Microzonation

Oxic Diagenesis Metals remobilized from sediments lying in the oxic zone. Remobilization likely occurs in anoxic microzones adjacent to nodules. Bioturbation is an important metal transport agent. Some nodules now found in oxic sediments were likely formed during times when the redox boundary was closer to the seafloor. 10-50 Todorokite (high Cu and Ni content) 32% 5-10 15-20... 

Three mechanisms have been proposed to explain how particulate metals could be transported within such sediments so as to support the growth of Fe-Mn nodules (1) anoxic microzones, (2) bioturbation, and (3) shifts in the depth of the redox boundary over time. Anoxic microzones are present within fecal pellets and the interiors of radiolarian shells where detrital POM is still present. Metals mobilized within these microzones should be able to diffuse through the sediments for substantial distances... 

These deposits probably harbor reducing microzones in which manganese could be resolubilized. 

Because of its high organic content, the marine snow acts as a microhabitat that supports enhanced rates of heterotrophic microbial activity. The associated nutrient remineralization causes the seawater within and aroimd the marine snow to be characterized by elevated nitrogen and phosphorus concentrations and low levels of O2. The importance of these suboxic and anoxic microzones to the marine cycling of the biolimiting elements is unknown but potentially significant. 

Microzone A small volume of water or sohd matter in which the redox environment is considerably different from that in the surrounding sediment or seawater. 

Based on its structure, a sensor is an analytical device consisting of two main parts a (bio)chemical microzone (recognition element) that is brought into contact with the sample and closely associated (connected or integrated)... 

Bio)chemical sensors can be active or passive according to whether they use a sensing microzone to accommodate a chemical or biochemical reaction and/or a biochemical e.g. immunological) or physico-chemical separation e.g. sorption). It should be noted that passive sensors e.g. a fibre-optic tip immersed in an industrial process stream) do not meet one of the essential requirements included in the definition of sensors as regeirds composition... 

The sensors discussed in this book are dealt with according to the classifications based on which the immobilized species is, the type of immobilization used and whether or not an additional separation is included. Thus, sensors have been included in three chapters according to the type of process that takes place in their sensing microzone, namely reaction-detection (Chapter 3), separation-detection (Chapter 4) and separation-reaction-detection (Chapter 5). 

No doubt, one of the most intuitive classifications of sensors is that based on the type of transducer used to reveal the physico-chemical changes that occur in the sensing microzone in the presence of the analyte. Figure 1.6 shows the principal types of transducing systems eire connected to or... 

Figure 1.14 — Classification of flow-through sensors according to external shape. SMZ sensing microzone D detector W waste. For details, see text.

The most salient feature of flow-through sensors is the way in which the sample is brought into contact with the sensitive microzone (see Fig. 1.14), which distinguishes them from probe and drop-planar sensors. In fact, the liquid (or gaseous) sample is passed over the microzone rather than dropped onto it or used to immerse the probe [1]. 

In broad terms, a flow-through sensor is an analytical device consisting of an active microzone where one or more chemical or biochemical reactions, in addition to a separation process, can take place. The microzone is connected to or incorporated into an optical, electric, thermal or mass transducer and must respond in a direct, reversible, continuous, expeditious and accurate manner to changes in the concentrations of chemical or biochemical species in the liquid or gaseous sample that is passed over it, whether forcefully (by aspiration or injection) or otherwise (gases). 

One possible classification is based on the type of physico-chemical phenomena that may occur in the sensor. Based on this criterion, there are passive flow-through sensors, which posses no reactive microzone and are... 

One other, very descriptive classification of flow-through sensors is based on the location of the active microzone and its relationship to the detector. Thus, the microzone can be connected (Figs 2.6. A and 2.6.B) or integrated (Fig. 2.6.C) with the measuring instrument. Sensors of the former type use optical or electric connections and are in fact probe sensors incorporated into flow-cells of continuous analytical systems they can be of two types depending on whether the active microzone is located at the probe end (e.g. see [17]) or is built into the flow-cell (e.g. see [18]) — in this latter case. 

Figure 2.6 — Classification of flow-through sensors according to the location of the active microzone relative to the measuring instrument (A,B) connected (C) built-in. (Reproduced from [1] with permission of the Royal Society of Chemistry).

Other possible classifications of flow-through sensors have been excluded from Fig. 2.4 because they are either of little consequence or dealt with in other sections below. Such is the case with the classification based on whether one or more of the active reaction ingredients (analyte, reagent, catalyst, reaction product) is immobilized temporarily or permanently on the active microzone. In addition, the immobilization process may involve one or several active components. 

Active flow-through (bio)chemical sensors include a microzone where a (bio)chemical reaction, a separation or both takes place. The active microzone may be located in the flow-cell itself (Figs 2.6.B and 2.6.C) or built into a probe sensor for insertion into a continuous-flow analytical system (Fig. 2.6.A). The external appearance of a sensitive microzone can be as widely different as the type of detector and process concerned. This is discussed in greater detail in the following section. 

The way in which the active microzone is retained also depends on its relationship to the detector (Fig. 2.6) and the type of interaction with the analyte or its reaction product. If the microzone is an integral part of the probe, an additional support (usually a membrane) is often required, so contact with the sample is hindered to some extent. On the other hand, a microzone located in a flow-cell can be retained in various ways. Thus, if the microzone consists of a porous solid or particle, the flow-cell is simply packed with two filters in order to avoid washing out (e.g. see [21]). Too finely divided solids (viz. particle sizes below 30-40 pm) should be avoided as they require pressures above atmospheric level, which complicates system design and precludes use of microzones with a high specific surface. Placing a separation membrane in a flow-cell is... 

Figure 2.7 — Types of species retained and immobilization at the active microzone of a flow-through sensor.

When the active species is to be reused many times, they must be immobilized permanently at the active microzone, which is how the reagent and catalyst are usually immobilized. An immobilized reagent must act in a reversible manner or be regenerable (usually by analyte removal or elution) on the other hand, a catalyst is self-regenerating, so no external action is required to make the sensor reversible. 

It should be noted that immobilization on the active microzone can occasionally be both permanent and temporary such is the case when two reagents (e.g. see [23]) or a catalyst plus the reaction product (e.g. see [24]) are to be immobilized. Double immobilization is also common practice when the inunobilized reagent retains the analyte and gives rise to a detectable alteration (a colour, fluorescence, mass or heat energy change) of the sensitive microzone (e.g. see [19]) all three processes (reaction, separation and detection) take place simultaneously rather than sequentially (see Chapter 5). 

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