The Atomic Absorption Process

If a continuum is used as a radiant energy source and is passed through an atomic vapor, such as sodium, the spectrum obtained will have narrow absorption lines corresponding to several of the easily excited states of sodium. The absorption lines will be very sharp, indicating that only the energy corresponding to the particular electronic transition involved is absorbed. Two important interrelated considerations result from these observations (1) The best emission source for atomic absorption measurements is a spectral line of the same wavelength as the analyte element and 

FIGURE 10-1. Atomic absorption of a spectral line and of a continuum. 

The magnitude of the atomic absorption signal is directly related to the number of ground state atoms in the optical path of the spectrometer. Ground state atoms are produced from the sample material, usually by evaporation of solvent and vaporization of the solid particle followed by decomposition of the molecular species into neutral atoms. Normally these steps are carried out using an aspirator and flame. These are the same processes that are involved in flame emission spectroscopy as described in Chapter 9. When ground state atoms are produced, some excited state atoms also occur and, for easily ionizable elements, some ions and electrons are produced. 

The relative number of atoms in a particular energy state can be determined by use of the Boltzmann equation [refer to equation (2-23)]. Walsh has calculated these ratios for the lowest excited states of several typical elements and several flame temperatures. Table 9-2 indicates that the number of atoms in the ground state is much greater than the number in the lowest excited state at temperatures commonly used in atomic absorption spectroscopy. 

Since atomic absorption spectroscopy utilizes the ground state atom population for its measurements, it would appear that atomic absorption has a great advantage over flame emission in terms of detection limits and sensitivities of detection. An inspection of Appendix VIII, where detection limits are given for a number of elements for flame emission and atomic absorption, indicates this is not true. The reason for this apparent discrepancy lies in the relative stabilities of ground state and excited state atoms. An excited atom has a lifetime of the order of 10 -10 sec, and thus emits its energy very quickly after being excited. The usual flame emission source has an upward velocity of from 1 to 10 m/sec, so the excited atom will move only about 10 -10 m between the time of excitation and emission. 

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The atomic absorption process can be summarized as follows radiant energy is emitted from a hollow cathode lamp and passed through a flame. The flame sampling system produces ground-state atoms from the sample. The intensity of radiation before and after sample atomization is measured and the result shown on a meter readout or digitally. 

Figure 1 is a sketch of the atomic absorption process. In lA, the emission spectrum of a hollow-cathode lamp is shown, with emission lines whose half-width is typically about 0.02 A. For most practical purposes, the desired element in the sample can be considered as being able to absorb only the "resonance lines, whose wavelengths correspond to transitions from the minimum energy state to some higher level. In IB, the sample is shown to absorb an amount "x which corresponds to the concentration of the element of interest. As seen in Figure 1C, after the flame, the resonance line is reduced while the others are unaflFected. In order to screen out the undesired emission, the radiation is now passed through a filter or monochromator (ID) which is tuned to pass the line... 

In the original paper by Walsh describing the atomic absorption process he advocated use of hollow cathode sources since this source seemed to meet most of the criteria for a good source as described above. Such lamps had been developed prior to the advent of atomic absorption spectroscopy and were known to produce sharp, intense line spectra. 

The X-ray spectrum observed in PIXE depends on the occurrence of several processes in the specimen. An ion is slowed by small inelastic scatterings with the electrons of the material, and it s energy is continuously reduced as a frmction of depth (see also the articles on RBS and ERS, where this part of the process is identical). The probability of ionizii an atomic shell of an element at a given depth of the material is proportional to the product of the cross section for subshell ionization by the ion at the reduced energy, the fluorescence yield, and the concentration of the element at the depth. The probability for X-ray emission from the ionized subshell is given by the fluorescence yield. The escape of X rays from the specimen and their detection by the spectrometer are controlled by the photoelectric absorption processes in the material and the energy-dependent efficiency of the spectrometer. 

