Mixed-mode capillary electrochromatographic separation of anionic analytes

In this work, mixed-mode capillary electrochromatography is introduced as a method for selectivity manipulation in the separation of charged analytes and is investigated for a number of analytes. This concept involves utilisi ng a component of the eluent to permit the chromatographic and capillary el ectrophoresis (CE) separation mechanisms to contribute in varying proportio ns to the separation. This approach was first investigated using a combinat ion of CE with reversed-phase liquid chromatography (RP-LC) for hydrophobic , charged analytes (aliphatic sulfonates), and using the concentration of o rganic modifier in the eluent to control the contributions of CE and RP-LC. However, the use of reversed-phase columns was found to be problematic for mobile phases with less then 50% organic modifier due to the hydrophobicit y of the stationary phase causing the column bed to overheat and dry, and l ow electroosmotic flow (EOF) values (mu less than or equal to 17.8 x 10(-9) m(2) V-1 s(-1)) caused additional restrictions. In a second case, ion-exch ange stationary phases were used, with the type and concentration of a comp eting anion in the eluent being used to control the contributions of ion ch romatography (IC) and CE to the separation. Nine common inorganic anions we re separated using a silica based anion-exchange column and phosphate (pH 7 .20) or sulfate (pH 8.2) as eluent with direct UV detection at 214 nm and 1 7 inorganic and small organic anions were separated using a nitrate eluent (pH 6.80) with indirect UV detection at 214 nm. The separation selectivity was shown to be a combination of IC and CE.