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Study Notes: Spectral Interferences

Interferences lead to a loss of absorbance signal or a change in absorbance signal that is due to the interfering factor and not the sample itself. There are two broad types of interferences - spectral and chemical. This Study Note covers Spectral Interferences.

Spectral Interferences are caused by particulate matter from the atomization process scattering the incident radiation from the source or where the emission of an interfering species overlaps or is close to the analyte wavelength.

Interference due to overlapping lines is rare because the emission lines of hollow cathode lamps are very narrow but may occur when the separation between the lines is around 0.01 nm. For example, Vanadium (Va) line at 308.211 interferes in an analysis based on the Aluminium (Al) line of 308.215 nm. This of course can be avoided by choosing another Al line such as 309.27 nm.

Spectral interferences may occur from either molecular combustion products that cause broad-band absorption or particulate products that scatter radiation. Both lead to a reduction in the intensity of the transmitted beam and hence both cause positive analytical errors. That is, the AAS determines that there is more of the species of interest in the sample than there really is! These can be corrected easily by absorbance measurements from the blank.

Sample matrix problems may be more difficult to handle. Concentrated solutions containing Titanium (Ti), Zirconium (Zr) and Tungsten (W) that form stable oxides are also a problem as the particles of stable oxide are often larger than the wavelength of the incident light and cause scattering of the incident beam.

Spectral interferences by matrix products are not common with flame atomisation and can usually be avoided by varying temperature and the fuel-to-oxidant ratio. Alternatively an excess of the interfering substance (if known) can be added to both the samples and standards and this will swamp out the minor contribution from the sample matrix. This addition is called a radiation buffer.

The matrix-interference is associated with electro-thermal atomisation and is on of the major cause of lower accuracy with non-flame methods (e.g. graphite furnace).

 

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