Sensors probe-type

Bio)chemical sensors can be used in both the batch and the continuous mode. While this is also true of probe-type sensors, flow-through sensors can only be used in a continuous regime coupled on-line to a continuous-flow configuration. 

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. 

Parameter Probe-type sensor Flow-cell type sensor... 

Figure 5.8 — Probe-type sensor based on continuous circulation of a stream containing an acid-base indicator for the batch determination of COj in sea water, (a) Reagent delivery capillary, (d) Reagent exit capillary, (c) Optical fibre from source, (d) Optical fibre to detector, (e) White silicone rubber membrane. (/) White silicone sealant, (g) Epoxy resin, (/i) 0-ring. (/) Sensor housing. (/) Optical cable. (Reproduced from [12] with permission of the American Chemical Society).

Recently, DeGrandpre [12] developed a probe-type sensor for the determination of PCO2 in sea water by direct immersion of the probe, which, however, has some connotations of flow-through sensor even though a pH indicator such as Phenol Red (piTj = 7.5) or Bromothymol Blue (pAn = 6.8) rather than the sample is circulated over the sensing microzone —the basic forms of these indicators have a high molar extinction coefficient at 560 and... 

Suzuki et al. [90] developed a diffuse refleetanee flow-through sensor for improving selectivity in the determination of lithium in human fluids, where sodium can occur at concentrations up to 10 000 times higher than that of the analyte. The mechanism on which the sensor action relies (Fig. 5.21.2) was previously exploited by the authors to develop a probe-type sensor for potassium [91]. The sensor response is based on an A -H exchange according to... 

In alternative energy processes, in most cases the goal is to eliminate all sampling systems and place the analyzer directly into the process. The "in-line" analyzer designs are widely available, and the various radiant energy and probe-type sensors operate without any sampling. 

TABLE 16.4 Advantages and Disadvantages of Various Insertion- or Probe-Type Sensors... 

R.H.-independent signal output has been achieved in thefour-probe type sensor shown in Fig. 36.4, where two additional Ag probes are inserted in the proton conductor bulk (AA) beneath the Pt electrodes. One of the Pt electrodes is covered by a layer of AA sheet, which acts as a sort of gas diffusion layer. The short-circuit current flowing between the two Pt electrodes is proportional to H2 concentration but dependent on R.H., just as in the previous amperometric sensor. On the other hand, the difference in potential between the two Ag probes (inner potential difference, AE g) with the outer Pt electrodes short-circuited is shown to be not only proportional to H2 concentration but also independent of R.H. as shown in Fig. 36.3b and Table 36.2. This mode of sensing has no precedence, and is noted as a new method to overcome the greatest difficulty in using proton conductor-based devices, i.e. their R.H. dependence. 

Fig. 36.3. (a) Dependence of short-circuit current (/) of the amperometric proton-conductor sensor on H2 concentration, and (b) dependence of (inner potential difference) of the four-probe type sensor on Hj concentration in air at different relative humidity (25 (reprinted by permission of The Electrochemical Society, Inc,),... 

In the potentiometric sensing of H2, (ii) is useful for H2 in inert gases while (i) is especially suited for H2 in air. On the other hand, only (iii) is applicable to the amperometric sensor and the four-probe type sensor. 

Fig. 36.4. Structure of the four-probe type sensor. AAD, antimonic acid disc AAS, antimonic acid sheet (reprinted by permission of The Electrochemical Society, Inc.).

The best way of monitoring pervaporated species is by inserting a probe-type sensor in the acceptor chamber with its active side facing the membrane. The most salient advantages of integrating pervaporation and detection are as follows ... 

The wire-type sensor (probe) can be a single-wire construction, or it may have two or three separate wires. With a three-wire sensor, all three velocity components can be determined. As well as wire-type sensors, there are hot-... 

Sandwich-type sensors are applicable for measuring large antigens that are capable of binding two different antibodies. Such sensors utilize an antibody that binds the analyte-antigen, which then binds an enzyme-labeled second antibody. After removal of the nonspecifically adsorbed label, the probe is placed into the substrate-containing solution, and the extent of the enzymatic reaction is monitored... 

Li YQ, Bricks JL, Resch-Genger U et al (2006) Bifunctional charge transfer operated fluorescent probes with acceptor and donor receptors. 2. Bifunctional cation coordination behavior of biphenyl-type sensor molecules incorporating 2, 2 6, 2"-terpyridine acceptors. J Phys Chem A 110 10972-10984... 

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. 

Table 4.1. Comparison of the performance of a probe-type and a flow-cell type sensor for fluoride...

The POT is a discrete measurement technique using pin type ionization probes as sensors for detecting the arrival time of detonation wave at pre-determined points and measuring propagation time between these points with the help of a high-speed oscilloscope. The schematic of instrumentation set-up is shown in Figure 3.17. 

Accuracy is improved if the flowing gas stream is directed at the probe by a venturi nozzle, or by placing the sensor in the throat of the venturi (Figure 3.74). The venturi ensures a smooth velocity profile and eliminates boundary layer effects while concentrating the flow onto the sensor. These units are available for both liquid and gas services. Other designs are of the insertion probe type. Their flow ranges are a function only of the size of the pipe into which they are inserted, and their performance is a function of the correctness of the insertion depth (as are all Pitot tubes). 

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