Zeeman correction method

The occurrence of molecular absorbance and scatter in AAS can be overcome by the use of background correction methods. Various types of correction procedures are common, e.g. continuum source, Smith-Hieftje and the Zeeman effect. In addition, other problems can occur and include those based on chemical, ionization, physical and spectral interferences. 

Cadmium is one of the most widely determined metals using the graphite furnace. A direct STPF method for Cd in urine detected less than 0.04 fig/L in the sample (Pruszkow-ska et al., 1983a). Zeeman correction was necessary because of the large background signals that accompanied the determination. The urine was diluted 1 + 4 in the matrix modifier. 

There is very little in the literature using modern furnace methods for Co in biological materials. Kimberly et al. (1987) determined Co in urine using mostly STPF conditions and Zeeman correction, except that the sample was deposited on the wall of the tube. They used 10, mL of a 1 -i- 1 dilution of urine and found no interferences. They reported a detection limit of 2.6, wg/L Co in the urine. We believe the detection limit can be reduced below 0.5, g/L if the platform and a somewhat larger sample are used. 

The determination of serum Pb with the furnace is difficult because the serum levels are very low. STPF methods and Zeeman correction are mandatory for serum Pb. If, as has been reported, normal serum Pb is less than 1 g/L, it will not be possible to measure these levels with confidence with a direct method. A 20- uL aliquot of a 1 -t- 1 dilution of serum in the diluent mentioned earlier will provide a detection limit a little lower than 1 IxqIL. For serum levels higher than this, the method can be used with confidence. 

Urine Pb down to the l- wg/L level can be measured in 20of a 1 + 1 dilution of urine in the matrix modifier. If a somewhat poorer detection limit is acceptable, a 1 -1- 3 dilution of urine is more reliably handled by the autosampler. Paschal and Kimberly (1985) used a very similar urine Pb method but altered the conditions to make the method applicable to non-Zeeman corrected instruments. 

Manganese is an ideal STPF analyte. The low levels of Mn that are often found in biological materials suggest that it should be determined in the furnace, usually by direct methods. The major problem with the determination of Mn is the control of contamination, which is discussed in an earlier section. Zeeman correction is particularly useful for Mn because, at the long wavelength used for Mn (279.5 nm) the continuum correctors are not very effective. 

Selenium is probably the furnace determination which most demands Zeeman correction STPF technology. Other methods are slow and prone to manipulative errors at the low concentrations that are typically of interest in biological materials. Nevertheless, the volatility of many Se compounds, especially organoselenium compounds, produces troubles. Both Fe and P cause severe overcorrection errors when Se is determined with continuum correction, making Zeeman correction mandatory for Se in biological materials. There are many papers in the literature that have not used Zeeman correction for Se but they rely on delicate timing of the thermal program so that Se is not volatilized at the same time as the interferent. The paper of Verlinden et al. (1981) on the MS determination of Se should be consulted. 

Background absorption can be compensated for or minimized by using sample-like standards or matrix modification, or by moving to an interference-free line if possible. However, the actual background correction methods are (i) Two line method (ii) Continuum source method (iii) Smith-Hieftje method (iv) Methods using the Zeeman effect. 

Graphite furnace atomic absorption spectrometry (GFAAS) is an excellent method to provide sub-ng/mL minimum detection limits [110]. Continuing advancements such as Zeeman correction, and stabilized temperature platform furnaces, have made GFAAS an effective analytical method for magnesium determination. Depending on the sample matrix, pretreatment can vary from direct analysis of fluids, to wet mineralization, dry ash, acid extraction, and by using PPRs (e.g., Triton X-100). 

Many of the published methods for the determination of metals in seawater are concerned with the determination of a single element. Single-element methods are discussed firstly in Sects. 5.2-5.73. However, much of the published work is concerned not only with the determination of a single element but with the determination of groups of elements (Sect. 5.74). This is particularly so in the case of techniques such as graphite furnace atomic absorption spectrometry, Zeeman background-corrected atomic absorption spectrometry, and inductively coupled plasma spectrometry. This also applies to other techniques, such as voltammetry, polarography, neutron activation analysis, X-ray fluroescence spectroscopy, and isotope dilution techniques. 

Pruszkowska et al. [135] described a simple and direct method for the determination of cadmium in coastal water utilizing a platform graphite furnace and Zeeman background correction. The furnace conditions are summarised in Table 5.1. These workers obtained a detection limit of 0.013 pg/1 in 12 pi samples, or about 0.16 pg cadmium in the coastal seawater sample. The characteristic integrated amount was 0.35 pg cadmium per 0.0044 A s. A matrix modifier containing di-ammonium hydrogen phosphate and nitric acid was used. Concentrations of cadmium in coastal seawater were calculated directly from a calibration curve. Standards contained sodium chloride and the same matrix modifier as the samples. No interference from the matrix was observed. 

Electrothermal atomic absorption spectrophotometry with Zeeman background correction was used by Zhang et al. [141] for the determination of cadmium in seawater. Citric acid was used as an organic matrix modifier and was found to be more effective than EDTA or ascorbic acid. The organic matrix modifier reduced the interferences from salts and other trace metals and gave a linear calibration curve for cadmium at concentrations < 1.6 pg/1. The method has a limit of detection of 0.019 pg/1 of cadmium and recoveries of 95-105% at the 0.2 pg of cadmium level. 

Maximum power heating, the L vov platform, gas stop, the smallest possible temperature step between thermal pretreatment and atomisation, peak area integration, and matrix modification have been applied in order to eliminate or at least reduce interferences in graphite furnace AAS. With Zeeman effect background correction, much better correction is achieved, making method development and trace metal determinations in samples containing high salt concentrations much simpler or even possible at all. 

Knowles M (1987) Varian atomic absorption no AA 71 methods for the determination of cadmium in seawater with Zeeman background correction... 

In the method described by Willie et al. [167] atomic absorption measurements were made with a Perkin-Elmer 5000 spectrometer fitted with a Model HGA 500 graphite furnace and Zeeman effect background correction system. Peak absorbance signals were recorded with a Perkin-Elmer PRS-10 printer-sequencer. A selenium electrodeless lamp (Perkin-Elmer Corp.) operated at 6W was used as the source. Absorption was measured at the 196.0nm line. The spectral band-pass was 0.7nm. Standard Perkin-Elmer pyrolytic graphite-coated tubes were used in all studies. 

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. 

Other background correction systems include the Zeeman effect and the Smith-Hieftje background correction. A detailed description of the operational principles of these methods is beyond the scope of this chapter and the required information can be found in the relevant literature.7,13 The advantages of these methods over deuterium lamps are that high background signals (up to 2.0 units) and structured backgrounds can easily be corrected for. 

With this technique, problems may arise with interference, such as background absorption—the nonspecific attenuation of radiation at the analyte wavelength caused by matrix components. To compensate for background absorption, correction techniques such as a continuous light source (D2-lamp) or the Zeeman or Smith-Hieftje method should be used. Enhanced matrix removal due to matrix modification may reduce background absorption. Nonspectral interference occurs when components of the sample matrix alter the vaporization behavior of the particles that contain the analyte. To compensate for this kind of interference, the method of standard addition can be used. Enhanced matrix removal by matrix modification or the use of a L vov platform can also reduce nonspectral interferences. Hollow cathode lamps are used for As, Cu, Cr, Ni, Pb, and Zn single-element lamps are preferred, but multielement lamps may be used if no spectral interference occurs. 

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