Direct Determination of Nonmetals --τ
in Solution with Atomic Spectrometry
David A. McGregor, Kevin B. Cull, Jay M. Gehlhausen, Anthony S. Viscomi, Mingin Wu, Liming Zhang, and Jon W. Carnahan Department of Chemistry Northern Illinois University DeKalb, IL 60115
In a 1984 REPORT, Browner and Boom discussed factors associated with sam ple introduction in atomic spectrosco py (2). Because of inherent problems that often restrict detection limits and produce interference effects, the au thors questioned whether sample in troduction was the Achilles' heel of atomic spectroscopy. It is also well known, but 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.
INSTRUMENTATION developed. The fundamental charac teristics 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 discus sion are based on equilibrium condi tions, and in some cases, significant de viations from equilibrium predictions occur. Some of these apparent devi ations will be noted as experimental methods are discussed. Excitation energies. Figure 1 illus trates the energies of the first excited
Fundamental limitations of nonmetal atomic spectrometry In general, atomic spectrometric tech niques are excellent methods for metal determinations. However, the ability to determine nonmetals is not as well 0003-2700/88/A360-1089/$01.50/0 © 1988 American Chemical Society
Figure 1. Energies of the 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 · 1089 A
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 at a greater energy above the ground state than metals. There fore, 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 di rectly proportional to emission intensi ty, X metal atoms should emit more photons than X nonmetal atoms (as suming equal transition probabilities). As an example, let us consider the population of the first excited states of CI, Zn, and Na under the same excita tion conditions, namely that popula tions of electronic states fit the Boltzmann distribution at 7500 K. This tem perature is chosen as a compromise between the highest temperatures not ed in analytical plasmas (10,000 K) and typical temperatures of flames (20003000 K). The ratio of excited atoms (N*) to ground state atoms (iV) is given by:
Atom Na Κ Mg Ca Cu Zn Ν 0 F Ρ S Cl Br I
Resonance level (eV) (2)
Emission wavelength (nm)
Ionization potential (eV) ( 3 )
Percent Ionized ( 4 ) a
2.10 1.61 4.35 2.93 3.78 5.80 10.33 9.52 12.98 6.94 6.86 8.92 8.04 6.77
589 770 285 422 327 214 120 130 95 178 181 139 163 183
5.14 4.34 7.65 6.11 7.73 9.39 14.53 13.62 17.42 10.49 10.36 12.97 11.81 10.45
99.5 99.5 98 99 90 75 0.1 0.1 0.0009 33 14 0.9 5 29
" Percent ionized figures based on a temperature of 7500 K and an electron density of 1 X 10* 5 / cm 3 as calculated by Houk (4). Values for Na and Κ have been recalculated to obtain more significant figures.
30 -
29.0 eV 2.6 -•«—-479.5 nm eV
-E*lkT
g
where g* and g are the degeneracy of the excited and ground states, respec tively, k is the Boltzmann constant, and E* is the energy difference be tween the excited and ground states. For the CI transition that produces line emission at 139.0 nm, g*/g = 3/2, 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*/N = 3.8 Χ ΙΟ' 4 . For Na at 589.6 nm, g*/g = 2, E* = 2.10 eV, and N*/N = 7.8 Χ 10" 2 . Values of Ν*/Νΐοτ Zn and Na are 250 and 52,000 times those of CI, respectively. Even when one considers that 99.5% of Na, 25% of Zn, and 1% of CI are ionized at this temperature and a typical plasma electron density of 1015/cm3, these cal culations show that excitation of nonmetals is difficult as a result of thermo dynamic considerations. A fundamen tal 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 wave lengths are 3.
Energy
N* Ν
Table 1. Characteristics of selected elements
15 13.0eV
104 eV
10
S
ev +
10.6 eV 10.4 eV = --725.7 nn 837.6 nm f t 8.9 eV
6.9 eV 5.8 eV
5
8.9 eV
-139.0 nn
180.7 nm
0
CI
Figure 2. Energy levels of the states and wavelengths of a few selected transitions of Na, Zn, S, and CI.
