Sensors, photometric chemical

Like any other type of sensor, the ideal photometric chemical sensor should not perturb the sample. In practice, this requires that the amount of analyte present to react with the indicator reagent should be small compared to the total amount of analyte in the sample. If this requirement is not met, then the contact of analyte with the reagent in the chemical transducer will result in a change in the analyte concentration in the sample. This, in turn, will result in a... 

The instrumental range is designed primarily for the clinical laboratories but 10 different wavelengths between 340 nm and 800 nm can be selected to match the test requirements. Sixteen photometric sensors are available. Serum quality data, including hpernia, icterus and remolysis, can be obtained in addition to the serum blank. All of this information can be obtained without sacrificing the number of channels available for chemical analysis. 

Figure 5.13 — Irreversible-reusable flow-through sensor for the kinetic multidetermination of phosphate and silicate based on integrated sorption of a reaction product, reaction (/ situ reduction) and photometric detection. (A) Microsensor block (1) and components (2). (B) Continuous-flow configuration coupled on-line to the sensor. P peristaltic pumps SV switching valve W waste. For details, see text. (Reproduced from [39] with permission of the American Chemical Society).

Sensing Volume. The sensing volume of a sensor is the volume where the air is actually monitored. The sensing volume is the reaction chamber of a flame photometric detector or a chemiluminescence device, the field of view of an open-path sensor, or the White cell of a reduced-pressure optical system. The residence time of the sample within the sensing volume ultimately limits the temporal resolution of most chemical sensors. 

Chemical sensors are designed to detect or measure the presence of specific chemical compounds. This category includes gas and electrochemical devices. Photometric sensors, which are optical sensors used to measure chemical presence, are also included in this category. 

Fig. 7.18. Low-pressure interfaces to detectors based on flow injection. (A) Interface to a photometric detector across a membrane. (Reproduced with permission of the American Chemical Society.) (B) Interface to a flow-through photometric sensor with prior derivatization by the modified Griess reaction. (Reproduced with permission of the American Chemical Society.) (C) Interface to a piezoelectric detector. P peristaltic pump, C collector, CUC clean-up column, DB debubbler, SA sulfamic acid, NEDD /V-( 1-naphthyl)ethylenediamine dihydrochloride, SV switching valve, W waste, DF displacement flask, IV injection valve, FC-PZ flow-cell-piezoelectric crystal, OC oscillator circuitry, F frequency counter, PC personal computer. (Reproduced with permission of Elsevier.)...

Amperometric Oxygen Sensors Calorimetric/Enthalpimetric Chemically Modified Electrodes Microorganism-Based Photometric... 

A photometric (or optical) sensor can be described as a device which can be employed for the detection and determination of physical and chemical parameters through measurements of optical property. Optical methods have played a dominant role and continue... 

When the chemical transduction medium used in a photometric sensor is opaque or when it transmits light only weakly, then measurements of the reflected light may be used. Reflection takes place when light infringes on a boundary surface, and two distinct types of reflection are possible. The first is the specular (or mirror type) reflection, which occurs at the interface of a medium with no transmission... 

The optical measmements of diffuse reflectance are dependent on the composition of the system. Several theoretical models have been proposed for diffuse reflectance, which are based on the radiative transfer theory, and all models consider that the incident hght is scattered by particles within the medium. The most widely used theory in photometric sensors is the Kubelka-Munk theory, in which it is assumed that the scattering layer is infinitively thick, which may, in practice, be the case with the chemical transducers utilized in photometric sensors. The absolute value of the reflectance R is related to the absorption coefficient K and the scattering coefficient S by the equation... 

Some analyte chemical species are themselves capable of exhibiting optical changes related to their concentrations, which can be measured photometrically using optical fibers. Such sensors, in which separate chemical transduction systems will not be required, are commonly referred to as plain-fiber sensors. Since many analytes do not themselves possess suitable optical characteristics, they may be sensed indirectly through their interaction with an appropriate chemical transduction system. The sensor response function in such chemical transducer-based optical sensors depends on the manner in which the analyte interacts with the reagent phase. For example, in a simple system where a reagent R reacts with an analyte species A forming a product AR can be represented by the reaction... 

Nonreversible chemical reactions may be employed in the photometric transduction systems, which can result in one shot type sensor devices. Though there will only be a limited merit in utilizing... 

A variety of transducer configurations that has been employed in photometric sensor devices fall into two sensor types extrinsic sensors and intrinsic sensors. While in the former sensor type the optical fiber merely acts as a light guide, conveying the optical information between the optical source and the chemical transducer and between the chemical transducer and the detector, in the latter sensor type the optical fiber, probably in some modified form, would become a part of the transducer. 

Initially fibre optic chemical sensors were simply devices for modifying the light path of a conventional spectrometer. The optical fibres were just used to conduct light to and from the sample. Sometimes conventional colorimetric reactions were carried out on the sample, and specialized probes could be used in conjunction with the optical fibres to make absorption measurements (for example). Such devices can be described as photometric or spectroscopic transducers. 

All these phenomena can be utilized in chemical sensors. Before discussing them, some photometric quantities must be defined. 

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