Photometric sensors

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 4.4 — (A) Flow-through cells for spectrofluorimetric sensors (a) fused silica tube (1.5 mm ID) packed with 1 mg of CM-Sephadex C-25 (b) micro-cell holder (c) side and (d) front view of a commercially available sensor. (Reproduced from [62] and [64] with permission of the Royal Society of Chemistry and Elsevier Science Publishers, respectively). (B) Flow-through cells for photometric sensors. Side and front views of two commercially available designs. For details, see text. (Reproduced from [80] and [83] with permission of Elsevier Science Publishers and the Royal Society of Chemistry, respectively).

Figure 4.6 — Manifold used and recording obtained with the flow-through photometric sensor for the determination of formaldehyde. PRA />-rosaniline q flow-rate P peristaltic pump M mixing point R reactor SV switching valve IV injection valve D detector W waste. (Reproduced from [89] with permission of Marcel Dekker, Inc.).

Figure 5.16 — Flow-through photometric sensor for the determination of traces of copper based on the immobilization of a chromogenic ligand (PAN) in a special flow-cell coupled on-line with a flow injection (A) or continuous-flow (B) configuration. IV injection valve SV switching valve W waste TGA thioglycollic acid. For details, see text. (Adapted from [42] with permission of Elsevier Science Publishers).

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.)...

This article will review the practical methodology as applied to photometric sensors where measurements are confined to the ultraviolet and visible region of the electromagnetic spectrum. A previous article in this series on Sensors has similarly discussed fluorescence measurement based sensors. 

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... 

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. 

One of the critical elements in the design of a photometric sensor is the configuration of the sensing terminus. Its configuration often determines detectability, linear dynamic range, rate of response, the effects of interferents, site specificity, and useful lifetime of the sensor. Most important design considerations include (1) the method of immobilization and (2) the shape and size of the terminus and the amount of reagent phase. 

The basic instrumentation associated with photometric sensors is simple and requires both optical and electrical components. Apart from the optical fiber, the instrumentation system involves a light soiu-ce, photodetector and associated display, optical components such as lenses, couplers and connectors, and monochromators or filters for wavelength selection. A typical instrumentation system employed in conjunction with photometric sensors is shown schematically in Figure 3. 

The light source must be able to provide a stable and intense optical radiation. Several types of sources have been employed in photometric sensors, namely incandescent lamps, gas lasers, LEDs, and laser... 

Hgure 3 Schematic diagram of a typical instrumentation employed with photometric sensors. 

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