16
the lander consisted of two pressure cases
containing the power supply, stepping motor control
and data logger units. LISA was deployed onto the lake bottom with a winch, and it was gently
landed at a vertical velocity of 2 cm s
-1
. Even though limnological information on the visualization
of steep ion gradients at the sediment surface was very significant, a strict correlation between the
ISEs and a reference method was missing.
More
recently, a setup for high resolution profiling in lakes based on all-solid-state ISEs
(incorporating carbon nanotubes as transducers) was reported.[14, 111] The approach was
comprised of a profiling ion analyser (PIA) that was expanded with ammonium ISEs embedded in
galvanically separated amplifiers (Figure 5).[43, 112] The device featured several units for
characterizing the water column (a photosynthetically active radiation sensor, amperometric
sulfide microsensor and a CTD probe and needle-type optodes for measuring dissolved oxygen
concentrations) and was designed for flexible online data acquisition, analysis and visualization
based on embedded computing.[111] Importantly, with the incorporation of all-solid-state
ammonium-selective electrodes into the PIA, the identification of hotspots as well as steep
gradients of ammonium concentrations in a lake by monitoring the electrode potential
online was
successfully demonstrated.[14, 78] Owing to the conversion of high-resolution concentration
profiles (Figure 5), fine-scale features were displayed, which would have been missed by
conventional limnological sampling followed by laboratory analyses.[14] The conversion of the
measured EMF to ammonium concentration was achieved with an algorithm that considers three
aspects - an
in situ
calibration of the ISEs, the main interferences and drift correction. The
in situ
calibration curve (plot of EMF values at respective depth points against the logarithm of the
ammonium concentration obtained from spectrophotometric analysis) was
plotted by high-
resolution sampling during profile recording using the syringe samples and thus excluding
spatiotemporal misalignment between measurements. The additional analysis of other ions in
these samples permitted one to determine the concentrations of interfering ions with selectivity
coefficients that were previously calculated, and, therefore, the measured EMF could be
corrected. Drift correction considers the
in situ
change of the standard potential
of the electrode
17
together with parallel drifts by minimizing the least-squares difference, leading to a single drift
value.[14] Undoubtedly, the extension of the presented setup to other selective potentiometric
sensors would supply relevant information regarding other targets of interest in pursuit of deep
and reliable study of biogeochemical processes that occur at the centimetre scale in freshwater
systems. Although the suitability of the device for acquiring continuous and reliable depth profiles
of ammonium was fully demonstrated, the applicability for long-term monitoring and therefore
temporal resolution was not evident. It is hence crucial to investigate in more detail the (bio)fouling
properties of membrane electrodes that, in principle, would be significant in eutrophic lakes.
Beyond the explored possibilities of ISEs for decentralized analysis of environmental waters,
certain types of centralized measurements are possible, permitting perspectives that were
previously inconceivable with other analytical techniques. This is
the case with concentration
gradient profiling in environmental biofilms, such as sediment cores. The use of appropriate
microsensors provides the required sub-millimetre and close-to-second resolution for this. The
works of the de Beer and Wehrli groups constitute a source of real microprofiles of ions (such as
NH
4
+
, NO
3
–
, NO
2
–
, CO
3
2
–
, Ca
2+
and pH) at the sediment-water surface that has much merit for the
study of photosynthesis and calcification/decalcification processes.[13, 38, 113-115] The
translation of the approach into an
in situ
concept would be a valuable step for these types of
environmental studies as preliminarily performed by Maerki et al.[3, 116]
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