Flow-through sensors active microzone

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 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. 

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

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. 

Figure 2.13 — Continuous configurations coupled on-line to flow-through biochemical sensors involving permanent immobilization of the reagent (R) at the active microzone. Symbol meanings are given in Fig. 2.12. For details, see text.

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