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Potentiometric Sensors
Are the simplest analytical systems in that they are comprised of a simple galvanic cell between
a WE and RE. The millivolt potential output from the cell is proportional to the analyte of interest
as long as the WE electrolyte chamber is separated from the ambient medium by an ion selective
(IS) coating. ISE based sensors provide a direct measure of ion activity and may therefore offer
the best proxy for bioactivity. However, reliable interpretation of such metal ion activity data
requires precise concurrent CTD measurement and their utility in open ocean systems may be
limited. Availability of field deployable and
in situ
pH and redox electrodes indicates that robust
fabrication technologies are in place to support electrode development. Additionally leveraging
technical advances in fabrication of solid state ISE for deployment on interplanetary missions
would enable developing compact field deployable ISE arrays for multiple metal ion analysis
systems (see S.
Kounaves presentation
). These electrode arrays could also be leveraged as
screening to facilitate and support needed research into the synthesis of novel ion-selective
coatings for critical metal species, the availability of which is currently the major obstacle
limiting the use of ISEs for metal monitoring. Investment in development of new IS coatings
could also benefit development of more sensitive and durable voltammetric microelectrodes.
Flow Injection Analysis
Traditional wet chemical analysis has a long history in aquatic science, particularly for
monitoring macro and micronutrient dynamics. Advent of continuous flow analysis (CFA) and
flow injection analysis (FIA) instrumentation was quickly adopted to promote high resolution
profiling and surface mapping (Johnson et al. 2000). FIA based analytical systems are comprised
of one to several pumps feeding samples, blanks and reagents to a mixing manifold and
subsequent passage of the analytical stream through a flow through detector to measure either
absorbance, fluorescence or chemiluminescence of the specific analyte+reagent reaction product.
Continuing advances in microfluidics and nanofabrication have enabled further size and power
reductions in these design of FIA system components making them compatible not only with on
site, but also
in situ
monitoring efforts. Application of FIA technology to metal analysis by the
research community has required inclusion of trace metal clean Teflon tubing reagent reservoirs
and on board cleaning capacities. Additionally, sensitivity for specific metal species has been
improved in oceanic systems by including solid phase preconcentration, long optical pathlength
detectors, or incorporation of detection reagents, yielding metal ion specific chemiluminescent or
fluorescent products for ambient level detection (10-12 - 10-9 M) of several metals (Fe, Co, Cu,
Mn, Zn) influencing biological activity. Alternation of acidifying and non-acidifying process
streams into the sample flow may in some cases, be used to distinguish between the labile and
total dissolved metal fractions. As with ISE and voltammetric techniques, investment in
identification of metal ion-specific detection reagents would enhance the adoption of this
technology by the monitoring community. As this approach is reagent dependent,
in situ
or on
site deployment intervals will ultimately be limited by reagent storage volumes and stability
under field conditions.
At present a FIA system for
in situ
measurement of dissolved iron II is commercially available
from SubChem Systems (
www.subchem.com
).
ACT Workshop on Trace Metal Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9