Approaching a million-fold sensitivity increase in capillary electrophoresis with direct ultraviolet detection: Cation-selective exhaustive injectionand sweeping

A novel method that combines two on-line concentration techniques in capill ary electrophoresis (CE), namely, sample stacking with electrokinetic injec tion (field-enhanced sample injection, FESI) and sweeping, afforded the det ection of positively chargeable analytes in parts per trillion (ppt) levels . The main idea is to selectively introduce by FESI as many molecules of ca tionic analytes as possible from a very dilute sample solution and focus th e resulting zone by sweeping. Limit of detection values (signal-to-noise ra tio 3) of 4.1 and 8.0 ppt-the lowest concentration reported by direct UV de tection in CE-with average plate numbers of 3.6 x 10(5) and 4.4 x 10(5) are obtained for laudanosine and naphthylamine (standard solutions), respectiv ely. This translates to improvements in peak heights compared with usual in jection approaching a million-fold. Optimization schemes and application to quantitative and qualitative analyses are also investigated.