Physical and chemical interferences

In AAS, the graphite furnace can be the cause of interfering emissions from the walls of the graphite rod. The compounds from the matrix can lead equally to unwanted absorptions. 

There is never total impossibility of the superimposition of two absorption lines such as that chosen for the measurement and that coming from a secondary line belonging to another element. Confusions are rare but it is sometimes advisable to undertake a second measurement at another wavelength. In atomic emission this problem is frequent, not least as the spectrum is more complex (cf. Chapter 14). 

A non-negligible fraction of the atoms of certain elements pass to the excited state thermally. These atoms emit photons of the same energy that those remaining in the fundamental state are able to absorb. In order to correct the measurement (the emitted intensity has to be subtracted) the HCL voltage is pulsed, which permits differentiation between the emission signal, which is constant, and the absorption signal, which is pulsed. It is also an easy way to account for instrumental variations and flame flicker . 

CHAPTER 13 - ATOMIC ABSORPTION AND FLAME EMISSION SPECTROSCOPY 

Finally, the use of very hot flames can provoke partial ionization of certain elements which decreases the concentration of free atoms in the flame. This phenomenon is corrected by addition of an ionization suppressor in the form of a cation whose ionization potential is lower than that of the analyte. A potassium salt of about 2 g/L is often employed for this purpose. 

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Advantage The major plus points of the standard addition method is that it tends to compensate for variations caused by physical and chemical interferences in the sample solution. 

The distillation method (see Alternate Protocol 1) involves recovering malonaldehyde from an acidified food product. It is similar to that of the direct heating approach (see below), except that TBA is reacted only with an aliquot of the distillate. Consequently, physical and chemical interference by extraneous food constituents in the reaction with TBA is minimized because the food is never directly in contact with TBA. Unfortunately, direct heating of a food under acidic conditions enhances the degradation of existing lipid hydroperoxides as malonaldehyde precursors, and generates additional reactive radicals and scission products other than malonaldehyde that can react with TBA (Raharjo and Sofos, 1993). More malonaldehyde or reactive substances will be gener-... 

Physical and chemical interferences elevate the XRF analysis detection limits for certain soil types, the actual detection limits may exceed these for an interference-free matrix shown in Table 3.10. The XFR detection limits are always higher than the detection limits for other elemental analysis techniques. The high detection limits may not be appropriate for some types of DQOs and are the greatest limitation of XRF screening. 

The following non-spectral (physical and chemical) interferences from sample matrix are often observed in the ICP-AES analysis of environmental matrices ... 

To detect and quantify the effects of physical and chemical interferences, the laboratories use a variety of techniques, such as standard addition, serial dilution or internal standard addition, which are described in Chapter 4.4.4.5. 

Laboratories use this test in ICP-AES and AA analysis of new and unknown or unusual matrices to determine if non-linear physical and chemical interferences may be obscuring the target analytes. 

If the difference between the two results exceeds the method criterion, physical and chemical interferences may be present, and laboratories should perform a post digestion spike addition to verify their presence. 

In AAS and AFS, limitations to the analytical accuracy are mostly related to physical and chemical interferences and are due less to spectral interferences. In furnace AAS thermochemical processes limit the achievable accuracy and necessitate temperature programs to be carefully worked out in order to cope with errors arising from thermochemical effects. In AFS and also in LEI, it is necessary to control matrix influences relating to quenching when analyzing real samples. 

Determination Application of appropriate method selected according to planning step, instrument optimization/utilization, necessary corrections for physical and chemical interferences, accurate/precise determination, calibration, data calculation and handling... 

In the general scheme of an analytical method as outlined in Table 2.2, the ninth step out of 12 is determination . As mentioned, determination refers to the application of an appropriate measurement method selected according to the planning step, to quantitating the amount of analyte in the sample and includes instrument opti-mization/utilization, necessary corrections for physical and chemical interferences, cal-... 

Table 4.21 Summary of Physical and Chemical Interferences in Atomic Absorption and Emission Spectroscopy...

Discuss why background correction is necessary in AA. Distinguish between physical and chemical interferences in AA. Explain what is meant by the term Smith Hieftje background correction and discuss how it differs from the kind of correction employed in the Perkin-Elmer Model 3110 AA. 

The degree of atomization of the sample in the flame and hence the sensitivity, detection limit and magnitude of potential physical and chemical interferences depend on a number of operational parameters of the flame. For example, the degree of atomization increases as flame temperature rises. Parallel to this, detection sensitivity is improved and the influence of chemical interferences is reduced. 

Interference in furnace AAS is much higher than in flame AAS. This applies both for physical and chemical interference. 

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