Sensing system

Detectors to be used in FIA should idealiy be endowed with a number of attributes such as low flow-cell volume and noise, flow-rate-independent signal, fast and linear response over a wide concentration range and high sensitivity. FIA methodology utilizes a variety of analytical detection techniques such as optical (spectroscopic and non-spectroscopic), electric (amperometric, po-tentiometrlc, conductimetric, coulometric) and thermochemical. 

The use of the voltammetric technique has fostered the development of a host of cells and electrodes, both solid and liquid (DME) [16-25], as well as the study of the principles describing the behaviour of such devices as parts of FIA systems [23-25]. 

Redox [26], ion-selective [27-30] and ISFET [31-32] potentiometric sensors have been widely used in FIA applications. In contrast, voltammetric [33,34] and potentiometric [35,36] stripping, couiometry [37,38] and conductimetry [39-41] have received less attention from FIA researchers. 

Optical detectors are by far the most common in FIA, even though only a few dedicated cells have been designed as most of the cells, particularly photometric and fluorimetric, manufactured for other flow methods can be readily adapted for use in FIA. On the other hand, other optical techniques such as chemiluminescence [42] and refractometry [43], of more limited use in this methodology, have favoured the development of new types of cell suited to specific needs. At this point it is worth noting the simplicity of the joint 

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The SPATE 9000, used in the experimental activity, is a remote sensing system able to detect... 

A measurement of particle moisture content will normally be taken at the exit of the dryer. This allows the process operators to make such adjustments as may be needed to maintain moisture within the desired range. Various instmments are used, none of which are entirely satisfactory, and periodic hand samples are used in some mills. Considering the importance of moisture sensing and control at the dryers, it is unfortunate that a truly efficient, consistent, and accurate sensing system is not yet available to the industry. The primary reasons for the difficulty of measuring moisture at the dryer exit are the extreme and adverse conditions of heat, dust, and moisture present at this location. 

H. S. Chen, Space Remote Sensing Systems An Introduction Academic Press, Inc., Orlando, Fla., 1985. 

Fig. 5.5. Schematic view of the deflection sensing system as used in the NanoScope III AFM (Digital Instruments, Santa Barbara, CA, USA). The deflection ofthe cantilever is amplified by a laser beam focused on the rear ofthe cantilever and reflected towards a split photodiode detector.

So far we have discussed the one-sensor/one-analyte approach. However, arrays of independent electrodes can offer much more analytical information and thus hold a great potential for many practical applications. These include the development of intelligent sensing systems capable of responding to changes in the chemical environment of the array. 

FIGURE 8.4 Configurations for several different kinds of optieal fiber sensing systems are shown. The eommon faetor in all these systems is the use of an optical fiber as an integral element in the system, either to earry light to and from diserete sensors (often referred to as optrodes), or as sensitive elements themselves (intrinsie fiber sensors). Courtesy, AT T Bell Laboratories. 

SCHEME 3 The electrochemical gene sensing system based on the formation of complementary sandwich-type complex, (a) Target DNA combines the ferrocenyl ODN with the probe ODN on the electrode. Redox currents due to the surface-confined ferrocenyl units should reflect the concentration of the target, (b) Ferrocenyl units are not deposited onto the electrode using nontarget DNA. 

Piezoelectric ceramics, which depend on lead compounds, are used to produce transducers and sensors which make possible ultrasound technologies used in wide-ranging medical and commercial applications, guidance and sensing systems used in defense and commerce, and in addition, new "smart materials" research projects. 

Several other organoboron polymers have been developed by various synthetic strategies and utilized to construct polymeric sensing systems for cations, dopamine, saccharides, and so on. Fabre and co-workers have reported the preparation of a conjugated trifluoroborate-substituted polythiophene system for sensing cations such as... 

Chan K., Ito H., Inaba H., Humio H., Remote sensing system for near-infrared differential absorption of methane gas using low-loss optical fiber link. Appl. Optics 1984 23 3415. 

Vast majority of the optical sensing systems are of the modulation type and its functional structure can be sketched as in Figure 1. 

Figure 1. Functional structure of a sensing system of the modulation type.

Many integrated optical sensing systems have been designed, realized and tested in the laboratories but only a few of them have reached the market. 

Optical fibre based optical detection systems offer a number of advantages over bulk optical sensing systems. The principal one is that a robust passive sensing head may be remoted from the monitoring station a factor giving particular advantages in severe environments. This also allows... 

T. Haruyama, S. Shiino, Y. Yanagida, E. Kobatake, and M. Aizawa, Two types of electrochemical nitric oxide (NO) sensing systems with heat-denatured Cyt C and radical scavenger PTIO. Biosens. Bioelectron. 13, 763—769 (1998). 

It is clear from the Nemst equation that the temperature of the solution affects the response slope (2.303A7//0 of the calibration curve. The electrode voltage changes linearly in relationship to changes in temperature at a given pH therefore, the pH of any solution is a function of its temperature. For example, the electrode response slope increases from 59.2mV/pH at 25°C to 61.5 mV/pH at a body temperature of 37°C. For modem pH sensing systems, a temperature probe is normally combined with the pH electrode. The pH meter with an automatic temperature compensation (ATC) function automatically corrects the pH value based on the temperature of the solution detected with the temperature probe. 

Such electrodes should be sufficient as a reference electrode for short-term usage or as a disposable electrode. However, the requirement of a pre-hydration time may limit its applications for fast measurements, such as POCT (the point-of-care testing), due to its slow response time. In fact, the lack of long-term stable microreference electrodes will continue to hamper the development of integrated pH sensing systems. 

Challenges remain in the development of lab-on-a-chip sensing systems. The overall lifetime of a sensor chip is always determined by the sensor with the shortest lifetime, which in most cases is the depletion of reference electrolytes. Measures to minimize cross-talking among sensors, especially when biosensors are integrated in the system, also should be implemented [122], The development of compatible deposition methods of various polymeric membranes on the same chip is another key step in the realization of multisensing devices. 

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