Non-resonance line

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. 

Establishing the analytical conditions. Wavelength selection will depend on analyte concentrations. Less sensitive non-resonance lines may be useful. Quantitation is then accomplished by using aqueous standards for peak area measurements. The use of a matrix modifier and a char step may be unnecessary. 

A) Monochromator does not resolve two non-resonance lines from the resonance line. (B) Concentrated sample absorbs entire resonance line. (C) Detector continues to see unabsorbed radiation... 

If the resonance detector is well-designed, the vast majority of the magnesium atoms are unexcited. The resonance lines from the magnesium hollow cathode lamp will cause the magnesium atoms in the resonance detector to fluoresce. Some of this fluorescence will fall on a photomultiplier detector placed at right angles to the optical path. The intensity of fluorescence is proportional to the intensity of emission. Non-resonant lines from the lamp or from the flame will have no effect on the resonance detector. Therefore, a system of narrow bandwidth is produced without the requirement of a monochromator. 

The use of an ICP as an excitation source makes it possible to correct interference due to light scatter by means of several different techniques. The double-beam technique is possible because ion and resonance lines can be obtained by the ICP which do not appear in cooler atomizers. In this technique, non-resonance lines near the resonance line of the analyte are employed. 

Two-line background correction The two-line correction method, which was proposed in the late 1970s [22], is based on measuring the absorption at a second, non-resonant line. This line should be close to the resonance Hne of the element that is measured but should not be absorbed by the analyte. If these conditions are met sufficiently well, it can be assumed that the attenuation at this second line is only due to the background absorption in the sample. 

Thus for quantitative analyses it is better to use the analytical lines which are cormected to the transitions on not ground state. For example, in industrial KCl fertilizers the self-absorption of the Na impurity becomes visible at approximately 2.0 % level (Fig. 6.16). It is clearly seen that non resonant line of Na peaking at 568.8 nm is much better correlated with Na concentration than famous resonant doublet at 589.9 and 589.6 nm. The same approach may be used for Mg analysis, where non-resonant Mg II lines peaking at 292.9 and 293.7 nm are less subjected to... 

In HR-CS AAS again these problems are essentially non-existent for the same reasons as given above. Firstly, because of the relatively constant, very intense emission of the primary radiation source, there are no weak lines, i.e. the same high SNR ratio will be obtained on all analytical lines, independent of their spectral origin. The only criteria that will have an influence will be the absorption coefficient and the population of the low excitation level when non-resonance lines are used. Secondly, because of the high resolution of the monochromator, and as the entire spectral environment of the analytical line becomes visible in HR-CS AAS, potential spectral interferences can easily be detected, and in addition cannot influence the actual measurement, except in the rare case of direct line overlap. But even in this case HR-CS AAS provides an appropriate solution, as will be discussed in Section 5.2.3. 

The proposed method has been applied successfully to determine phosphorus in cast iron, in pine needles and in super phosphate fertilizer. The reported detection limit of 1.3 mg/L is almost two orders of magnitude better than that obtained by LS AAS in a nitrous oxide/acetylene flame at the 213.618 nm non-resonance line for phosphorus, and it is quite comparable with the LOD reported for GF AAS and ICPOES, except for those that are obtained in the vacuum-UV at 177.495 nm [82]. 

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