Spectral interferences flame photometry

In environmental analysis, flame photometry is most widely used for the determination of potassium, which emits at 766.5 nm. It is also often used for the determination of sodium at 589.0 nm, although spectral interference problems (see Chapter 3) then may be encountered in the presence of excess calcium because of emission from calcium-containing polyatomic species. Molecular species are more likely to be found in cooler flames than in hotter flames. Some instruments use single, interchangeable filters, while others have three or more filters, for example for the determinations of potassium, sodium and lithium,... 

Sodium is still often determined by flame photometry, measuring the emission intensity of the doublet at around 589 nm, but care is necessary to make sure that excess calcium does not cause spectral interference (from molecular emission). This is unlikely to be a problem if AES is used, with a narrow spectral band-pass, and the intensity of emission at 589.0 nm from an air-acetylene flame is measured. However, at low determinant concentrations it is then advisable to add 2-5 mg ml 1 potassium or caesium as an ionization buffer. This is even more true if a nitrous oxide-acetylene flame is used for FES, although its use is rarely justified in environmental analyses because the additional sensitivity gained is rarely necessary. 

The development of fast and accurate procedures for the determination of calcium in biological materials represents one of the important early achievements of atomic absorption spectroscopy. The diflBculties encountered with calcium in emission flame photometry are well known (Dll, L6, S6, SIO), but spectral interferences and extreme dependency on flame temperature, serious obstacles in emission, are either nonexistent or of lower importance in absorption. Chemical interferences, however. 

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]. 

Regarding historical insight and descriptions of principles and fundamentals of flame atomic emission spectrometry, a chapter on flame photometry appeared in the first edition of Treatise on Analytical Chemistry (Vallee and Thiers 1965) covering the flame and burner, photometer/spec-trometer, fundamental discussion of excitation and processes within the flame, cation and anion interferences and handling of analytical samples. In an analogous, expanded, detailed and excellent treatment of EAES in the second edition of the Treatise on Analytical Chemistry, Syty (1981) discusses types of flames used for excitation, processes within flames, spectral, chemical and physical interferences and remedies. 

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]. 

Among the spectrometric methods used to determine metal concentrations, FAAS is particularly useful to perform water analysis (Figure 11.1). It is a relatively inexpensive method, which presents an adequate sensitivity sufficiently high for the determination of major metals in most of aquatic systems. Considering that most of atomic absorption instruments are also equipped to operate in an emission mode, large number of alkali metals (i.e., Na, K) are typically determined by flame photometry or flame atomic emission spectrometry (FAES) due to their relatively low excitation and simplicity of the emission techniques. This technique is relatively free from spectral interferences, and considering its versatility and simplicity of operation, it has become the most extensively used method for the determination of metals within water samples. 

The atomic absorption method is in some ways superior to the flame photometry method since spectral radiation interferences do not occur. 

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