A study of the effects of injection mode, on-capillary stacking and off-line concentration on the capillary electrophoresis limits of detection for four structural types of industrial dyes

The limits of detection of four structural types of industrial dyes - Malachite Green (MG), Indigo Carmine (IC), Cibacron Blue 3GA (CB) and Remazol Black B (RB)- by capillary zone electrophoresis (CZE) were investigated by both hydrodynamic and electrokinetic injection. The LODs obtained using a 30 s hydrodynamic injection were compared to those achieved using a 30 s + 10 kV electrokinetic injection and a 30 s -10 kV injection. Two of the anionic dyes IC and RB were studied using a 0.02 mol dm(-3) sodium tetraborate (pH 9.26) run buffer, and a 0.05 mol dm(-3) citric acid (pH 2.23) run buffer was used for the anionic CB and the cationic MG. The LOD for MG, IC and RE using 30 s hydrodynamic injection was found to be 5 x 10(-7) mol dm(-3) and 5 x 10(-6) mol dm(-3) for CB. By applying a + 10 kV injection potential in the electrokinetic mode the LOD for MG was decreased to 5 x 10(-8) mol dm(-3), whilst a negative injection potential (-10 kV) decreased the LOD of CB to 1 x 10(-6) mol dm(-3) and RB to 1 x 10(-9) mol dm(-3). Electrokinetic injection for IC gave a much higher LOD of 1 x 10(-5) mol dm(-3) for both injection potentials. Reproducibilities, calibration plots and efficiencies for the four dyes were compared for both modes of injection. Optimum separation of a mixture of the four dyes was performed using a run buffer of 0.02 mol dm(-3) sodium tetraborate buffer (pH 9.26) with 10% acetonitrile, showing adequate resolution of monopositively charged, di-, tri-and tetra-negatively charged species. LOD improvement for hydrodynamic injections was observed by resort to on-capillary stacking using multiple 30 s injections with sample solvent removal and stacking in the presence of the electroosmotic flow modifier diethylenetriamine (DETA). The dye IC had its LOD improved by this technique from 5 x 10(-7) mol dm(-3) to 5 x 10(-8) mol dm(-3). An off-line method for LOD improvement using concentration on a SAX Bond Elut cartridge (strong anion exchanging quaternary amine) was also effective for the environmentally significant hydrolysed forms of RE and three other Remazol dyes in that LOD values for large volume samples (1 dm(3)) were lowered 20-50 fold when compared with LODs using on-capillary stacking with a run electrolyte of 0.05 mol dm(-3) citric acid, pH 2.23 with 10% acetonitrile and 0.006 mol dm(-3) DETA using 30 s hydrodynamic injections in the reverse polarity mode.