Detectors diffraction grating monochromator

The intensity of fluorescence is directly proportional to the concentration of the fluorescent compound. If the target compound is not fluorescent, then it is converted into a fluorescent derivative by reaction with a suitable (nonfluorescent) reagent. The fluorescence emitted by the fluorescent compound is measured using a spectrofluorometer [6]. Most of the modem spectrofluorometers employ diffraction grating monochromators to select the appropriate wavelengths for maximum excitation and emission. The basic components of a fluorometer are a light source, an excitation monochromator, a sample holder, an emission monochromator, and a fluorescence detector as shown in Fig. 6.6. 

The first requirement is a source of infrared radiation that emits all frequencies of the spectral range being studied. This polychromatic beam is analyzed by a monochromator, formerly a system of prisms, today diffraction gratings. The movement of the monochromator causes the spectrum from the source to scan across an exit slit onto the detector. This kind of spectrometer in which the range of wavelengths is swept as a function of time and monochromator movement is called the dispersive type. 

Figure 8.28 shows how the X-rays fall on the solid or liquid sample which then emits X-ray fluorescence in the region 0.2-20 A. The fluorescence is dispersed by a flat crystal, often of lithium fluoride, which acts as a diffraction grating (rather like the quartz crystal in the X-ray monochromator in Figure 8.3). The fluorescence may be detected by a scintillation counter, a semiconductor detector or a gas flow proportional detector in which the X-rays ionize a gas such as argon and the resulting ions are counted.

The basic instrumentation used for spectrometric measurements has already been described in the previous chapter (p. 277). Methods of excitation, monochromators and detectors used in atomic emission and absorption techniques are included in Table 8.1. Sources of radiation physically separated from the sample are required for atomic absorption, atomic fluorescence and X-ray fluorescence spectrometry (cf. molecular absorption spectrometry), whereas in flame photometry, arc/spark and plasma emission techniques, the sample is excited directly by thermal means. Diffraction gratings or prism monochromators are used for dispersion in all the techniques including X-ray fluorescence where a single crystal of appropriate lattice dimensions acts as a grating. Atomic fluorescence spectra are sufficiently simple to allow the use of an interference filter in many instances. Photomultiplier detectors are used in every technique except X-ray fluorescence where proportional counting or scintillation devices are employed. Photographic recording of a complete spectrum facilitates qualitative analysis by optical emission spectrometry, but is now rarely used. 

Fig. 6. Diagrammatic representation of a spectrofluorimeter. 1, radiation source 2, excitation monochromator (filter, prism, or diffraction grating) 3, quartz sample cell 4, emission monochromator 5, photomultiplier tube detector 6, amplifier 7,...

---