Flame emission and absorption

Spectrometers comparable to those available from Perkin-Elmer Corporation, Instrument Division, Main Avenue MS-12, Norwalk, Connecticut-06856, 

Varian Associates, 611 Hansen Way, Palo Alto, California-94303, and Instrumentation Laboratories, Inc., Jonspin Road, Wilmington, Massachussets-01887, will provide adequate signals for those elements that can be determined by AES or AAS using air + C2H2 or N20 + C2H2 flames. 

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Dissociation equilibria that involve anions other than oxygen may ilso influence flame emission and absorption. Tor example, the line intensity for stxlium is markedly decreased by the presence of I K l. A likely explanation is the mass-action effect on the equilibrium... 

The symbols used are as follows / = angle of incidence, 0 = angle of diffraction (or reflectance), p = blaze angle of the grating, d = grating spacing. (Modified from Dean, J.A. and Rains, T.C., eds.. Flame Emission and Absorption Spectrometry, Vol. 2, Marcel Dekker, Inc., New York, 1971. Used with permission.)... 

All of these methods require that the sample be dissolved, which necessarily causes some dilution and a consequent loss of sensitivity. In addition, matrix effects can cause serious error unless standards similar in composition to the sample can be prepared. The use of organic solvents, rather than water, often improves the limits of detection, occasionally by an order of magnitude or more flame emission and absorption measurements are often especially useful in the determination of impurities following solvent extraction. 

From J. A. Dean and T. C. Rains, Standard Solutions for Flame Spectrometry, in Flame Emission and Atomic Absorption Spectrometry, J. A. Dean and T. C. Rains (Eds.), Vol. 2, Chap. 13, Marcel Dekker, New York, 1971. 

Nitric oxide and OF2 inflame on contact emission and absorption spectra of the flame have been studied (24). Oxygen difluoride oxidizes SO2 to SO, but under the influence of uv kradiation it forms sulfuryl fluoride [2699-79-8] SO2F2, and pyrosulfuryl fluoride [37240-33-8] S20 F2 (25). Photolysis of SO —OF2 mixtures yields the peroxy compound FSO2OOF [13997-94-9] (25,26). 

Both emission and absorption spectra are affected in a complex way by variations in atomisation temperature. The means of excitation contributes to the complexity of the spectra. Thermal excitation by flames (1500-3000 K) only results in a limited number of lines and simple spectra. Higher temperatures increase the total atom population of the flame, and thus the sensitivity. With certain elements, however, the increase in atom population is more than offset by the loss of atoms as a result of ionisation. Temperature also determines the relative number of excited and unexcited atoms in a source. The number of unexcited atoms in a typical flame exceeds the number of excited ones by a factor of 103 to 1010 or more. At higher temperatures (up to 10 000 K), in plasmas and electrical discharges, more complex spectra result, owing to the excitation to more and higher levels, and contributions of ionised species. On the other hand, atomic absorption and atomic fluorescence spectrometry, which require excitation by absorption of UV/VIS radiation, mainly involve resonance transitions, and result in very simple spectra. 

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. 

Dean, J.A. and T.C. Bains Eds., Flame Emission Atomic Absorption Spectroscopy , Vol. II., Components... 

One often unsuspected source of error can arise from interference by the substances originating in the sample which are present in addition to the analyte, and which are collectively termed the matrix. The matrix components could enhance, diminish or have no effect on the measured reading, when present within the normal range of concentrations. Atomic absorption spectrophotometry is particularly susceptible to this type of interference, especially with electrothermal atomization. Flame AAS may also be affected by the flame emission or absorption spectrum, even using ac modulated hollow cathode lamp emission and detection (Faithfull, 1971b, 1975). 

Analysis. The bright scarlet flame color of Sr indicates that atomic emission and absorption methods wiU be good for its analysis. Sr is quantitatively determined by colorimetry down to 200 ppm using chloranrlic acid, by atomic absorption spectroscopy (AAS) to 100 ppb, to 1 ppb by electrothermal absorption spectroscopy (ETAS), and to 0.1 ppb by inductively-coupled plasma emission spectroscopy (ICPES) and inductively-coupled plasma mass spectroscopy (ICPMS). A spot test for Sr which extends to 40 ppm is provided by K2Cr04 and sodium rhodizonate. 

The flame is a complex medium in dynamic equilibrium that must be perfectly controlled. It is characterised by its chemical reactivity for a given maximum temperature (Table 14.2) and by its spectrum. Free radicals present in the flame have an emission and absorption spectrum in the near UV and this can sometimes interfere with the measurement of some elements. Thus, the observation height of the flame must be adjusted for some elements. 

D. The measurement of Li in brine (salt water) is used by geochemists to help determine the origin of this fluid in oil fields. Flame atomic emission and absorption of Li are subject to interference by scattering, ionization, and overlapping spectral emission from other elements. Atomic absorption analysis of replicate samples of a marine sediment gave the results in the table below. 

Potassium is one of the more easily ionized metals and the type of flame used will be of even greater influence than in sodium work. Since the degree of ionization depends also on other solution constituents, i.e., alkali metals, significant interference from sodium can be expected. The relative enhancement of potassium emission and absorption is shown by Baker and Garton (B2). Atomization of potassium in the fiame, however, is not only reduced by ionization but also by compound formation, notably hydroxide. 

Analytical applications have been found for all parts of the electromagnetic spectrum ranging from microwaves through visible radiation to gamma (y) rays (Table 1). The emission and absorption of electromagnetic radiation are specific to atomic and molecular processes and provide the basis for sensitive and rapid methods of analysis. There are two general analytical approaches. In one, the sample is the source of the radiation in the other, there is an external source and the absorption or scattering of radiation by the sample is measured. Emission from the sample may be spontaneous, as in radioactive decay, or stimulated by thermal or other means, as in flame photometry and fluorimetry. Both approaches can be used to provide qualitative and quantitative information about the atoms present in, or the molecular structure of, the sample. 

It remained, however, for Gustav Kirchhoff and Robert Wilhelm Bunsen in 1859 and 1860 to explain the origin of the Fraunhofer lines. Bunsen had invented his famous burner (Figure 24F-2) a few years earlier, which made possible spectral observations of emission and absorption phenomena in a nearly transparent flame. Kirchhoff con-... 

Both emission and absorption spectra are affected in a complex way by variations in flame temperature. In both cases, higher temperatures increase the total atom population of the flame and thus the sensitivity. With certain elements, such as the alkali metals, however, this increase in atom population is more than offset by the loss of atoms by ionization. 

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