The separation of four toxic metal ions (Cr3+, Pb2+, Hg2+, Ni2+) was achieved by optimizing the composition of the histidine/tartaric acid background electrolyte. An on-column preconcentration technique, viz. field amplified sample injection, was performed to improve the sensitivity. This method afforded an enhancement factor of up to 91 800 fold with the LODs ranging from 0.005 to 2.32 mu g/L, which were well below the maximum contaminant levels set by the United States Environmental Protection Agency. The robustness of this method was demonstrated with its application to the analysis of real samples including tap water, drain water, and reservoir water with recoveries between 90 and 120%.