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Theory

ASV is one of the most sensitive, convenient, and cost effective analytical methods for detection and determination of metal ion contaminants, such as lead, in water whether from rivers, lakes, process streams or drinking sources. With ASV, it is possible to analyze simultaneously very low levels of several metals such as Pb, Cu, Cd and Zn. ASV can detect levels in the range of part-per-million (ppm) or even part-per-billion (ppb) (i.e., ~10-10 M). Example linear sweep ASVs of Pb are shown in Figure1.

ASV involves a deposition step (often called pre-concentration) at an applied negative potential for a specified period of time. The metals, as ions in solution, are concentrated by plating them onto the electrode in their metallic form. Then the electrode is scanned linearly toward positive potentials so that the metals, one at a time, are stripped from the electrode and re-oxidized at a potential characteristic of each metal. The resulting current-voltage peaks can be compared with those in a calibration curve done with standard solutions of known quantities of metal ions. Besides determining which metals are present, the quantity of each metal can be correlated to the peak height of the current or the integrated charge under the peak.

Figure 1. Linear scan ASV of Pb using a thin Hg film electrode on glassy carbon. Experimental conditions: deposition time 2.0 minutes a with 30 second rest period before scanning. Initial potential at -1,000 mV and final potential at +200 mV. Scan rate = 2 V/s with the Hg film co-deposited with Pb during the deposition step. Data files were exported to Excel and plotted with the potential scale set from –200 to –600 mV. If you are unfamiliar with the use of Excel program, please consult the Technical Note on Excel in this eChem Manual.

ASV was originally developed with a hanging Hg drop electrode where the metals often form an amalgam. However, to limit the quantity of toxic Hg used, thin Hg films can be pre-deposited onto an electrode such as glassy carbon or co-deposited simultaneously with the analyte metal ions. The amount of Hg deposited is very little and can be readily removed at potentials less negative than those of the analyte metals, such as Pb, Cd and Cu. References 1-5 include web-based online ASV articles and hardcopy references on the subject.