Absorption of Radiant Energy by Atoms

Each element has a specific number of electrons located in an orbital structure that is unique to each element. The lowest energy electronic configuration of an atom is called 

The calculated wavelengths Ai, A2, and A3 all arise from transitions from the ground state to excited states. Absorption lines due to transitions from the ground state are called resonance lines. It is possible for an electron in an excited state to absorb radiant energy and move to an even higher excited state in that case, we use the AE values for the appropriate energy levels involved. As we will see, in AAS most absorptions do arise from the ground state. 

Under the temperatures encountered in the atomizers used in commercial AAS systems, a large majority of the atoms exist in their lowest possible energy state, the ground state. Very few atoms are normally in the higher energy states. The ratio of atoms in an upper excited state to a lower energy state can be calculated from the Maxwell-Boltzmann equation (also called the Boltzmann distribution)  

For example, it can be calculated from the Boltzmann distribution that if zinc vapor (Zn° gas) with resonance absorption at 213.9 nm is heated to 3000 K, there will be only one atom in the first excited state for every lO atoms in the ground state. Zinc atoms need a considerable amount of energy to become excited. On the other hand, sodium 

AAS is useful for the analysis of approximately 70 elements, almost all of them metal or metalloid elements. Grotrian diagrams correctly predict that the energy required to reach even the first excited state of nonmetals is so great that they cannot be excited by normal UV radiation ( 190 nm). The resonance hnes of nonmetals lie in the vacuum UV region. Commercial AAS systems generally have air in the optical path, and the most common atomizer, the flame, must operate in air. Consequently, using flame atomizers, atomic absorption cannot be used for the direct determination of nonmetals. However, nonmetals have been determined by indirect methods, as will be discussed in the applications section. 

is the number of atoms in the upper state No is the number of atoms in the lower state 

AE is the energy difference between the upper and lower states (J) k is the Boltzmann constant = 1.381 x 10 J/K T is the absolute temperature (K) 

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Each element has a unique set of permitted electronic energy levels because of its unique electronic structure. The wavelengths of light absorbed or emitted by atoms of an element are characteristic of that element. The absorption of radiant energy by atoms forms the basis of AAS, discussed in Chapter 6. The absorption of energy and the subsequent emission of radiant energy by excited atoms form the basis of AES and atomic fluorescence spectroscopy, discussed in Chapter 7. 

Walsh, in 1955, described the theoretical principles of atomic absorption spectroscopy10). Briefly, it can be defined as the absorption of radiant energy by ground state atomic vapor. There are several ways of obtaining atomic vapor, but aspiration of a solution into a flame is the most conventient and most widely used method. 

Step-VI The absorption of radiant energy by some atoms into their higher energy levels enable them to radiate energy (atomic absorption) measured by Atomic Absorption Spectroscopy (AAS). 

Although atomic absorption is not entirely analogous to absorption of radiant energy by, for example, a colored solution, the Beer-Lambert relation is valid for atomic absorption processes. The Beer-Lambert law relating absorption of radiant energy to path length and concentration is... 

Excited state. An energy-rich state of an atom or a molecule, produced by the absorption of radiant energy. 

Absorption spectroscopy records depletion by the sample of radiant energy from a continuous or frequency-tunable source, at resonance frequencies that are characteristic of various energy levels ia atoms or molecules. The basic law of absorption, credited to Bouguer-Lambert-Beer, states that ia terms of the iacident, Jq, and transmitted, light iatensities, the absorbance, M (or transmittance, T), is given by equation 1 ... 

I. The Existence of Stationary Stales. An atomic system can exist in certain stationary states, each one corresponding to a definite value of the energy W of the system and transition from one stationary state to another is accompanied by the emission or absorption as radiant energy, or the transfer to or from another system, of an amount of energy equal to the difference in energy of the two states. 

Flame atomic absorption spectrometry requires a means by which an aqueous solution containing metal ions can be aspirated into a reducing flame environment by which atomic Mg or Ca vapor is formed. Photons from the characteristic Mg emission of a hollow-cathode lamp (HCL) are absorbed by ground-state Mg atoms present in the approximately 2300°C air-acetylene flame. The amount of radiant energy absorbed as a function of concentration of an element in the flame is the basis of AA and follows Beer s law. In contrast to molecular absorption in solution, atomic spectra consist of lines and originate either from atomic absorption or atomic emission processes, which are depicted schematically below (1, Chap. 9) ... 

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