Lead(II) nitrate was obtained from Aldrich (Germany). Cadmium(II) sulfate was obtained from Baker Analyzed (USA). Copper(II) sulphate was obtained from BDH (England). Standard stock solutions of all analytes were freshly prepared each day. The running buffer solution (BGE) for separation was prepared from 2-morpholinoethanesulfonic acid (MES), (s)-2-amiono-3-(4-imidazyl) propionic acid (l-histidine) which were obtained from Fluka (USA). Sodium hydroxide and Hydrochloric acid were purchased from Merck (Germany), and were used for pH adjustment of the electrolytic buffer solution. All reagents were used without further purification. Pure deionized water (Millipore, USA) was used to prepare all aqueous solutions. The sample solutions were prepared by diluting corresponding stock solutions in running buffer. All solutions for electrophoresis experiments were filtered through a 0.45