Ozin et al. 107,108) performed matrix, optical experiments that resulted in the identification of the dimers of these first-row, transition metals. For Sc and Ti (4s 3d and 4s 3d, respectively), a facile dimerization process was observed in argon. It was found that, for Sc, the atomic absorptions were blue-shifted 500-1000 cm with respect to gas-phase data, whereas the extinction coefficients for both Sc and Scj were of the same order of magnitude, a feature also deduced for Ti and Ti2. The optical transitions and tentative assignments (based on EHMO calculations) are summarized in Table I. 

In the vicinity of the atomic absorption edges, the participation of free and bound excited states in the scattering process can no longer be ignored. The first term in the interaction Hamiltonian of Eq. (1.11) leads, in second-order perturbation theory, to a resonance scattering contribution (in units of classical electron scattering) equal to (Gerward et al. 1979, Blume 1994)4... 

An on-stream atomic absorption method for the determination of zinc and manganese in flotation liquors containing calcium sulphate has been reported [27]. The nebuliser to an air/acetylene flame was continuously fed, by gravity, with a portion of the process stream buffered with EDTA to overcome calcium sulphate interference. The atomic absorption monitor operated continuously for 3h before salting-up of the burner occurred. 

Radiation absorbed by atoms under conditions used in atomic absorption spectrometry may be re-emitted as fluorescence. The fluorescent radiation is characteristic of the atoms which have absorbed the primary radiation and is emitted 1n all directions. It may be monitored in any direction other than in a direct line with radiation from the hollow-cathode lamp which ensures that tha detector will not respond to the primury absorption process nor to unabsorbed radiation from the lamp. The intensity of fluorescent emission is directly proportional to the concentration of the absorbing atoms but it is diminished by collisions between excited atoms and other species within the flame, a process known as quenching. Nitrogen and hydrocarbons enhance quenching, and flames incorporating either should be avoided or their effect modified by dilution with argon. 

It is important to note that in solids distances between nearest atoms can vary in different directions, and hence the minimum of the valley may not occur at kx=ky = k = 0, but at some point defining a specific direction, as shown in Figure 2.4B for a crystalline solid. In an optical transition, both energy and momentum must be conserved. Because the momentum of a photon, h/X (X is the wavelength of light which is typically thousands of angstroms), is very small compared to the crystal momentum h/a (a is the lattice constant, typically a few Angstroms), the photon-absorption process should conserve the electron momentum. 

The various states of coordination arc associated with the surface cxcitons at different energies. If the oxide ion were completely isolated from the lattice, the optical absorption process would correspond to the simple electronic excitation of the oxide ion Eq. (1). However, the experimental results shown in Table I (66) indicate that the energy required for this process decreases as the atomic number of the cation increases, which is consistent with the observation that a charge transfer is involved [(Eq. 2)]. Such a process may involve more than one cation because of the delocalization of the electron, but it may be increasingly regarded as localized as the coordination of the ions is reduced. 

When the energy supplied is sufficient to excite and ionise some of the atoms, the atomisation process docs not necessarily come to an end with the atoms in their ground state. Ions absorb at different wavelengths so the atomic absorption at the resonance wavelength is reduced. Consequently, when an excessive amount of energy eliminates atoms in their ground state, there is ionisation interference. 

The nebulizer and burner system is probably the most important component of the atomic-absorption or emission spectrophotometer, because it is imperative that neutral (un-ionized) atoms of the test element be presented to the optical system. When the sample solution passes into the flame, it must be in the form of small droplets. The process of breaking down a solution into a fine spray is known as nebulization. Nebulization is generally carried out with the support or oxidant gas. 

The size of these deviations fid E) from the atomic absorption fiod E) amounts to roughly 10% of the edge jump A d, which describes the difference between the mean absorption at energies above the edge and below the edge. This 10% corresponds to the part of elastically backscattered electrons. The other part of the excited electrons experiences forward scattering or are lost by inelastic scattering processes. They do not contribute to variations in the absorption. 

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