Several experimental arrangements are possible when exploiting the use of the Zeeman Effect in atomic absorption. Firstly, the field can be applied to either the lamp or to the sample since the Zeeman Effect applies both to absorbing and emitting species. Standard hollow cathode lamps do not operate satisfactorily in the strong magnetic fields required. Even if acceptable operation were achievable, the split lamp emission profile would result in the background absorption being measured at wavelengths slightly removed from the analyte absorption wavelength. Consequently, if the background contained discrete spectral features, this approach could lead to inaccurate background correction. By applying the field directly to the absorbing sample, standard hollow cathode lamps can be used and the background absorbance is measured at the same wavelength as that of the analyte and so correction for structured background features is optimized.
Accepting the preference for applying the magnetic field to the absorbing sample, it is possible to use either a fixed or modulated field. With a magnetic field fixed in time, the sample and background measurements are discerned on the basis of their polarization characteristics by using a modulated polarizer. Such an approach does not achieve optimum sensitivity for those atomic transitions exhibiting the
Anomalous Zeeman effect, and experimental problems associated with the modulation of the polarizer lead to a degradation in the background correction accuracy.
With a modulated magnetic field, a fixed polarizer is included and oriented so as to reject the component of the hollow cathode lamp emission polarized parallel to the magnetic field. With the field off, the total absorbance of sample plus background is measured whereas when the field is on, the perpendicularly polarized light
transmitted by the optical system can only be absorbed by the σ components and, in the ideal case, these are sufficiently shifted from the lamp resonance wavelength to result in zero atomic absorbance. However, the background species, being
insensitive to the presence of the magnetic field, continue to absorb providing the background absorbance. The field off and field on absorbances are subtracted to yield the background corrected atomic absorbance. Furthermore, in taking this difference of absorbances, the absolute magnitude of the hollow cathode lamp intensity is cancelled out and so the Zeeman approach to atomic absorption provides, in a single beam optical system, the attributes of a double beam system. That is, changes in the source intensity that are slow relative to the rate at which the field is modulated are automatically compensated. Furthermore, since the π component of the absorbing transition is not used, no loss in sensitivity is encountered with the Anomalous Zeeman Effect.
This arrangement results in the best combination of analytical sensitivity and background correction accuracy. Figure 11 shows a schematic representation of the SpectrAA-30/40 Zeeman configuration.
Figure 11. Optical configuration of Varian SpectrAA-40 Zeeman
Analytical Characteristics of Zeeman Atomic Absorption
As discussed above, with the magnetic field energized, the atomic absorbance is ideally zero and only the background absorbance is measured. However, in practice there may be some small residual atomic absorption due to:
a The incomplete removal of the component of the hollow cathode lamp
emission polarized parallel to the magnetic field.
b Insufficiently intense magnetic fields being used such that the
components are not adequately shifted.
c The complex interaction of the hyperfine (ie. isotopic) structure of a
given spectral transition.
Detector Monochromator Prism polarizer Lamp Furnace Magnet
In the SpectrAA-30/40 Zeeman a high quality crystal polarizer has been used so the rejection of the π component is, to all practical purposes, complete. In addition, the magnetic field intensity is high enough to minimize the effects of problems (b) and (c). Nevertheless, there is often an unavoidable but slight loss in sensitivity and the ratio of the Zeeman sensitivity to that of normal atomic absorption is defined as the magnetic sensitivity ratio (MSR). MSR values for the most common atomic
absorption lines are listed in Chapter Four and it will be apparent that for the majority of elements the loss in sensitivity with the Zeeman system is minimal and is more than compensated for by the superior background correction accuracy. Calibration curves in conventional atomic absorption, generally asymptote towards a limiting absorbance at high concentrations (Figure 12). This is due to differences in the emission and absorption profiles, the presence of nearby non absorbable spectral lines passed by the monochromator, and residual stray light within the spectrometer's optical system. In Zeeman atomic absorption, there is the further complication that when the field is applied, there may still be some small residual atomic absorption which can result in the analytical curve bending over at high concentrations (322). This phenomenon of reflex curvature means that two different concentrations can give the same absorbance, a clearly unacceptable situation. The SpectrAA-30/40 Zeeman system constantly monitors the peak absorbance value during the atomization process and indicates an error when the maximum permissible absorbance, slightly below the reflex point, is exceeded. Conceptually, the reflex curvature phenomenon in Zeeman atomic absorption can be understood by viewing the Zeeman corrected absorbance as the result of
subtracting the Zeeman background absorbance from the total Zeeman absorbance. As a function of concentration, the total absorbance measurement asymptotes to a limiting value at a greater rate than does the background measurement and the net result of subtracting these two curves results in a reflexing of the analytical curve.
Figure 12. Calibration curve for normal and Zeeman atomic absorption
In choosing the analytical wavelength in atomic absorption, the major
considerations are the measured sensitivity and the analytical working range. However, using Zeeman atomic absorption there are the additional complications of MSR and reflex curvature. Different lines of a given element may exhibit
significantly different values of MSR or degrees of calibration curvature and these must be also considered in choosing the optimum analytical wavelength. In some cases, the selected wavelength will be different from that preferred for conventional atomic absorption. In Zeeman AA, for example, the 327.4 nm wavelength for copper yields comparable sensitivity and more linear calibration curves compared with the 324.7 nm wavelength preferred for non-Zeeman AA.
Ab sorb ance Normal AA Zeeman AA Reflex point Concentration
Finally, the measurements of the total and background absorbances are made sequentially, thereby admitting the possibility of dynamic measurement errors associated with rapidly changing absorbance levels. To minimize these effects the magnetic field is modulated at twice the mains frequency (100 or 120 Hz) and furthermore the SpectrAA-30/40 Zeeman software incorporates a polynomial interpolation routine such that the two absorbance measurements are effectively made at the same point in time (323).
3. Development of Analytical Programs
General
The graphite tube atomizer system has facilities for the programming of a number of parameters including temperature, time, gas flow rate and type. In addition, memory facilities for the storage of programs are a feature of the instrument. A complete description of the atomizer controls and instructions for programming the atomizer are contained in the Operation Manual. The DS-15 unit incorporates a CRT display, which is used to display the parameters selected for the operation of the furnace as well as giving a graphical presentation of the variation of absorbance with time during any selected port ion of the temperature cycle.
The recommended conditions (wavelength. lamp current. spectral band pass. Maximum absorbance) are given in Chapter Four; specific methods are given in Chapter Seven.
The installation of the atomizer and alignment in the light path are described in the Operation Manual and are not repeated here. The most important point to note in the alignment of the atomizer is that the position of the atomizer must be adjusted to give the maximum intensity of transmitted light from the hollow cathode lamp.