Monochromators flame photometry

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. 

Flame spectrophotometry uses a monochromator to resolve the light. Flame "spec-trography is emission spectrography using flame excitation and photographic recording Refs 1) F. Burriel-Marti 8c J. Ramirez-Munoz, "Flame Photometry , Elsevier, NY (1957) 2) R. Herrmann 8c C.T.J. Alkemade,... 

Filter monochromators are now used almost only for flame photometry. They make use of interference filters, which may have a fairly low spectral bandpass (less than a few nm). However, it is also possible to use such filters for dynamic measurements of line and background intensities, and for transient signals, as occur in gas chromatography. The use of oscillating filters has been described, where the wavelength bandpass is slightly shifted by inclining them towards the radiation beam [65]. 

Filler monochromators are of use only for flame photometry. They make use of interference filters, which often have a spectral bandpass of a few nanometers or less. Multiplex spectrometers include Hadamard transform spectrometers and Fourier transform spectrometers, and are especially useful where very stable sources are needed. Hadamard transform instruments make use of a coding of the spectrum produced by recombining the information with the aid of a slit mask which scans the spectrum [48]. 

This is practically similar to that of emission flame photometry. An important point of difference is the need to have a radiation source. It is practically impossible to isolate a particular resonance wavelength from a continuous source by using a prism or a diffraction grating or both simultaneously. This problem was solved with the development of hollow-cathode discharge lamps. Such lamps produce monochromatic radiation characteristic of the element analyzed. In these lamps the cathode is a hollow tube which is lined by the element in question. The lamp will thus emit monoehromatic radiation characteristic of the emission spectrum of the element Involved. Such lamps have now become commercially available for a long range of elements. In less sophisticated instruments, a continuous discharge lamp with double monochromators is used. 

It can be seen now that although calcium presents many problems in flame photometry, methods can usually be found provided suitable equipment is available. A fairly hot or hot flame is always indicated and ideally a recording monochromator instrument is required if samples of any complexity are to be examined. 

The coloured radiations produced when the salts of certain elements are introduced into a flame have been used for identification purposes for many years. In flame photometry a solution of a salt is introduced continuously and at a constant rate so that the intensity of the radiation is a constant value which when measured forms the basis of a quantitative procedure. Many instruments have been built for this purpose and the range of work which can be accomplished depends on the design. In the simplest equipment a cool air/coal gas or air/propane flame is used so that few elements are excited and from the simple spectra the required radiations can be isolated by means of filters. The intensity of the transmitted radiation is then measured with a photocell coupled to a galvanometer or amplifier and meter. Such instruments are used for the determination of the alkali metals and in certain cases the alkaline earths. The more versatile instruments employ a monochromator for isolating the required wavelengths and so hotter flames can be used which excite many more elements and therefore produce much more complex spectra. A sensitive detector is required and a photomultiplier coupled to an amplifier satisfies this need. 

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