Recent Advances in Ion-Selective Membrane Electrodes for In Situ Environmental Water Analysis
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configuration. The presented device operates autonomously with measurements, calibration, fluidic control and acquisition triggers all integrated into a custom-built self-contained instrument.[94] A careful design of the potentiometric flow cell integrates all the fluidics components.[103] Specifically, six concentric channels drilled in an acrylic cell connect the sample with an outer solution placed in a beaker that contains a constantly renewed 3 M KCl solution (reference solution) (Figure 4). Notably, this configuration, based on using an external reference solution, has the inconvenience of consuming a large volume of concentrated KCl solution. Yet, advantageously, the proposed fluidics design allows the calibration of the potentiometric sensors between sample measurements (through a two-point bracket approach), providing more reliable data as demonstrated with the accomplished external validation.[103] Nonetheless, the frequent calibrations limit the continuity of the obtained profiles (Figure 4). The required time for the potentiometric measurements at one depth is additionally restricted by the use of an automated monitoring cycle for sampling. A deeper evaluation of the real need for electrode re-calibration 14 would be indispensable for possibly reducing the total analysis time and then improving the spatial and temporal resolution of the approach with respect to dynamic profiles. In a recent work, nitrate, ammonium, carbonate, Ca 2+ and pH levels were monitored in a lake at different depths and over 90 hours, supplying valuable 2D profiles.[103] Intriguing patterns were found for the mentioned ions, like the influence of the anoxic region on nitrate levels or day/night cycles of calcium. The extension of this approach to other natural water systems with biogeochemical interpretation may represent a promising challenge along these lines. Nevertheless, the proposed configuration is not compatible with in situ measurements because of the inner filling configuration of the electrodes (backpressure issues). Further efforts may be directed toward the translation of this concept into an all-solid-state configuration for the potentiometric sensors as previously commented herein, simplification and miniaturization of the fluidics and instrumentation as well as designing submersible housing while maintaining the flow mode. An all-solid-state reference electrode would be beneficial so as to avoid huge amounts of KCl necessary as the reference solution (for more background on all-solid- state reference electrodes, see Zuliani et al.[24], Hu et al.[39] and Michalska et al.[41]) Although on-board and on-site configurations just necessitate the use of systems for water retrieval together with portable analytical devices, which seems simple a priori , the creation of deployable devices for in situ measurements is more complex. The development of submersible pressure resistant housings for the hardware utilized for the analytical readout is one of the key steps for this purpose. Today, this technology is very advanced and submersible multi-parametric probes are commercially available for in situ water measurements, with various features, like temperature, pressure, conductivity, dissolved oxygen, pH, turbidity, dissolved CO 2 , nitrate, ammonium, chloride, having been successful implemented.[104-108] While in at early stages, commercial probes were mainly restricted to glass pH electrodes and dissolved CO 2 measurements, new designs also incorporated Cl – , nitrate and ammonium ISEs (the type of electrode unknown and restricted to freshwater and 17 m depths, [107]). Note that 15 dissolved CO 2 generally uses sensors based on gas-permeable membranes that suffer from (bio)fouling, and their employed is limited to freshwater based on the fact that the high amount of Cl – present in seawater severe interferes with the sensor.[109] Custom-built submersible probes are also reported in the literature. The incorporation of a pH microelectrode together with other sensors within a small remote-operated vehicle (ROV) was described for the analysis of pore water.[33] The pH electrode was partially embedded in a cylindrical pressure housing containing the ROV controller, analogue signal processing boards and batteries. A long waterproof cable was connected to the potentiometer that was placed on a ship. Other pH electrodes have also been incorporated into different platforms, such as a those with a stainless steel frame connected to a winch with durable rope,[101] a rugged enclosure with an AM transmitter,[93] and a buoy additionally containing the potentiometer and a radio antenna to transmit data to a shore-based computer.[100] The profiles elicited are commonly based on pH values at different depths or for long-term monitoring. Camusso et al. [12] employed a Cu(II) jalpaite ion-selective electrode (Orion Model 94-29) and NH 3 -ISE (Orion Model 95-12) attached to a conductivity, temperature and depth (CTD) probe from Idronaut SRL (Italy).[106] Despite this contribution being outside of the focused scope of this review (for crystalline/glass ISE electrodes, refer to De Marco [20]), it is worth mentioning as it represents an early demonstration of potentiometric approaches for in situ ion detection of environmentally important species using solid-state ISEs. The electrode-equipped CTD was deployed at a velocity of 10 m min -1 , and a display monitor (Idronaut Model 401) featured the registered average values as a function of depth.[12] Muller and Wehrli, pioneers of studies related to mineralization in surface sediments, carried out tải về 1.01 Mb. Chia sẻ với bạn bè của bạn:
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