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The implementation of such metals criteria is not straightforward due to the site-specific nature
of metals toxicity in aquatic environments. Compounding the challenges to metal monitoring
imposed by their inherent chemical properties are the plethora of monitoring goals (i.e., total
maximum daily loads (TMDLs), Maximum Daily Effluent Limits (MDEL) wastewater effluent
monitoring, compliance and ambient monitoring), many of which are expressed as site-specific
objectives, criteria (total recoverable versus total dissolved), sampling constraints (clean
technique, volume and frequency) and ex situ analytical turnaround. Furthermore regional
variation in metal contaminant loads in underlying sediments may contribute to continued WQ
degradation even when direct discharges have been mitigated. While powerful analytical tools
are available for high precision quantification of metal ion content in environmental samples (e.g.,
graphite furnace atomic absorption spectroscopy, GFAA; inductively coupled plasma mass
spectroscopy, ICP-MS; atomic emission spectrometry, AES; X-ray spectroscopy, neutron
activation analysis) their utility in monitoring programs is ultimately constrained by the nature of
ex-situ sampling and potential for sample alteration and contamination during handling. While
these established technologies offer the advantage of high precision analysis of an array of
elements with parts per trillion detection limits, they suffer the critical limitations of high
operational costs, low portability and only provide measurement of total metal concentration in
the processed sample fraction. Analysis of metal speciation by these methods, a parameter
important to the interpretation of biological effects of metal loading, can only be accomplished
by coupling upstream separation and extraction procedures, thereby not only adding cost, but also
increasing the risk of altering sample composition from the ambient state. The development of
in situ
tools for monitoring ambient metal loads would clearly enhance all aspects of current
monitoring efforts.
Technologies Available For In Situ Metal Analysis
Desirable features for
in situ
and on site metal analysis should include the capacity to measure not
only total dissolved or extractable metal content, but also metal concentrations within the
dynamic or labile species fractions (potentially representing biologically available forms) and free
metal ion species (representing the most biologically active form). In order to provide the most
accurate assessment of the true chemical state, the targeted technologies should provide analytical
results with minimal sample manipulation. It is useful to distinguish technological approaches
based on whether they function as sensors, directly measuring the metal analyte of interest or
analytical systems which incorporate sensor types into a sample processing stream. In the
following, technology base summaries for: Spectroscopic Techniques, Electrochemical
Techniques, Voltammetric Techniques, Potentiometric Sensors, Flow Injection Analysis,
Diffusive Gradients in Thin Gels (DGT), and Biosensors, are presented in order of standard usage
in WQ studies and application history in the field of metal analysis.
Spectroscopic Techniques
While existing spectroscopic techniques (e.g., mass spectrometry, emission spectrometry, atomic
absorption spectrometry) offer high precision and sensitive analysis of total metal content orders
of magnitude below existing WQ criteria, the portability and infrastructure costs of these
analytical systems need to benefit from advances in nanofabrication; ultimately their utility in
monitoring metal bioactivity is limited by the requirement to couple these detection systems with
ACT Workshop on Trace Metal Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7