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in Solution with Atomic Spectrometry
David A. McGregor, Kevin B. cull, Jav M. Gehlhausen. Anthonv S. Vi&", Mingin Wu,LimingZhang, and Jon W. Carnahan Department of Chemistry Northern illinois University DeKalb, IL 60115 In a 1984 REPORT, Browner and Boorn discussed factors associated with sample introduction in atomic spectroscopy ( 1 ) . Because of inherent problems that often restrict detection limits ani produce interference effects, the au thors questioned whether sample introduction was the Achilles' heel of atomic spectroscopy. I t is also well known, hut less often discussed, that another chink exists in the armor of this class of techniques. This chink is characterized by the difficulty of nonmetal determinations with solution samples. In this article, solution nonmetal determinations are addressed on a fundamental level, research in this direction is Characterized, and future implications are discussed.
Fundamental limitations of nonmetal atomic spectrometry In aeneral, atomic spectrometric techniques are'excellentmethodsfor metal determinations. However, the to determine nonmetals is not as well 0O03-27O0/86/A360-1O89~$01.5O/O
@ 1988 American Chemical Society
//j/SrRuME/VrA~/O/j/ developed. The fundamental characteristics of nonmetal atoms lie at the root of these problems. Because the species giving rise to metal and nonmetal elemental analysis methods are usually ions, ground state atoms, and/ or atoms in lower energy excited states, an understanding of the populations and characteristics of these species is
essential. Parts of the following a w m sion are based on equilibrium conditions, and in some cases, significant deviations from equilibrium predictions occur. Some of these apparent deviations will be noted as experimental methods are discussed. Excitation energies. Figure 1 illustrates the energies of the first excited
Figure 1. Energies ? first excited states of the atoms. Wavelengths correspond to the energy Of the transition from the first excited state to the ground state. the resonance transitions. ANALYTICAL CHEMISTRY, VOL. 60, NO. 19, OCTOBER 1, 1988 * 1089A
electronic states of the elements, and Table I provides other characteristics of a few select elements. In general, the lowest energy excited states of nonmetal atoms lie a t a greater energy above the ground state than metals. Therefore, if equal numbers of free nonmetal and metal atoms are exposed to the same temperature environment, a greater population of excited states will exist in the metal system. Because the population of the excited state is directly proportional to emission intensity, X metal atoms should emit more photons than X nonmetal atoms (assuming equal transition probabilities). As an example, let us consider the population of the first excited states of CI, Zn, and Na under the same excitation conditions, namely that populations of electronic states fit the Boltzmann distribution a t 7500 K. This temperature is chosen as a compromise between the highest temperatures noted in analytical plasmas (10,000 K)and typical temperatures of flames (ZOOO3000 K). The ratio of excited atoms (N') to ground state atoms (N) is given by:
where g* and g are the degeneracy of the excited and ground states, respectively, k is the Boltzmann constant, and E' is the energy difference between the excited and ground states. For the C1 transition that produces line emission a t 139.0 nm, g*/g = 312, E* = 8.92 eV, and N*/N = 1.5 X 10-6. For the Zn transition at 213.9 nm,g*/g = 3, E* = 5.80 eV, and N*IN = 3.8 X ForNaat589.6nm,ga/g=2,E*=2.10 eV, and N*/N = 7.8 X Values of N'lNfor Zn and Na are 250 and 52,000 times those of C1, respectively. Even when one considers that 99.5% of Na, 25%of Zn, and 1%of C1 are ionized a t this temperature and a typical plasma electron density of 1015/cm3,these calculations show that excitation of nonmetals is difficult as a result of thermodynamic considerations. A fundamental limitation of lower excited atom populations exists. Spectral accessibility. Because the atomic resonance transitions involve greater energy in nonmetals than in metals, line emissions from nonmetal resonance transitions lie in shorter wavelength regions of the spectrum. More often than not, these wavelengths are
