Question

What are the sources of interference in AAS? What methods are used to minimise these interferences?

Answer 1

Interferences

Interferences in atomic absorption spectroscopy fall into two basic categories, namely, non-spectral and spectral. Interference is a phenomenon that leads to changes in intensity of the analyte signal in spectroscopy.

Non-spectral interferences affect the formation of analyte items and spectral interferences result in higher light absorption due to presence of absorbing species other than the analyte element.

Non-spectral interferences

Matrix interference

When a sample is more viscous or has different surface tension than the standard it can result in differences in sample uptake rate due to changes in nebulization efficiency. Such interferences are minimized by matching as closely as possible the matrix composition of standard and sample

Chemical interference

If a sample contains a species which forms a thermally stable compound with the analyte that is not completely decomposed by the energy available in the flame then chemical interference exists. Analysis of such elements can be carried out at higher flame temperatures using nitrous oxide – acetylene flame instead of air-acetylene to provide higher dissociation energy. Alternately an excess of another element or compound can be added e.g. Ca in presence of phosphate produces stable calcium phosphate which reduces absorption due to Ca ion. If an excess of lanthanum is added it forms a thermally stable compound with phosphate and calcium absorption is not affected.

Ionization interference

Ionization interference is more common in hot flames. The dissociation process does not stop at formation of ground state atoms. Excess energy of the flame can lead to excitation of ground state atoms to ionic state by loss of electrons thereby resulting in depletion of ground state atoms. In cooler flames such interference is encountered with easily ionized elements such as alkali metals and alkaline earths.

Ionisation interference is eliminated by adding an excess of an element which is easily ionized thereby creating a large number of free electrons in the flame and suppressing ionization of the analyte. Salts of such elements as K, Rb and Cs are commonly used as ionization suppressants.

Spectral Interferences

Spectral interferences are caused by presence of another atomic absorption line or a molecular absorbance band close to the spectral line of element of interest. Most common spectral interferences are due to molecular emissions from oxides of other elements in the sample.

The main cause of background absorption is presence of undissociated molecules of matrix that have broad band absorption spectra and tiny solid particles, unvaporized solvent droplets or molecular species in the flame which may scatter light over a wide wavelength region. When this type of non-specific adsorption overlaps the atomic absorption of the analyte, background absorption occurs. The problem is overcome by measuring and subtracting the background absorption from the total measured absorption to determine the true atomic absorption.