Basic HTML Version
ACT Workshop on Trace Metal Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8
additional analytical separation procedures (e.g., chemical modification and extraction, HPLC,
ion chromatography). Numerous systems are available through a variety of laboratory equipment
vendors.
Electrochemical Techniques
Associated with their compatibility with redox chemistries, metal detection has been a target for
development of electrochemical analytical techniques for over 30 years. Two broad analytical
approaches have been applied to the problem of metal detection in environmental samples:
voltammetric and potentiometric systems. The low power requirements and flexible design with
standard electronic components make electrochemistry a flexible and relatively low cost target for
field deployable metal analysis systems.
Voltammetric Techniques
Monitor the current (i) flowing between an auxiliary electrode (AE) and a working electrode
(WE) resulting from the reduction or oxidation of a metal species at applied potentials (E)
supplied to the WE relative to a reference electrode (RE). Depending on the electrode
composition and configuration, each metal analyte will have unique i vs. E profiles and the
current magnitude is proportional to analyte concentration. Due to the flexibility of this approach,
a range of protocols have been developed based on the potential scan rate format (linear sweep,
cyclic voltammetry, differential pulse or square wave) and WE configuration (macro- vs. stirring
independent microelectrodes (r < 10
F
m)). Direct reduction methods generally have higher
detection limits (>10
-8
M) than adsorptive methods such as ASV (anodic stripping voltammetry)
and AdSV (adsorptive SV), which employ a high WE potential to pre-concentrate the target
metal(s) in the mercury film on the WE surface, thereby lowering the detection limits in
environmental samples (< 10
-12
M). While the inclusion of a metal ion specific ligand in AdSV
techniques can expand the range of metal analyte species targets, this required addition of
exogenous co-analytes will limit its applicability for
in situ
monitoring applications. Electrode
sensor design has also been improved for field applications by refinement of techniques for
fabrication of microWEs, which eliminate dependence on sample stirring. Further improvements
in electrode durability have been achieved by inclusion of agarose gel impregnated surface
microelectrodes (GIME), which help to stabilize the Hg films and reduce sensitivity to
macromolecular fouling components (Buffle and Tercier-Waeber 2005). While microWEs
incorporate small amounts of Hg, acceptance of these systems would be enhanced by further
research on development of Hg-free electrodes.
Field deployable voltammetric analytical systems have been developed for biogeochemical
research applications in extreme environments (Luther et al, 2001; Nuzzio et al, 2002; see G.
Luther presentation). Several voltammetric analytical systems are now commercially available
for on-site metal analysis (GAT TEA 4000 MP,
www.rudolphinst.com/trace_metal_analyzer.html
; Nano-BandTM Explorer,
www.tracedetect.com
) and a single system is available with capacity
for
in situ
trace element profiling and monitoring in aquatic systems with nanomolar sensitivity
(VIP System
www.idronaut.it
). The VIP System incorporates a sensor package for concurrent
measurements of critical ambient WQ parameters of conductivity, temperature and depth (CTD),
dissolved oxygen, pH and redox. Thus, enabling more rigorous interpretation of the metal
speciation data.