Background correction continuum source method

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. 

Instrumental correction for background absorption using a double beam instrument or a continuum source has already been discussed (p. 325). An alternative is to assess the background absorption on a non-resonance line two or three band-passes away from the analytical line and to correct the sample absorption accordingly. This method assumes the molecular absorption to be constant over several band passes. The elimination of spectral interference from the emission of radiation by the heated sample and matrix has been discussed on page 324 et seq. 

One of the main practical problems with the use of AAS is the occurrence of molecular species that coincide with the atomic signal. One approach to remove this molecular absorbance is by the use of background correction methods. Several approaches are possible, but the most common is based on the use of a continuum source, D2. In the atomization cell (e.g. flame) absorption is possible from both atomic species and from molecular species (unwanted interference). By measuring the absorption that occurs from the radiation source (HCL) and comparing it with the absorbance that occurs from the continuum source (D2) a corrected absorption signal can be obtained. This is because the atomic species of interest absorb the specific radiation associated with the HCL source, whereas the absorption of radiation by the continuum source for the same atomic species will be negligible. 

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. 

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). 

Molecular absorption methods described in the literature are based on the use of continuum light sources (deuterium lamp with a thermal cathode or hollow cathode) or line-like radiation sources (hollow cathode lamps). Measurements using a continuum light source are carried out with a dual-channel instrument. The other channel is needed for the background correction. With a line-like radiation source, a conventional AA spectrometer can be used. In this technique the non-specific absorption is measured with a continuum radiation source. 

In spite of these limitations, continuum source background correction may be used with good accuracy for many analyses. It offers low cost, wide applicability, operation at high frequencies, and little degradation in detection limits or linear dynamic range. It is commonly found in commercial instrumentation alone or with other methods of correction. 

The Continuum-Source Correction Method Figure 9-14 illustrates a second method for background corrections that is widely used. In this technique, a deuterium lamp provides a source of continuum radiation throughout the ultraviolet region. The configuration of the chopper is such that radiation from the continuum source and the hollow-cathode lamp arc passed alternately through the electrothermal atomizer. The absorbance of the deuterium radiation is then subtracted from that of the analvte beam. 

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