Detector obscuration

Figure 4.2.4 Optical smoke detector - obscuration type (Manual of Firemanship. Courtesy The Controller, the Stationery Office)...

Since 1970 the subject of amoiphous semiconductors, in particular silicon, has progressed from obscurity to product commercialisation such as flat-panel hquid crystal displays, linear sensor arrays for facsimile machines, inexpensive solar panels, electrophotography, etc. Many other appHcations are at the developmental stage such as nuclear particle detectors, medical imaging, spatial light modulators for optical computing, and switches in neural networks (1,2). 

This is a method which is very attractive in principle and which has been applied to yield approximate barriers for a number of molecules. There are, however, difficulties in its use. In the first place, it is not easy to measure the intensities of microwave lines with accuracy. There are unsolved problems of saturation, reflections in the wave guide, and variation of detector efficiency with frequency which are presumably reponsible for the fact that measurements made with ordinary wave guide spectrometers are not very reproducible. In addition, both the spectral lines may be split into components by tunnelling from one potential minimum to another and this splitting, even though it is not resolved, can alter the apparent intensity. Furthermore, it is often difficult to find pairs of lines such that neither is obscured by Stark lobes from the other. 

Detector Sensitivity or the Minimum Detectable Concentration has been defined as the minimum concentration of an eluted solute that can be differentiated unambiguously from the noise. The ratio of the signal to the noise for a peak that is considered decisively identifiable has been arbitrarily chosen to be two. This ratio originated from electronic theory and has been transposed to LC. Nevertheless, the ratio is realistic and any peak having a signal to noise ratio of less than two is seriously obscured by the noise. Thus, the minimum detectable concentration is that concentration that provides a signal equivalent to twice the noise level. Unfortunately, the concentration that will provide a signal equivalent to twice the noise level will usually depend on the physical properties of the solute used for measurement. Consequently, the detector sensitivity, or minimum detectable concentration, must be quoted in conjunction with the solute that is used for measurement. 

The reason for separating benzene and benzyl acetate in 2.6 seconds remains (to say the least) obscure and figure 6 is obviously an example of "Chromatography Show Biz". Nevertheless, it does demonstrate that columns can be designed and detectors developed that can provide extremely fast analyses. 

Smoke detectors are employed where the type of fire anticipated and equipment protection needs a faster response time than heat detectors. A smoke detector will detect the generation of the invisible and visible products of combustion before temperature changes are sufficient to activate heat detectors. The ability of a smoke detector to sense a fire is dependent on the rise, spread, rate-of-bum, coagulation and air movement of the smoke itself. Where the safety of personnel is a concern, it is crucial to detect a fire incident at its early stages because of the toxic gases, lack of oxygen that may develop, and obscuration of escape routes. Smoke detection systems should be considered when these factors are present. 

Photoelectric detectors are of the spot type or light-scattering type. In each, visible products of combustion partially obscure or reflect a beam of light between its source and a photoelectric receiving element. The disruption of the light source is detected by the receiving unit and as a result... 

Often chemisorption produces reaction products which are swept out of the sample cell to the detector. In such cases, to prevent obscuring the adsorption signal, it is necessary to remove the reaction products from the flow stream. Depending upon their nature they can often be removed chemically or by condensation in a cold trap. 

The increase or appearance of peaks has several possible explanations. Addition of contaminants by the base during extraction is minimal, as proven by GC analysis of blank ether extractions of the buffer used in the procedure. The base could convert compounds and thus account for some of the new peaks seen. However, the hump obscures the normal column base line, and the removal of the hump by base extraction may cause previously undetected or unintegrated compounds to be measured. The hump-associated retention times have more detector noise than the other retention times in the chromatograms. The integrator usually defines the detector noise as a peak unless the sensitivity of the integrator is decreased (22). 

HPLC with immunoassay detection is useful for the analysis of those metabolites which are difficult to isolate from biological fluids by extraction (e.g. glucuronides). Such polar compounds cannot be observed at low concentrations with conventional detectors as they are obscured by endogenous compounds. HPLC with immunoassay detection can be applied to the determination of cannabin-oids, opiates, lysergide, and cardiac glycosides in biological fluids (A. C. Mof at,Analyl. Proc., 1981, IS, 115-116). 

Spectroscopy. Infrared and UV spectra were run on the 5 latex samples described in Table I to provide information for interpretation of subsequent application test results, and as a guide for GPC detector settings. In both cases, the absorption of the ferrocene moiety is largely obscured by the absorption of the parent polymer, thus limiting these techniques for definite analy-... 

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