Chemical sensors signal processing

Another important field of application for chemical sensors is process control. Here the sen- sor is expected to deliver some crucial signal to the actuators (valves, pumps, etc.) that control the actual process. Fully automated process control is feasible with certain types of feedback circuits. Since key chemical parameters in many chemical or biochemical processes are not subject to sufficiently accurate direct determination by the human senses, or even via the detour of a physical parameter, sensor technology becomes the key to automatic process and quality control. Often the performance of an entire industrial process depends on the quality and reliability of the sensors employed. Successful adaptation of a batch process to flow-through reactor production technology... 

Fig. 5 shows the instrumental arrangement of the commercially most successful optical chemical sensor between 1984 and 2000. It is used in about 70% of all critical care operations in the US to monitor pH, pC02 and p02 in the cardiopulmonary bypass operations35. It contains 3 fluorescent spots, each sensitive for one parameter, in contact with blood. Fluorescence intensity is measured at two wavelengths and the signals are then submitted to internal referencing and data processing. 

No doubt, computer science in general and chemometrics in particular offer a vast potential for Analytical Chemistry to dramatically raise the amount and quality of information that can be abstracted from raw data. The use of smart signal-processing systems is one of the more salient trends in the context of (bio)chemical sensors (Fig. 1.17.4). 

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). 

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 requirements for chemical sensors suitable for use in eddy correlation direct measurements of surface fluxes are examined. The resolution of chemical sensors is examined and defined in terms of surface flux and commonly measured micrometeorological parameters. Aspects of the design and operation of sensor systems are considered. In particular, the effects of the inlet ductingy the sensing volume, and the signal processing on the ability to measure surface fluxes were analyzed. 

The fundamental operation of an optochemical sensor consists of three main steps the analyte-recognizing element interaction by means of any of the different mechanisms that are schematized in Fig. 1 [3] the detection and transduction of any physical or chemical variation caused by the recognizing reactions and the signal processing and the acquisition of results. 

R 18] [A 1] Each module is equipped with a heater (H3-H8) and a fluidic cooling (C03-C06). Temperature sensors integrated in the modules deliver the sensor signals for the heater control. Fluidic data such as flow and pressure are measured integrally outside the micro structured devices by laboratory-made flow sensors manufactured by silicon machining. The micro structured pressure sensor can tolerate up to 10 bar at 200 °C with a small dead volume of only 0.5 pi. The micro structured mass flow sensor relies on the Coriolis principle and is positioned behind the pumps in Figure 4.59 (FIC). For more detailed information about the product quality it was recommended to use optical flow cells inline with the chemical process combined with an NIR analytic or a Raman spectrometer. 

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