Peak compression effects in capillary electrochromatography.

Peak compression in CEC is a phenomenon that can generate very narrow peaks with extremely high efficiencies that defy current chromatographic theory. This review article summarises the content of publications in this area up to this date. Two main types of peak compression effects have been observed in the literature. First, an irreproducible and hard to control focusing effect of unclear origin, observed on strong cation exchangers. Second, a reproducible continuous stacking effect caused by sample composition induced system zones demonstrated on several types of stationary phases.

Improved quantification limits in chiral capillary electrochromatography by peak compression effects.

The peak compression effect has been applied to improve quantification limits in chiral capillary electrochromatography (CEC). A stationary phase based on the chiral selector vancomycin (Chirobiotic V) was used for separations of the enantiomers of mianserin. By adding solvents with a low dielectric constant, e.g. 2-propanol or tetrahydrofuran, to the sample solution, peak compression could be induced. The plate numbers for the minor enantiomer increased from approximately 100,000 to 1.4-1.6 million plates/m, when the composition of the mobile phase was adjusted so that the analyte eluted within either one of two system zones originating from the sample solution. A 10-fold improvement in the quantification limit for the minor enantiomer was obtained compared to elution under non-focused conditions.

Evaluation of generic chiral liquid chromatography screens for pharmaceutical analysis.

Two different automated generic liquid chromatography screens for the separation of chiral compounds of pharmaceutical interest have been evaluated. The test set comprised 53 chemically diverse chiral compounds involving 55 enantiomeric pairs from the pharmaceutical industry (i.e. starting materials, synthetic intermediates and drug substances). The first screen utilised four polysaccharide-based columns with five mobile phases and showed enantioselectivity for 87% of the test compounds. The second screen employed three macrocyclic glycopeptide columns with two mobile phases and showed enantioselectivity for 65% of the test compounds. Merging of the two screening procedures resulted in an enantioselectivity for 96% of the chiral compounds. It is anticipated that the systematic use of the automated chiral HPLC screens described in this report will substantially reduce the necessary time for method development of pharmaceutically related chiral analytical methods.

Peak compression effects in capillary electrochromatography of basic drug substances using a strong cation-exchanger.

Peak compression effects in capillary electrochromatography of basic drug substances using a strong cation-exchanger have been studied. Extremely narrow peaks with apparent efficiencies of several million plates per meter could be obtained when the composition of the sample zone differed from that of the mobile phase. The increased efficiencies were predominately observed when the analyte had an elution time similar to that of the electroosmotic flow marker. Peak compression was found to be reproducible and could be obtained for all investigated basic drug substances by altering the composition of the mobile phase in such a way that the analyte co-eluted with the sample zone. An explanation of the observed phenomena is proposed. A sample zone differing in composition from the mobile phase will disturb the equilibrium between the stationary and mobile phase. The elution rate of an analyte will consequently be different when residing inside the sample zone. If the analyte migrates through the sample zone at a higher speed than the rest of the mobile phase and is strongly retained after passing through a boundary in the sample zone, a continuous stacking can be obtained trapping the analyte as a very narrow band.