Coatings vapor detectors

Acoustic Wave Sensors. Another emerging physical transduction technique involves the use of acoustic waves to detect the accumulation of species in or on a chemically sensitive film. This technique originated with the use of quartz resonators excited into thickness-shear resonance to monitor vacuum deposition of metals (11). The device is operated in an oscillator configuration. Changes in resonant frequency are simply related to the areal mass density accumulated on the crystal face. These sensors, often referred to as quartz crystal microbalances (QCMs), have been coated with chemically sensitive films to produce gas and vapor detectors (12), and have been operated in solution as Hquid-phase microbalances (13). A dual QCM that has one smooth surface and one textured surface can be used to measure both the density and viscosity of many Hquids in real time (14). 

Microsensor Vapor Detectors Based on Coating Films of Phthalocyanine and Several of Its Metal Complexes... 

In previous sections, carbon materials serve either as sensing materials or as transducers. Many studies also use carbon-polymer composites, where carbon materials serve as both support and transducer. The polymer-coated FET discussed above is one example. Arrays of carbon black/organic polymer composite have also been demonstrated in many studies as chemiresistive vapor detectors... 

As an example, both monofunctional and multifunctional polymeric mercapto-esters were deposited onto optically smooth silicon wafers coated with vapor-deposited copper. The copper had been oxidized to Cu20, as verified by XPS. Infrared reflectance (RAIRS) at 81° (4 cm-1 resolution, 2000 scans) using an MCT detector yielded information on both the nature and the durability of the mercaptoester bond to the metal oxide film. A 16 cm l shift (1740— 1724 cm-1) was observed in the carbonyl absorption of stearyl thioglycolate (STG) deposited onto the Cu20 mirror. The absorption spectrum of the carbonyl region is illustrated in Fig. 11, both for the pure STG and the reacted monolayer. 

Host stabilizers are relatively nonvolatile so they do not vaporize during a thermal curing process. Unfortunately, their low volatility make GC analysis impossible for many stabilizers. HPLC works well for the UVA, but HALS are not easily detected by conventional UV or fluorescent detectors. High resolution capillary SFC was shown to be an ideal separation method for twenty-one polymer additives (17). We chose SFC to characterize stabilizers contained in automotive coatings. 

Figure 3. Bar chart display, detector coated with several exposed to various Structured rectangles reflect electrical vapor on a per mole basis for...

An electrochemical flow detector, based on four working electrodes coated with different permselective film types, for use in flow-injection measurements of multiple nitroaromatic explosives was recently developed [18]. The resulting array response (Fig. 4) offers unique fingerprints of such explosive compounds. Electrochemical devices are extremely attractive for designing such e-nose-type multi-electrode arrays that combine several partially selective electrodes and lead to a distinct response pattern (signature) for mixtures of organic vapors without prior separation [2],... 

The response of this detector is based on the fact that the frequency output from piezoelectric material is influenced by the weight of the coatings or layers on its surface. This effect has been used for many years to measure trace concentrations of water vapor in a gas and xylene vapor in air has been detected by this means at concentrations as low as lO g/ml. It was first introduced as a GC detecting system by King [22]. The detector consists of a quartz crystal (coated with a high boiling liquid) that is appropriately situated in an electronic circuit that causes it to oscillate at its natural frequency. The oscillation frequency is continuously monitored by a separate circuit. 

As already mentioned under transport detectors, Dugger [6] modified the moving wire detector to detect tritium and carbon. To detect carbon, the solute coated on the wire after evaporation of the solvent was oxidized to carbon dioxide and water. The radioactive carbon dioxide was passed to a Geiger counter and detected in the same manner as that described by James and Piper [7] which was discussed under GC radioactivity detectors in an earlier chapter. Tritium could be detected by passing the water vapor from the oxidation process over heated iron to reduce it to hydrogen and tritium, which was then also passed through a Geiger counter. 

Microsensor Systems, Inc. (Fairfax, VA) offers SAW devices and coatings for organophosphorus compounds, aromatic hydrocarbons, water vapor, and others. A four-sensor array consisting of four 158 MHz dual-delay-line oscillators and a portable gas chromatograph with an SAW detector are also available. 

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