DYNAMICS AND PERFORMANCE OF FAST LINEAR SCAN ANODIC-STRIPPING VOLTAMMETRY OF CD, PB, AND CU USING IN SITU-GENERATED ULTRATHIN MERCURY FILMS

The dynamics of fast linear scan (LS) ASV for the simultaneous detecti on of Cd, Pb, and Cu was investigated at various scan rates (0.5-10 V/ s) and at different metal ion concentrations (50-800 nM) utilizing ult rathin mercury films (9 nm) at a conventional size (d(0) = 1 mm) elect rode. Results of the investigation show that when the thin films were utilized, diffusion of metals through the mercury film was not the rat e-limiting step of the stripping process at moderate to fast scan rate s (0.5-10 V/s). A fairly linear relationship between the peak height a nd scan rate was observed at scan rates (0.5-10 V/s) beyond the upper limit of the theoretical model for the behavior of LS-ASV. In addition , peak width at half-height (b(1/2)) as low as 33 mV was achieved at 0 .5 V/s. The behavior of LS-ASV in terms of peak width at these scan ra tes is thus different from what the theoretical model of LS-ASV would have predicted. For the ultrathin mercury films, at least two addition al factors, kinetics and concentration, have significant effects on pr actical LS-ASV. Experimental results show that the stripping process o f Cu was primarily kinetic-controlled for fast scans, while those for Cd and Pb were dependent on both scan rates and concentrations. The ul trathin mercury film resulted in a significant enhancement of the rati o of signal-to-baseline slope (i(p)/Delta i(b), a ratio used to measur e the effectiveness of discrimination of the peak signal against the s teep sloping baseline in LS-ASV) for Cd and Pb stripping peaks, but on ly a slight enhancement for Cu stripping peaks. The optimal performanc e of LS-ASV in terms of sensitivity, peak width, and enhancement of th e i(p)/Delta i(b) ratio for the three metals was achieved at 2 V/s. Be cause of the high reproducibility of the background currents of the st able in situ MTFs, background subtraction was carried out at 2 V/s wit h little hysteresis. This feature, combined with the enhancement of th e i(p)/Delta i(b) ratio at the fast scan rate of 2 V/s, allowed for th e detection of sub-ppb levels of Cd, Pb, and Cu at a deposition time o f 2 min.