1090 A · ANALYTICAL CHEMISTRY, VOL. 60, NO. 19, OCTOBER 1, 1988
Figure 3. Configurations and most common samples and analytes of atomic spectrometric techniques. lines typically lie at wavelengths below 200 nm, nonresonance emission lines from 200 to 1200 nm are often ob served. These lines arise as a result of transitions from higher upper state en ergy levels that drop not to the ground state but to some intermediate energy level. Figure 2 illustrates this point. For Na and Zn, the lines arise from a transi tion from the first excited state to the ground state; Na emits at 589.6 nm and Zn at 213.9 nm. However, for S and CI, the emission from the first excited state to the ground state transition ap pears at 180.7 and 139.0 nm, respec tively. For these elements, nonresonant atom lines may be observed in the red and near-IR spectral regions and se lected ion transitions fall in the visible spectral region. Because ion lines typi cally arise from states even higher in energy above the ground state ion than the first excited atom states, and be cause fewer nonmetal ions than atoms typically exist in plasmas, the upper state populations of nonmetal ions should be correspondingly smaller. This lessens the intensity of the moni tored atomic emission. Ionization energies. In inductively coupled plasma (ICP) and microwaveinduced plasma (MIP) optical emission spectrometries, the most intense metal emission lines often arise not from at oms, but from ions. Furthermore, plas mas serve as excellent ionization sources for mass spectrometry (MS). These phenomena occur because the ionization energies of most metals are in a 4-9 eV range, and they are easily ionized in high-temperature plasmas. Nonmetals, which have ionization en
ergies ranging from 10 to 17 eV, are significantly less ionized at typical op erating conditions, as shown in Table I. Therefore significantly lower ion popu lations are predicted. Status of nonmetals and atomic spectroscopic techniques Common spectroscopic techniques for nonmetal determinations and selected studies are discussed below. We have cited the recent literature, and the ref erences in these papers may be used to extend the library on this topic. An overview of common spectroscopic techniques is shown in Figure 3. Absorption and flame emission spectrometry. Atomic absorption spectrometry (AAS) is performed by passing radiation of an appropriate wavelength (generally corresponding to an atomic resonance transition) through an atom reservoir (usually a flame or graphite furnace) where the analyte absorbs a fraction of the radia tion. This method is commonly used for the elemental detection of ΙΑ, ΙΙΑ, ΠΙΑ, and transition metals. In atomic emission spectroscopy (AES), electron ic transitions of the analyte from high er to lower energy states produce radia tion of known wavelengths that can be monitored. Flame AES yields best re sults for elements of the ΙΑ, ΙΙΑ, and ΠΙΑ groups, because these elements are excited to a significant extent at flame temperatures of 1500-3000 K. Although AAS and flame AES have been used successfully in metal deter minations, applications to nonmetals such as F, CI, Br, I, S, P, and C have been quite limited. Primary reasons for
limited successes have been the lack of suitable nonmetal line sources, the aforementioned wavelength problem associated with resonance transitions, and the inability of flames to extensive ly excite nonmetals. However, a number of interesting re sults have been obtained. One of these is the determination of sulfur by AAS using electrodeless discharge lamps as line sources. In combination with a laminar flow nitrous oxide-acetylene flame, Kirkbright and co-workers (6) obtained detection limits of 12-30 ppm. Taylor, Gibson, and Skogerboe (7) reported using MIPs for AAS, AES, and atomic fluorescence spectrometry (AFS) of sulfur. Using discrete sam ples, micro- to submicrogram detection limits were obtained. Shahwan and Heithmar (8) used AFS with an air-H2 flame and excitation radiation at 182.0 nm from an xenon lamp for the deter mination of SO2 and H2S volatilized from aqueous samples. The authors re port a sulfur detection limit of 2 ng/s. This method was found to be less sensi tive than was hoped, mainly because the xenon lamp is not well suited as an AFS excitation source at this wave length. Another interesting approach has fo cused on the formation and spectral characteristics of metal halides. Molec ular absorption and emission bands have been used for halide determina tions (9-12). Tsunoda et al. (9, 10) demonstrated molecular absorption sensitivities great enough for the detec tion of subnanogram quantities of ha lides with tungsten lamp radiation sources and graphite furnace atom res ervoirs. Of the alkaline earth molecular species, absorption of SrF at 662.9 nm provided the best sensitivity for fluo ride (0.38 ng/1% abs.), whereas CaCl at 621.2 nm was the most sensitive for chloride (0.64 ng/1% abs.) (9). These sensitivities were similar to those ob tained with the monohalides of alumi num (10). Dittrich et al. (11) and Dagnall et al. (12) obtained similar results with molecular absorption bands of IIIB monohalides. Although these techniques are inter esting, they present problems: lack of good radiation sources and the suscep tibility of the molecular species to ma trix effects. These impediments make it unlikely that the techniques will be widely adopted. Plasma AES. Another important elemental analysis technique is plasma AES. With typical plasma tempera tures in the range of 4000 to 10,000 K, analyte atomization, ionization, and excitation are generally much more ex tensive than with flame techniques. Since the mid-1970s, the determina tion of metals in solution using plasma excitation sources has become com monplace. Three plasma-based atomic
ANALYTICAL CHEMISTRY, VOL. 60, NO. 19, OCTOBER 1, 1988 · 1091 A
emission spectroscopic techniques have dominated the investigations of nonmetal determinations: argon (Ar)and helium (He)-ICPs and He-MIPs. The results with nonmetals in gasphase samples discussed below indi cate the potential of exciting nonmet als in liquid samples. Plasmas and gas-phase nonmetals. Most nonmetal plasma emission spec trometry has been performed on gasphase samples. Typical sample intro duction modes include gas chromatog raphy (GC) (13-17), electrothermal vaporization (18,19), and analyte vola tilization reactions (20, 21). Several re views addressing these topics have ap peared in the literature (22-24). The optimized GC/He-MIP system yields subnanogram detection limits for nonmetals (13,14,22-24) that have been obtained by monitoring line emis sion in the ultraviolet (UV) and visible (vis) spectral regions. Atom lines of ele ments such as C, N, F, and Ο are ob
served, and the most intense lines for many nonmetals (CI, Br, etc.) are those of ions. That these ion lines are so in tense indicates that He-MIPs are far from thermodynamic equilibrium. Cer tain nonmetal high-energy states are overpopulated. Although excitation processes are not well understood, it is highly possible that a very efficient en ergy transfer process (perhaps a reso nance energy transfer) is occurring. Even though these processes are of great interest to plasma spectroscopists, discussions of these mechanisms are beyond the scope of this article. In troductions to these areas may be found in two recent reviews (22, 23). He-MIPs for GC detectors are typi cally produced by the application of microwave power (2450 MHz) of 100 W or less to a flowing stream of helium. The dimensions of atmospheric pres sure plasmas are generally
