Instrumentation
Steven D. Brown Laboratory for Chemometrics Department of Chemistry University of Washington Seattle, Wash. 98195
Zeeman Effect-Based Background Correction in Atomic Absorption Spectrometry During the last seven years, the development of flameless atomization techniques has made it possible to directly analyze for a variety of elements in many matrices by atomic absorption spectrometry (AAS). Direct analysis is desirable because each addition of reagent and each manipulation of the sample add contaminants (1,2). However, direct analysis by AAS, especially flameless AAS, requires efficient background correction to compensate for the strong absorption of the matrix. Procedures such as nonresonant line correction (3), continuumarc source correction (4), and continuum-source, wavelength-modulated echelle monochromator (5) techniques have been used previously in attempts to remove background absorption. Recently, a pair of new methods based on the Zeeman effect have been used to perform background correction. The older of the two, Zeeman sourceshifted background correction, is similar to the older nonresonant line technique, in that it generates a pair of nonresonant lines very near the resonance line, while the newer of the two, Zeeman analyte-shifted background correction, utilizes the polarization characteristics of the analyte resonance line to perform background correction. This paper examines the theoretical basis for these techniques and reviews the experimental procedures utilizing the Zeeman effect to perform background correction.
Principles of Zeeman Effect The quantum states of an atom undergo drastic changes when that atom is placed in a magnetic field; states that were degenerate may separate,
and spectral lines may split into three or more components. The separation of states that arises from the interaction of the magnetic field with the magnetic moment of the electronic system of the atom can be fairly easily shown (6, 7) to generate the new energy states E, so that: E = E° + unBMjg
(1)
where MB is the Bohr magnetron (eft/2 me), B is the magnetic induction in gauss, Mj is the magnetic quantum number, and g is the Lande g factor, g = 1 + \[J(J+ 1)+S(S+ \)-L(L + l)]/[2 J ( J + 1)]| and where E° is the original energy of the state, in c m - 1 with L, S, and J being the usual orbital, spin, and total angular momentum quantum numbers, respectively. Transitions between these new states are given by the usual selection rule AMj = 0, ±1
(2)
which holds for both emission and absorption (7). The AM j • 0 components (called the x components) have their electric vectors linearly polarized parallel to the magnetic field, while the AMj = ±1 components (called the o-± components for senkrecht, or perpendicular) have their electric vectors linearly polarized perpendicular to the magnetic field. The 201|- lg, 204 Hg
-9.98
-5.34
20C Hg
198H
9 : 01 Hg
3.96 4.83
199 H ( 3,201Hg
5.10 X 10~ 4 nm 4 Atomized sample 1.634 X 10~ nm
S^rco
10 kG field
11.8
Field-Free Frequency Shift, GHz Figure 4. Zeeman effect in mercury 1272 A • ANALYTICAL CHEMISTRY, VOL. 49, NO. 14, DECEMBER 1977
3.79 X 10~ 3 nm
Source ~ 1 0 - 1 1 nm 4 Atomized sample 2.63 X 10" nm * Calculated using formulas 3 , 5 , and 11 from ref. 11. with data taken from ref. 14, assuming a collision cross section a2 = 10 X 1 0 - ' 6 c m - 2t
Normalized Intensity j ' •Ki»j antWiM m£ii. ...LV.'-.-^-
-15
Figure 5a. Line broadening effects—lamp in magnetic field, TLAMP = 550 K, TSAMPLE " 2310 K, Ca 422.7-nm line in 10
kG field, both curves normalized to 1 for clarity
shift. The Zeeman shift dominates, although broadening is pronounced, when the field either is applied to a Ca hollow cathode lamp or to a heated cell containing Ca atoms, as is shown in Figures 5a and 5b. In both of the cases above, the x component coincides with the resonance line, while the
