Monochromator, fluorescence spectrometry

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. 

Spectrometers that use phototubes or photomultiplier tubes (or diode arrays) as detectors are generally called spectrophotometers, and the corresponding measurement is called spectrophotometry. More strictly speaking, the journal Analytical Chemistry defines a spectrophotometer as a spectrometer that measures the ratio of the radiant power of two beams, that is, PIPq, and so it can record absorbance. The two beams may be measured simultaneously or separately, as in a double-beam or a single-beam instrument—see below. Phototube and photomultiplier instruments in practice are almost always used in this maimer. An exception is when the radiation source is replaced by a radiating sample whose spectrum and intensity are to be measured, as in fluorescence spectrometry—see below. If the prism or grating monochromator in a spectrophotometer is replaced by an optical filter that passes a narrow band of wavelengths, the instrument may be called a photometer. 

Fluorescence excitation and emission spectra of the two sodium D lines in an air-acetylene flame, (a) In the excitation spectrum, the laser was scanned, (to) In the emission spectrum, the monochromator was scanned. The monochromator slit width was the same for both spectra. [From s. J. Weeks, H. Haraguchl, and J. D. Wlnefordner, Improvement of Detection Limits in Laser-Excited Atomic Fluorescence Flame Spectrometry," Anal. Chem. 1976t 50,360.]... 

The stability of the wavelength setting of a monochromator can be a problem in high resolution spectrometry. This difficulty has been overcome by the use of the resonance monochromator (S24), consisting of a hollow cathode lamp modified to produce only an atomic vapor. The vapor is irradiated with the light to be analyzed and fluorescence occurs at the resonant wavelength of the cathode element. The intensity of the fluorescence is proportional to the component of that wavelength in the primary radiation. 

In the case of atomic absorption and atomic fluorescence the selectivity is thus already partly realized by the radiation source delivering the primary radiation, which in most cases is a line source (hollow cathode lamp, laser, etc.). Therefore, the spectral bandpass of the monochromator is not as critical as it is in atomic emission work. This is especially true for laser based methods, where in some cases of atomic fluorescence a filter is sufficient, or for laser induced ionization spectrometry where no spectral isolation is required at all. 

Raman spectra of many pure colorless compounds can easily be measured with an instrument incorporating a visible laser, scanning double monochromator, and PMT. However, when spectroscopists attempted to measure the corresponding spectra of real-world samples with this type of instrument, good spectra were rarely obtained. The root cause of this difficulty was fluorescence by the sample, either because of its intrinsic electronic spectrum or, more likely, because of low levels of fluorescent impurities. Even for nonfluorescent samples, it often took at least 30 minutes to measure a reasonably noise-free Raman spectrum. Thus, with the exception of a few spectroscopists in industrial labs who could obtain their information in no other way, Raman spectrometry was considered to be largely... 

The instrumentation for detecting and measuring fluorescence is similar to that for absorption spectrometry, except that two dispersion monochromators are needed, one for the excitation wavelength and the other for analyzing the resulting fluorescence. Note that the emitted radiation is detected at 90° to the excitation radiation, as shown in Figure 4. 

Some instruments have been developed for both atomic absorption and atomic fluorescence. However, a powerful source e.g. a laser is required for the latter spectrometry. In principle, when a gaseous metal atom is excited by absorption of radiation, it emits fluorescence radiation when it reverts to the ground state. This can be recorded in a monochromator/detector set up, not unlike atomic absorption. (J.Chem. Educ., 59, 1982,909 895 AnalChem., 53,1981,332A 1448A 54, 1082, 553, 1006A). 

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