Principal Detector Characteristics

In order to evaluate a detector for use in liquid chromatography (LC), accurate performance criteria or specifications must be provided by the manufacturers. This is necessary to assess the pertinence of a given detector for a particular chromatographic separation, and/or to permit a rational comparison with other detectors. More important, such performance criteria will allow the optimum column to be designed to achieve a particular separation for which the detector is to be used. It follows that the performance data provided for each detector must be presented in a standard form and given in standard units which will be consistent between detectors that function on widely different principles. The principal characteristics of a detector that will fulfill these requirements are given as follows. 

In the past, there has been much confusion in the field of LC, not only with respect to the units in which the above specifications should be given, but even in the exact definition of these criteria. This confusion has resulted partly from the use of criteria developed for other instrumental devices which are not applicable to LC detectors and partly due to some manufacturers selecting ambiguous criteria in order to present their products in the best possible light. It follows that the various criteria and specifications of a given detector must be unambiguously defined and very carefully described. 

As already stated, a detector can be either a mass sensitive or concentration sensitive device, that is to say, the detector 

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Every mass spectrometer consists of four principal components (Fig 1) (1) the source, where a beam of gaseous ions are produced from the sample (2) the analyzer, where the ion beam is resolved into its characteristic mass species (3) the detector, where the ions are detected and their intensities measured (4) the sample introduction system to vaporize and admit the sample into the ion source. There is a wide variety in each of these components and only those types which are relevant to analytical and organic mass spectrometry will be emphasized in this survey. The instrumentation... 

Principles and Characteristics Mass-spectral analysis methods may be either indirect or direct. Indirect mass-spectral analysis usually requires some pretreatment (normally extraction and separation) of the material, to separate the organic additives from the polymers and inorganic fillers. The mass spectrometer is then used as a detector. Direct mass-spectrometric methods have to compete with separation techniques such as GC, LC and SFC that are more commonly used for quantitative analysis of polymer additives. The principal advantage of direct mass-spectrometric examination of compounded polymers (or their extracts) is speed of analysis. However, quite often more information can be... 

In this final section, we summarize the operation and characteristics of the principal vacuum tube and solid state detectors that are available for red/near-IR fluorescence studies. These include conventional photomultipliers, microchannel plate versions, streak cameras, and various types of photodiodes. Detector applicability to both steady-state and time-resolved studies will be considered. However, emphasis will be placed on photon counting capabilities as this provides the ultimate sensitivity in steady-state fluorescence measurements as well as permitting lifetime studies. 

In addition to these principal requirements, there are further characteristics that affect the performance and applicability of analysers and detectors. The signal should depend as little as possible on the liquid flow-rate and temperature. With continuous monitors, the sensor should be selective for the test component. On the other hand, with chromatographic detectors, the sensor should respond to all eluted components with the same sensitivity. 

Except when fitting peaks containing very few counts, the assumption of a gaus-sian peak shape is clearly inadequate. Figure 6.7 illustrates the principal effects contributing to the peak shape. If the resolution broadening caused by the x-ray spectrometer is deconvoluted from the spectrum, the characteristic x-ray line approaches a delta function with a width limited only by the natural line width. This narrow line is superimposed on the background from the specimen. Occasionally, when the characteristic photon is detected in the Si(Li) detector, not all... 

Nowadays, electronic noses have an increasingly prominent role in the field of perfume analysis. These instruments are devices composed of an array of nonselective gas sensors that can act in a manner similar to that of real biological noses. In this way, after exposure to analyte vapors, the analyte molecules diffuse and pass over the detectors, producing characteristic signal patterns, which are conveniently processed using multivariate data analysis (principal... 

Today, activation analysis procedures rely almost exclusively on the detection of gamma rays for simultaneous qualitative identification and quantitative recording of the decay of the activation products. Exceptions are in the use of purely or predominantly -emitting activation products, such as or Si, in conjunction with radiochemical separation to achieve specificity (see O Sect. 30.5). INAA procedures utilize a variety of gamma-ray spectrometry systems the principal components of which are illustrated in O Fig. 30.3. In this section, the components and characteristics of gamma-ray spectrometers used for INAA will be briefly described. More technical details can be found in the excellent monographs available (Knoll 2000 Gilmore 2008 Debertin and Helmer 1988) and in O Chap. 48 of Vol. 5 on radiation detectors. 

The flame photometric detector is the principal component in the determination of sulphur compounds for which it offers a selectivity of about five orders of magnitude with respect to hydrocarbons. The selective sulphur detection is based on the formation of electronically excited S2 molecules in a hydrogen-rich flame. These short-lived species revert to their ground state and emit characteristic molecular band spectra with peak wavelengths at 384 and 394 nm. This chemiluminescent radiation passes an optical filter and is monitored by a UV-sensitive photomultiplier. 

Figure 1 is a diagram of the apparatus we use which has the characteristics to do what we have described above, it consists of three principal parts the detonation vacuum chamber, the molecular beam skimmer and differential pumping chambers, and the quadrupole mass spectrometer detector. 

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