Separation and sensitive determination of metal ions by capillary zone electrophoresis with 2-(5-bromo-2-pyridylazo)-5-(N-propyl-N-sulfopropylamino)phenol

Capillary electrophoresis of 22 metal ions was examined with a visible light-absorbing chelating agent, 2-(5-bromo-2-pyridylazo)-5-(N-propyl-N-sulfopropylamino)phenol (5-Br-PAPS). The metal chelates separated in a capillary were measured at 550 nm by on-column visible absorption detection. In a standard procedure, carriers (electrophoretic buffers) containing the chelating agent and pH buffer components were used, and sample solutions containing excess amounts of the chelating agent were injected using a vacuum injection method (about 1 × 10–8 dm3). Fourteen metal ions were detected at pH 8.7, and the apparent electrophoretic mobilities (–µep′, 10–4 cm V–1 s–1) were as follows: Ag+, –4.21; Co2+(Co3+), 1.37; VIV, 1.74; Pd2+, 2.36; Mn2+ and Hg2+, 2.43; Ni2+, 2.48; Fe2+, Fe3+ and Cu2+, 2.50; Cd2+, 2.55; Zn2+, 2.58; Pb2+, 2.62; and TiIV, 3.27. By lowering the pH of the carrier, the separation of metal ions was improved, and the apparent electrophoretic mobilities at pH 4.9 were as follows: Ag+, –6.50; Cd2+, –3.78; Zn2+, –2.78; Pb2+, –2.21; VIV, 0.17; Mn2+, 0.48; Hg2+, 0.57; Cu2+, 0.60; Co2+(Co3+), 1.37; VV, 2.01; Ni2+, 2.34; Fe2+, 2.47; and Pd2+, 2.55. At pH 4.9, these metal ions were separated within 7 min by using a fused-silica capillary (50 µm i.d.) of a 50 cm effective length at an applied voltage of 30 kV. The detection limits obtained by the standard procedure at pH 4.9 were as follows (mol dm–3): Fe2+ and Co2+(Co3+), 5 × 10–8; Cu2+, 3 × 10–7; Zn2+, Pb2+ and VIV, 5 × 10–7; and Cd2+, 1 × 10–6. The method was applied to the determination of trace amounts of cobalt contained as impurities in nickel salts.