Background correction continuum source Zeeman

Zeeman background correction. Continuum source (D2) may be applied for plasma/ serum matrices... 

A spectrometer with rapid response electronics should be used for electrothermal atomization, as it must follow the transient absorption event in the tube. Automatic simultaneous background correction (see Section 2.2.5.2) is virtually essential, as non-specific absorption problems are very severe. It is important that the continuum light follows exactly the same path through the furnace as the radiation from the line source (assuming a deuterium lamp is being used rather than Smith-Hieftje or Zeeman effect). The time interval between the two source pulses should be as short as possible (a chopping frequency of at least 50 Hz) because of the transient nature of the signal. 

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. 

Almost no real samples can be run without background correction. After an instrument has been in service for some time, the continuum background correction lamps do not always remain in adjustment and correction can introduce errors. This is, beyond doubt, the major advantage of Zeeman correction. Since the same source and optical system are used for both analysis and correction, nothing can go out of adjustment. For this reason Zeeman corrected systems can accommodate much higher backgrounds which produces greater accuracy, as well as lower detection limits, in real samples. 

Aside from the extensively studied volatility interferences, it has been demonstrated that the conventional method of background correction which is based on the use of a continuum source (D2), is subject to spectral interferences from iron and for phosphate decomposition products (presumably PO and P2) (Saeed and Thomassen, 1981). Even though these spectral interferences are highly reduced by matrix modification with either nickel, a nickel/platinum or a nickel/palladium matrix modifier, the use of a Zeeman based instrument is highly recommended (Bauslaugh et al., 1984 Radziuk and Thomassen, 1992 Hoenig, 1991). 

All of the flicker noises can be effectively eliminated by the use of double-beam optics in conjunction with a background correction system such as Zeeman splitting or a well-aligned (or wavelength-modulated) continuum source. Thus the ultimate limiting noise in atomic absorption is source shot noise, which can be reduced (relative to total source intensity or I, ) by increasing the source intensity, up to the point of optical saturation. 

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. 

A significant effort was necessary to obtain accurate results for selenium in blood, plasma, and erythrocytes [27]. Iron causes a spectral interference at the selenium line at 196 nm if a continuum source background corrector is used, causing erroneous results particularly in blood and erythrocytes. Zeeman effect background correction is therefore mandatory for the determination of selenium in clinical samples. In addition, some of the previously recommended chemical modifiers were found to stabilize the different selenium species differently, so that some of them may be lost in the pyrolysis stage. A mixture of palladium and magnesium nitrates was found to solve the problem and prevent any preatomization losses [28]. 

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