Flow-through sensors transient signals

Figure 2.18 shows the most relevant and/or usual types of transient signals provided by the flow-through (bio)chemical sensors used in the continuous configurations depicted in Figs 2.12-2.16 and the regeneration modes illustrated in Fig. 2.17. The two sequential steps (1 and 2) affecting the sensitive microzone of the flow-through sensor are distinguished.

On the other hand, its should be emphasized that such basic analytical properties as precision, sensitivity and selectivity are influenced by the kinetic connotations of the sensor. Measurement repeatability and reproducibility depend largely on constancy of the hydrodynamic properties of the continuous system used and on whether or not the chemical and separation processes involved reach complete equilibrium (otherwise, measurements made under unstable conditions may result in substantial errors). Reaction rate measurements boost selectivity as they provide differential (incremental) rather than absolute values, so any interferences from the sample matrix are considerably reduced. Because flow-through sensors enable simultaneous concentration and detection, they can be used to develop kinetic methodologies based on the slope of the initial portion of the transient signal, thereby indirectly increasing the sensitivity without the need for the large sample volumes typically used by classical preconcentration methods. 

The transient signals provided by flow-through (bio)chemical sensors can be processed in various ways in order to draw information that can be directly related to the analyte concentration in the sample. Figure 2.19 shows the more frequently used approaches in this respect, classified according to whether they rely on direct (A) or kinetic measurements (B). 

Figure 2.19 — Types of measurements available on the transient signals provided by flow-through (bio)chemical sensors. (A) Ordinary measurements. (B) Kinetic measurements. For details, see text.

Rule 1. The first rule is the requirement of the closed electrical circuit. This means that at least two electrodes must be present in the electrochemical cell. From a purely electrical point of view, it means that we have a sensor electrode (the working electrode) and a signal return electrode (often called the auxiliary electrode). This requirement does not necessarily mean that a DC electrical current will flow in a closed circuit. Obviously, if we consider an ideal capacitor C in series with a resistor R (Appendix C), a DC voltage will appear across the capacitor, but only as a transient DC current will not flow through it. On the other hand, if an AC voltage is applied to the cell, a continuous displacement charging current will flow. 

---