Effect of on-capillary large volume sample stacking on limits of detection in the capillary zone electrophoretic determination of selected drugs, dyes and metal chelates

Abstract

On-capillary large volume sample stacking (LVSS) with hydrodynamic injections of up to 240 s can be used to significantly decrease the limits of detection (LODs) of selected cationic basic drugs, such as clenbuterol, narcotine, flurazepam, codeine and pethidine, by capillary zone electrophoresis (CZE) by up to two orders of magnitude. This technique for these cations requires the use of the electroosmotic flow (EOF) modifier, cetyltrimethylammonium bromide (CTAB), employed at a concentration of 0.002 mol l−1 in a run buffer of 0.05 mol l−1 disodium tetraborate adjusted to pH 2.2 with orthophosphoric acid. In addition, polarity switching has to be employed after stacking and sample solvent removal in order to effect migration to the UV detector set at 210 nm. Similarily, but using visible detection and a run buffer of 0.05 mol l−1 sodium acetate, 0.002 mol l−1 CTAB and 1.0·10−7 mol l−1 of chelating agent 2-(5′-bromo-2-pyridylazo)-5-diethylamino phenol (5-Br-PADAP) at pH 8, cobalt may be determined as its cationic chelate by LVSS. An LOD of 5·10−8 mol l−1 cobalt can be achieved. Anionic species, such as dyes containing SO3− groups, can also be subjected to LVSS using the EOF modifier, diethylenetriamine (DETA) at a concentration of 0.006 mol l−1 in a run electrolyte of 0.05 mol l−1 citric acid (pH 2.23). Using a 30 s hydrodynamic injection and reversed polarity CZE without post-stacking polarity switching, LODs can be decreased by up to 20 times. The anionic chelate of boron with the ligand Azomethine H can be subjected to LVSS using the same EOF modifier and run electrolyte. A decrease in LOD is again found, down to a value of 5·10−8 mol l−1. This method can be applied to the selective determination of boron at realistic concentrations in an artificial river water matrix containing some 20 trace metal ions.