Transducer thermal

Thermal Transducers Infrared radiation generally does not have sufficient energy to produce a measurable current when using a photon transducer. A thermal transducer, therefore, is used for infrared spectroscopy. The absorption of infrared photons by a thermal transducer increases its temperature, changing one or more of its characteristic properties. The pneumatic transducer, for example. 

A photon detector produces a current or voltage as a result of the emission of electrons from a photosensitive surface when struck by photons. A heat detector consists of a darkened surface to absorb infrared radiation and produce a temperature increase. A thermal transducer produces an electrical signal whose magnitude is related to the temperature and thus the intensity of the infrared radiation. 

The integrated system, including transducer and enzyme reactor, provides improved reliability and stability in multianalyte determinations, as compared with discrete thermal sensor systems. In addition, application of micromachining and IC technologies is of benefit for the manufacture of uniform, cheap thermal transducers with flexible shape, size, and resistance, as well as delicate microstructure on the chips. The good thermal insulation of the transducers from the flow stream eliminates interference from the reactants on the transducers, and the intrinsic stability of the transducers obviates the need for frequent recalibration of the sensors. 

The distinction between photon and heat transducers is important because shot noise often limits (he behavior of photon transducers and thermal noise frequently limits thermal transducers. As shown in Section -SB-2. Ihe indeterminate errors associated with Ihe two types of transducers are fundaiiienlally differenl-... 

The convenient phototransducers just considered arc generally not applicable in the infrared because photons in this region lack the energy to cause photocmis-sion of electrons. Thus, thermal transducers or photo-conductive transducers (see Section 7l -4) must be used. Neither of these is as satisfactory as photon transducers. 

In thermal transducers.- the radiation impinges on and is absorbed b a small blackbodv the resultant... 

IR transducers are of three general types (1) pyroelectric transducers. (2) photoconducting transducers, and (3) thermal transducers. The tirst Ls found in photometers, some FTIR spectrometers, and dispersive speclropholometers. Photoconducting transducers arc found in many F TIR inslrumonis. Thermal detectors arc found in older dispersive instruments but arc too slow to be used in FTIR spectrometers. 

Thermal transducers, whose responses depend on the heating elfcct of radiation, are found in older dispersive spectrometers for detection of all but the shortest IR wavelengths. With lliese devices, the radiation is absorbed by a small blackbody and the resultant lem-peralure rise is measured. The radiant power level from a speciropliolonicter boain is minute (10 lo 10 W), so that the heat capacity of ilic absorbing cl-... 

The problem of measuring IR radiation by thermal means is compounded by thermal noise from the surroundings. For this reason, thermal transducers are housed in a vacuum and are carefully shielded from thermal radiation emitted by other nearby objects. To further minimize the effects of extraneous heat sources, the beam from the source is always chopped. In this way, the analyte signal, after transduction, has the frequency of the chopper and can be separated electronically from extraneous noise signals, which are ordinarily broad band or vary only slowly with time. 

Figure 7-27 shows the relative spectral response of the various kinds of transducers that arc useful for UV, visible, and IR spectroscopy. The ordinate function is inversely related to the noise of the transducers and directly related to the square root of its surface area. Note that the relative sensitivity of the thermal transducers (curves H and /) is independent of wavelength but significantly lower than the sensitivity of photoelectric transducers. On the other hand, photon transducers are often far from ideal with respect to constant response versus wavelength. 

In addition to electrochemical and optical detectors, there have been a very wide variety of other kinds of detector systems that have been employed in experimental biosensors as noted in Fig. 1, including surface wave detectors, which are essentially mass detectors, conductivity, and thermal transducers. However, because of the need for reliability, flexibility, and sensitivity, electrochemical transducers are usually the system of choice because of the extensive research and manufacturing experience available. 

Finally, both coupling and transformation reactions can be detected using enzymatic labelling of immunoagents with, for example, peroxidases. The label ensures that there is sufficient consumption of a substrate, or the generation of a product, for detection by potentiometric, optical or thermal transducers. 

The transducer must also make optimal use of the physicochemical modification resulting firom the biological reaction. If this reaction gives rise to a variation in enthalpy, then a voy weak increase in local temperature will occur (of the order of 1/lOOth of a degree). Here a thermistor makes a better thermal transducer than a thermocouple. 

A real thermal enzyme sensor must involve, as its name suggests, a thermal transducer (thermistors or thermopiles) with a layer of immolnlized enzyme. The enzyme transforms die substrate and ensures the liberadon or consumption of calories that are detectable in situ by the transducer. 

The total variation in enthalpy, AH3 = AH) + AH2 43 kcal, is the value detected by the thermal transducer. 

The following reaction can be catalysed by immobilized urease on a thermal transducer... 

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