Continuous configurations

One of the most valuable assets of flow-through (bio)chemical sensors is their compatibility with unsegmented-flow configurations, which endows them with major advantages over probe-type sensors including higher flexibility and automatability in addition to wider applicability to real rather than academic problems — the former are rarely addressed by using sensors. 

It should be noted that Figs 2.12-2.16 do not include every possible type of configuration, as shown in subsequent chapters. Rather, they provide an overview of the wide variety of on-line coupled flow-through sensors and continuous unsegmented-flow analytical configurations. 

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Compatibility between sensors and automatic and automated analytical systems is crucial as it allows two Analytical Chemistry trends to be combined (see Fig. 1.1). Probe-type and planar sensors can be used in automated batch systems including robot stations, as well as in continuous (mixed in-line/on-line) systems. On the other hand, flow-through sensors are only compatible with continuous configurations. 

Figure 2.11 — Functions and implicit advantages over conventional probe sensors of continuous configurations coupled on-line to a flow-through (bio)chemical sensor.

Figure 2.12 — Continuous configurations coupled on-line to flow-through sensors involving transient immobilization of the analyte (A) contained in the sample (S). P pump. C carrier. RC regenerating carrier. R reagent. IV injection valve. SV switching valve. W waste. For details, see text.

Figure 2.15 — Variants of coupled continuous configurations and sensors accommodating an immobilized catalyst (Cat). (A) Conventional system. (B) Stopped-flow system. (C) Configuration with iterative reversal of the flow direction. (D) Open-closed circuit configuration. Symbol meanings are given in Fig. 2.14. For details, see text.

Figure 2.17 compares the different ways of regenerating flow-through sensors with the normal procedure for probe sensors the probe is successively immersed in the sample and buffer solution and removed from it prior to immersion into the next sample, which hinders automated functioning unless a robot station is lised (Fig. 2.17.A). On the other hand, on-line coupled flow-through sensors in continuous configurations lend themselves readily to convenient, automated regeneration.

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.

Kinetic measurements are based on signal increments over preset intervals and have the advantage of their relative rather than absolute nature, which avoids interferences from the sample matrix. Figure 2.19.B shows the different variants of kinetic measurements in this context, which depend on the type of sensor and coupled continuous configuration used. The most immediate variant involves halting the flow over an interval At when the sample plug reaches the detector (Fig. 2.19.B.2), where the (bio)chemical reaction is allowed to developed while the product of interest is monitored simultaneously. The other two variants... 

One of the most topical ways of approaching this type of system, where separation and detection take place sequentially in space and time, to current trends in Science and Technology (e.g. automation and miniaturization) involves integrating both steps. Integrated systems of this type meet the requirements of chemical sensors [7,8] and differ clearly from conventional flow systems, where detection and mass transfer take place at different locations in the continuous configuration. In fact, the characteristic mass transfer of separation techniques takes place simultaneously with detection... 

In the best of cases, the carrier itself acts as regenerator otherwise, a continuous configuration (usually an FI manifold) is the most convenient choice for this purpose. 

The continuous configurations depicted in Figs 5.12.D1 and 5.12.D2 were designed by Nieman s group for application of this sensor to the determination of sucrose (and glucose) in soft drinks, breakfast cereal and cake mix [36]. The analyte is converted into /3-D-glucose, to which the sensor is responsive, in two reaction steps that are catalysed by invertase (INV) and mutarotase (MUT) ... 

Heat transfer can occur in either batch or continuous configurations. Both types of processes require fluid motion to obtain an effective heat transfer to the bulk of the fluid. In batch processing using jacketed vessels, helical coils, or coils in a baffle configuration, for example, sufficient agitation is required for heat transfer through the medium while continuous systems rely on flow rate to achieve effective heat transfer to satisfy process requirements. Effective heat transfer in batch operations for structured liquid detergents may require scrapers or anchor-type impellers to increase heat transfer coefficients in jacketed vessels. 

Unlike with discrete or batch configurations, the nomenclature used with continuous configurations Is not quite correct taking into account the clear distinction between the terms analyeie and determination , establlehed by Pardue [9] in his hierarchical view of Analytical Chemistry (see Chapter 1). Thue, terms such as continuous-flow analysis , segmented-flow analysis or flow-injection analysis are not meant to describe the overall analytical process insofar as they do not include the preliminary sampling and sample treat-... 

Conhol room operating personnel are directly responsible for safe operation of the plant, including its continued configuration control. They should be informed (by means of a work permit procedure, for example) of aU MS I work before it is commenced, of any changes to the plant that this work entails, and of the return of plant systems to the control of the operator. During the performance of such work, adequate communication should be maintained between the relevant personnel and control room operating personnel. 

Therefore, in an attempt to obtain simple analytic expressions for the distribution functions of stiff polymer chains, condition (5.2a) is relaxed. The relaxation of this condition is in the original spirit of the use of Wiener integrals. If this condition were imposed for flexible polymer chains, the Wiener measure would be 2[t s)] exp (—3L/2/) and would give equal weight (measure) to all continuous configurations of the polymer. Thus the use of (5.2a) would not yield the correct gaussian distribution for flexible chains. 

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