Zeeman effect instruments

The need for improved background correction performance has generated considerable interest in applying the Zeeman effect, where the atomic spectral line is split into several polarised components by the application of a magnetic field. With a Zeeman effect instrument background correction is performed at, or very close to, the analyte wavelength without the need for auxiliary light sources. An additional benefit is that double-beam operation is achieved with a very simple optical system. 

In assessing overall performance with a Zeeman effect instrument, the subject of analytical range must also be considered. For most normal class transitions, a component will be completely separated at sufficiently high magnetic fields. Consequently, the analytical curves will generally be similar to those obtained by standard AAS. However, for certain anomalous transitions some overlap may occur. In these cases, curvature will be greater and may be so severe as to produce double-valued analytical curves. Figure 11.3, which shows calibration curves for copper, illustrates the reason for this behaviour. The Zeeman pattern... 

A second type of Zeeman effect instrument has been designed in which a magnet surrounds the hollow-cathode source. Here, it is the emission spectrum of the source that is split rather than the absorption spectrum of the sample. This instrument configuration provides an analogous correction. To date, most instruments are of the type illustrated in Figure 9-15. 

Zeeman effect instruments provide a more accurate correction for background than the methods dc.scribed earlier. These instruments are particularly useful for electrothermal atomizers and permit the direct determination of elements in samples such as urine and blood. The decomposition of organic material in these samples leads to large background corrections (background A > 1) and, as a result, susceptibility to significant error. 

Zeeman effect splitting of atomic absorption lines. (Redrawn from Concepts Instrumentation and Techniques in Atomic Absorption Spectrophotometry (R. D. Beaty and J. D. Kerber). Perkin-Elmer,... 

Figure 14.13—Zeeman effect correction. Instrument showing the principle used for correction of absorbance by the Zeeman effect. Two solutions are applicable I) magnetic field, B, switched alternately on and off and a fixed polariser 2) fixed magnetic field, B, and a rotating polariser.

The second set-up uses an alternating on/off field and a fixed polariser with an orientation that suppresses detection of the tt component. This system is equivalent to a double beam instrument with a common path. The atoms are affected by the Zeeman effect but particles in suspension are not (Fig. 14.14). 

An A AS method is employed for the determination of lead (Pb) in a sample of adulterated paprika by the introduction of lead oxide (of the same colour). An electrothermal atomic absorption instrument that provides a background correction based upon the Zeeman effect is used. 

Multielement analysis will become more important in industrial hygiene analysis as the number of elements per sample and the numbers of samples increases. Additional requirements that will push development of atomic absorption techniques and may encourage the use of new techniques are lower detction and sample speciation. Sample speciation will probably require the use of a chromatographic technique coupled to the spectroscopic instrumentation as an elemental detector. This type of instrumental marriage will not be seen in routine analysis. The use of Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) (17), Zeeman-effect atomic absorption spectroscopy (ZAA) (18), and X-ray fluorescence (XRF) (19) will increase in industrial hygiene laboratories because they each offer advantages or detection that AAS does not. 

One new instrument is worthy of note. This is the Hitachi Zeeman effect atomic absorption spectrometer, model 170-70. It provides background correction for nonatomic absorption at all wavelengths through use of the Zeeman effect. It is presently offered only in a carbon furnace conflguration. The cost of the instrument is considerably higher than conventional instruments. 

Instrumentation for diode laser based AAS is now commercially available and the method certainly will expand as diode lasers penetrating further into the UV range become available, especially because of their analytical figures of merit that have been discussed and also because of their price. In diode laser AAS the use of monochromators for spectral isolation of the analyte lines becomes completely superfluous and correction for non-element specific absorption no longer requires techniques such as Zeeman-effect background correction atomic absorption or the use of broad band sources such as deuterium lamps. 

Figure 13.4 Examples of calibration graphs in AAS. Left, a straight calibration line at sub-ppb concentrations obtained with an instrument equipped with a Zeeman effect device (see section 13.7) for the quantification of sodium. Right, a quadratic cirrve for the measurement for zinc at concentrations in the ppm range with a burner type instnrment. This second graph reveals that when concentrations increase, the absorbance is no longer linear. The quantitative analysis software for AAS provides several types of calibration curves.

In order to determine Bi by GF AAS under stabilized temperature platform furnace (STPF) conditions using the Pd-Mg modifier, a pyrolysis temperature of 1200 °C must be applied (Hiltenkamp and Werth 1988). The optimum atomization temperature under these conditions is 1900 °C the characteristic mass with Zeeman effect background correction (BC) is 28 pg, while in a non-Zeeman instrument it is about 20 pg. 

The use of the Zeeman effect for background correction in AAS was initially proposed in 1969 [23]. Six years later, the first commercial instruments became available [24]. Zeeman background correction is nowadays widely used in ET-AAS instruments. It is capable of reliably handling even high nonspecific absorption. In particular the more volatile elements Pb and Cd are preferably measured by ET-AAS with Zeeman background correction, since the relative volatility of the elements does not allow high temperatures to be used in the pyrolysis step to remove interferences. 

By utilizing the Zeeman effect it is also possible to obtain an (optical) single-beam instrument with double-beam characteristics. When the magnetic field at the atomizer is switched on, the analyte ions cannot absorb the radiation from the source, while they can absorb normally when the magnetic field is switched off. Since the radiation with and without magnetic field has the same intensity, drifts in the source or detector are eliminated, as with a double-beam instrument. 

Application of the Zeeman effect to atomic absorption instruments is based on the differing response of the two types of absorption lines to polarized radiation. The Tt line absorbs only that radiation that is plane-qrolarized in a direction parallel to the external magnetic field the a lines, in contrast, absorb only radiation polarized at 90° to the field. 

Figure 9-15 shows details of an electrothermal atomic absorption instrument, which uses the Zeeman effect for background correction. Unpolarized radiation from an ordinary hollow-cathode source A is passed through a rotating polarizer B. which separates the beam into two components that are plane-polarized at 90° to one another C. These beams pass into a lube-type graphite furnace similar to the one shown in Fig-... 

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