile concentration, or [ACN], increased; with increasing [ACN], these analytes diffused at similar or slightly faster rates and eluted at slightly later times. TheN's of strongly retained analytes decreased rapidly as [ACN] increased; with increasing [ACN], these analytes diffused faster and eluted at much later times. The later elution time affectedN more than the faster diffusion. The strongly retained analytes eluted at later times in ACN-containing buffers because the micellar electrophoretic mobility decreased less rapidly with increasing [ACN] than did the electroosmosis, enabling the micelles to migrate upstream with increasing success. This difference of migration rate caused the micelles and highly solubilized analytes to drift slowly toward the detector. Although conductivity measurements showed that SDS micelles were more charged in ACN's presence than in its absence, the micellar electrophoretic mobility varied inversely with buffer viscosity. In contrast, the electro-osmosis decreased more rapidly with increasing [ACN] than could be explained by only changes of buffer viscosity.