Detectors surface sensing

Figure 21.6 The three basic configurations for electrochemical detectors. SS, sensing surface. (Reprinted by permission of Elsevier.)...

In general, detectors can be classified according to the way in which the analyte or reaction product is probed, i.e., whether it is done by bulk or surface sensing. It is also possible to draw a distinction between flow-through detectors measuring an intrinsic property of the analyte and those requiring prior online derivatization. From the point of view of instrumental requirements, it is more appropriate to... 

The hydrodynamically well-defined conditions of flow systems are an ideal environment for electrochemical detectors, resulting in enhanced performance characteristics. The surface sensing properties of most electrochemical methods require particular attention in the construction of suitable flow-through cells. Efficient and repeatable mass transport toward the electrode surface is necessary, and dead volumes should be small. Various flow-through cells have been designed for electrochemical detection, all of which can be derived from the basic configurations depicted in Figure 4. 

Thermal desorption can be performed with a rather simple UHV setup, consisting of a heater for raising the temperature of the sample surface, a detector for sensing the desorption species, and a gas doser for preparing the sample surface. A mass spectrometer is usually used as a detector and is helpful in identifying the desorbed species based on their mass spectrum. A multiplexer is often used to detect several species simultaneously (strictly speaking, sequentially with repeated quick scans) with a single temperature rise. It is necessary to minimize the temperature rise of the materials contained in the UHV system other than the specimen. 

A biosensor s detector typically consists of chemical receptors attached or immobilized to a material surface (typically the transducer surface). These are often themselves biomolecules. Detection involves an interaction between these immobilized molecules and biomolecules from solution that approach the detector surface. In this sense, both the fabrication of, and detection using, biosensors involve biomolecules at interfaces. 

Conventional ion-selective electrodes have been used as detectors for immunoassays. Antibody binding measurements can be made with hapten-selective electrodes such as the trimethylphenylammonium ion electrode Enzyme immunoassays in which the enzyme label catalyzes the production of a product that is detected by an ion-selective or gas-sensing electrode take advantage of the amplification effect of enzyme catalysis in order to reach lower detection limits. Systems for hepatitis B surface antigen and estradiol use horseradish peroxidase as the enzyme label and... 

The broadband analysis was confirmed by the experimental results mentioned in Sect. 5.4.1. This method can also be further enhanced by some of the techniques described in Sects. 5.4.2 and 5.4.3. The conclusion is that these methods of microcavity-enhanced optical absorption sensing provide compact, inexpensive, and sensitive detectors for molecular species in the ambient gas or liquid, and that further increases in sensitivity can be implemented to make them even more competitive. The molecular-transition specificity that is implicit in absorption spectroscopy is a limiting restriction, but the surface-enhanced Raman sensing that is enabled by metallic nanoparticles on the microresonator surface can significantly increase the number of molecular species that could be detected. 

Sodium contamination and drift effects have traditionally been measured using static bias-temperature stress on metal-oxide-silicon (MOS) capacitors (7). This technique depends upon the perfection of the oxidized silicon interface to permit its use as a sensitive detector of charges induced in the silicon surface as a result of the density and distribution of mobile ions in the oxide above it. To measure the sodium ion barrier properties of another insulator by an analogous procedure, oxidized silicon samples would be coated with the film in question, a measured amount of sodium contamination would be placed on the surface, and a top electrode would be affixed to attempt to drift the sodium through the film with an applied dc bias voltage. Resulting inward motion of the sodium would be sensed by shifts in the MOS capacitance-voltage characteristic. 

Gamma-ray spectrometers use scintillator detectors. These spectrometers sense y-rays from all directions, and hence have large "footprints" (commonly hundreds of kilometers diameter) with sizes determined by orbital elevation above the surface. The y-rays come from depths of less than a meter in the target material. 

There are two categories of remote sensing, active and passive. Passive techniques utilise electromagnetic radiation emitted from or transmitted through the atmosphere, the radiation source being for example the black body emission from the earth s surface or solar and stellar irradiances. The most critical part of a passive remote sensing instrument is its detector. In contrast, active remote sensing systems have their own radiation source and a detector, for example, radar and lidar techniques. 

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