The surface structure of mercury thin films (MTFs) on a glassy carbon electrode (GCE) and the changes of MTFs associated with anodic stripping voltammetry (ASV) were studied using ex situ optical microscopy and linear scan voltammetry. The results of microscopic examinations showed that, in contrary to tbe early observations of large scattered microdroplets on GCE surfaces, the original state of MTFs was close to a thin film, The surface coverage of the mercury microdroplets was continuous microscopically, and the size of the droplets ranged from less than 0.1 pm (unidentifiable under 400X magnification) to about 1 pm, depending on the total amount of mercury on the GCE surface. Under the condition of ASV scanning (2 V/s) with in situ MTFs, the stability of a mercury coating was shown to depend on the total amount of mercury on GCE. It was also shown that the severe noise spikes and the erratic current behavior were associated with the surface structural changes from a state of fine and smooth microdroplets to a state of large scattered microdroplets. The transformation from one state to another is believed to be driven by the surface perturbation of rapid potential changes under the ASV condition.We report here the surface conditions of mercury coatings on a glassy carbon electrode (GCE) and the surface changes associated with anodic stripping voltammetry (ASV). ASV is a sensitive tool for the analyses of trace amounts of heavy metals.'-' Perhaps the most widely used electrode form is the mercury thin film (MTF) on a glassy carbon substrate because of the versatility, the ease of preparation, and the wide potential window for a n a l y s e~.~-~l~ In particular, ASV with in situ formed MTF provides high sensitivity, low detection limit, and high resolution of neighboring peaks, as demonstrated by F l~r e n c e .~ Since their initial usage, the nature of mercury coatings on GCE has been the subject of many s t~d i e s~-~,~,~J~ because of the importance of a stable and reproducible mercury coating to ASV and the interesting properties of mercury thin films as a form of electrode. The model of homogeneous thin film has been used in theoretical treatments9-13 of metal diffusion (1) Vydra, F.; Stulik, K.; Julakova, E. in and out of the mercury coating and in experimental applications in ASV,435 with the film thickness ranging from 1 to 10 nm for in situ formed MTFs4 and from 0.1 to 1 pm for ex situ formed MTFs.~ On the other hand, mercury coatings on the GC surface have been shown, by various optical microscopic and SEM i m g a g e~,~J~J~ to be inhomogeneous with scattered microdroplets. Despite these reports, the nature of mercury coatings is still not at all certain because of the dynamics involved in stripping processes and the inconsistent behavior of ASV with various MTFs. Thus, to what degree these mercury coatings can be considered as thin films remains unanswered.The objectives of the present investigation of mercury coatings on GCE have been to understand the nature of mercury coatings under various conditio...

The dynamics of fast linear scan (LS) ASV for the simultaneous detection 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 ultrathin mercury films (9 nm) at a conventional size (d(0) = 1 mm) electrode. Results of the investigation show that when the thin films were utilized, diffusion of metals through the mercury film was not the rate-limiting step of the stripping process at moderate to fast scan rates (0.5-10 V/s). A fairly linear relationship between the peak height and 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 rates is thus different from what the theoretical model of LS-ASV would have predicted. For the ultrathin mercury films, at least two additional factors, kinetics and concentration, have significant effects on practical LS-ASV. Experimental results show that the stripping process of Cu was primarily kinetic-controlled for fast scans, while those for Cd and Pb were dependent on both scan rates and concentrations. The ultrathin mercury film resulted in a significant enhancement of the ratio of signal-to-baseline slope (i(p)/Δi(b), a ratio used to measure the effectiveness of discrimination of the peak signal against the steep sloping baseline in LS-ASV) for Cd and Pb stripping peaks, but only a slight enhancement for Cu stripping peaks. The optimal performance of LS-ASV in terms of sensitivity, peak width, and enhancement of the i(p)/Δi(b) ratio for the three metals was achieved at 2 V/s. Because of the high reproducibility of the background currents of the stable in situ MTFs, background subtraction was carried out at 2 V/s with little hysteresis. This feature, combined with the enhancement of the i(p)/Δi(b) ratio at the fast scan rate of 2 V/s, allowed for the detection of sub-ppb levels of Cd, Pb, and Cu at a deposition time of 2 min.

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