Anodic and Cathodic Pulse Amperometric Detection of Metal Ions Separated by Capillary Electrophoresis

The pulse amperometric detection (PAD) of electroactive metal ions separated in 25-μm-i.d. capillaries has been investigated with a bipolar waveform. The metal ions were detected at Au and Pt 25-μm disk electrodes via both cathodic deposition and anodic stripping. Analyte response was averaged over multiples of 24 ms to minimize 42-Hz noise associated with the detection system; the frequency of the applied waveform was 5−8 Hz. The response of freshly polished electrodes was constant within 30 min of application of the pulse waveform. Comparison with constant-voltage detection showed that the pulsed detection system gave superior stability, and detector stability was consistent within 5% over a 14-h period. Detection limits in cathodic analysis for Tl(I), Pb(II), Zn(II), Ni(II), and Co(II) were 2−3 μmol/L; for Cd(II) and Hg(II), 5−7 μmol/L; for Ag(I), 11 μmol/L; and for Cu(II), 20 μmol/L. The maximum change in response factors was 10% from 50 μmol/L to 1000 μmol/L. For anodic analysis, detection limits for Tl(I), Pb(II), Zn(II), and Cd(II) were 0.2−0.5 μmol/L; and for Co(II) and Ni(II), 0.6−1.0 μmol/L. Response factors were constant to within 10% over the range of 5−500 μmol/L. Sample stacking to enhance detection for metal ions was also studied, with optimal conditions for linear calibration at electrolyte concentrations about 7−8 times less than that used for the electrophoretic separations. Response factors were constant to <±5% over 1.0−100.0 μmol/L, and detection limits were more than 10-fold smaller than those for nonstacking analysis. A comparison of PAD for spiked and real snow samples with atomic absorption spectrometry showed good agreement.