Interferences in Atomic Absorption Spectroscopy

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

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.

Interference in Atomic Absorption Spectroscopy

Interference in Atomic Absorption Spectroscopy

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. Refractory elements such as Ti, W, Zr, Mo and Al may combine with oxygen to form thermally stable oxides. 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.

Interferences are corrected using background correction techniques which are discussed in the next module.

About Dr. Deepak Bhanot

Dr Deepak Bhanot is a seasoned professional having nearly 30 years expertise beginning from sales and product support of analytical instruments. After completing his graduation and post graduation from Delhi University and IIT Delhi he went on to Loughborough University of Technology, UK for doctorate research in analytical chemistry. His mission is to develop training programs on analytical techniques and share his experiences with broad spectrum of users ranging from professionals engaged in analytical development and research as well as young enthusiasts fresh from academics who wish to embark upon a career in analytical industry.

Comments

  1. Roan Birgen says:

    great site to polish and get deeper into the analytical field.keep it up

  2. al-amin says:

    I want to know about physical interference & refractory compound

    • Hello Al-Amin,
      Interferences in AAS analysis are basically spectroscopic and non-spectroscopic.I am not very clear on what you are refering to when you say physical interferences.Kindly elaborate so that I may comment.
      Refractory compounds,particularly oxides are those that do not decompose at normal air -acetylene flame temperatures and require higher temperatures to decompose and form free atoms.These temperatures are achieved by using nitrous oxide-acetylene flames.

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