Abstract: A moving neutralization boundary (MNB) was developed as a novel model of visual offline sample stacking for the trace analysis of heavy metal ions (HMIs) by capillary zone electrophoresis (CZE). In the stacking system, the motion direction of MNB to cathode was used with 2.1 mmol/L HCl-98 mmol/L KCl-trace metal ions in the anodic solution and 4.0 mmol/L NaOH-96 mmol/L KCl in the cathodic solution. The voltage was constant at 180 V and the flow rate of the anolyte and catholyte was 1 mL/min. The metal ions in the gel after stacking were detected by capillary electrophoresis. The calibration curves showed good linear relationship (r≥0.9985) in the concentration range used in the experiments. The pre-concentration factors were up to 80~150 and the limits of detection （LODs） were 0.163, 0.256, 0.077, 0.153, 0.203, 0.062 and 0.142 mg/L for Cu(II), Zn(II), Ni(II), Mg(II), Ca(II), Cr(III) and Fe(III), respectively, obviously lower than the national standards. The intra-day and inter-day assay precisions were good (the relative standard deviations (RSDs) less than 7.42%). Finally, the developed method has been successfully used for the stacking and the detection of heavy metal ions in electroplate waste water.

Key words: electroplate waste water, heavy metal ions, moving neutralization boundary, sample stacking, trace analyses, capillary zone electrophoresis （CZE）