Combination of flow injection with capillary electrophoresis. Part 6.: A bias-free sample introduction system based on electroosmotic-flow traction

Abstract

A bias-free sample injection method based on electroosmotic-flow (EOF) traction was developed for the combined flow injection-capillary electrophoresis system. A Nafion joint implemented at a capillary fracture close to the capillary inlet was used for isolating the electric field during the split-flow sample injection, the sample solution being drawn into the capillary by EOF without bias effects. An electronic time relay system was adopted to control the switching of high voltage between the capillary inlet end and the joint. The peak area ratios of two model compounds, dimethyl sulfoxide and benzoic acid obtained using the proposed system were identical to those with gravity injection. Relative standard deviations of 0.5–2.5% ( n = 32) were achieved for peak height and peak area evaluation in the consecutive injections of a series of replicate sample solutions at a sampling frequency of 15 h −1 . Similar performance of the system was obtained with a reversed EOF background electrolyte after reversing the electrode polarity for a mixture of the anions, Cl −, SO 4 2−, NO 3 − and F − using indirect UV detection. The performance of the system for the analysis of real samples was demonstrated by bias-free sample injection in the separation of caffeine, theobromine and theophylline in beverage samples with satisfactory recoveries in the 92–107% range.