Interference and Types of Interference

 INTERFERENCES

An increase and decrease in the size of the signal obtained from the analyte as a result of the presence of some other known or unknown component in a sample is called interference.

It is the phenomenon in which two waves superimpose to form a resultant wave of greater or lower amplitude. It decreases the intensity of absorption of light.

Various factors may lead to interference with the determination of the concentration of a given element.

Types of Interference

Ø Spectral interferences

Ø Matrix Interference

Ø Chemical interferences

Ø Ionization interferences

Spectral Interferences

This interference is caused by overlapping any radiation with the sample's radiation.

The interfering radiation may be due to another element, radical, molecule unresolved spectra, or general background emission from flame or solvent.

Example:

Vanadium (Va) line at 308.211nm interface in analysis based with the Aluminum (Al) line at 308.225nm. This can be avoided by choosing another (Al) line such as 309.270nm.

How to overcome it?

This can be overcome by selecting spectral lines properly or by chemical separation.

Spectral interferences also result from the presence of combustion products that exhibit broadband absorption or particulate products that scatter radiation. Both reduce the power of the transmitted beam and lead to positive analytical errors.

When the source of these products is the fuel and oxidant mixture alone the analytical data can be corrected by making absorption measurements while a blank is aspirated into the flame.

This correction must be used with both double-beam and single-beam instruments because the reference beam of a double-beam instrument does not pass through the flame.

Matrix Interference:

Spectral interference is also observed when the source of absorption or scattering originates in the sample matrix.

The power of the transmitted beam P is reduced by the matrix components but the incident beam power Po is not: a positive error in absorbance and thus concentration results.

Example: Determination of Ba in alkaline-earth mixtures. The wavelength of the barium line used for atomic absorption analysis appears in the center of a broad absorption band for CaOH. Calcium is expected to interfere in barium determinations, but the effect is easily eliminated by substituting nitrous oxide for air as the oxidant. The higher temperature of the nitrous oxide flame decomposes the Ca(OH) and eliminates the absorption band.

 Chemical Interferences:

Chemical interference occurs when an analyte is not totally decomposed in flame. There are fewer atoms present. Dissociation leads to low absorption.

Due to anions: Phosphate ions interfere with the determination of Mg and Ca. The absorption due to Mg and Ca is appreciably weaker in the presence of PO₄^3- ions. This is due to the formation of stable phosphates of  Mg and Ca which do not readily split up into the respective atoms in the flame.

How to overcome it?

 The addition of an excess of strontium (Sr) or lanthanum (La), or thorium (Th) ions minimizes the interference of PO₄^3-   ion in the determination of Mg and Ca by replacing the analyte in the compound formed with the respective interfering species. In short, these ions combine preferentially with PO₄^3-  ions.

 

Due to Cations: In certain cases, cations also interfere in atomic absorption measurements. For instance: Boron interferes with Mg and Ca whereas aluminum interferes with alkaline earth elements.

Protective Agents

 These agents are found to inhibit the interferences by virtue of their ability to form relatively stable but volatile species with the respective analyte.

Ionization interference:

It is observed that the ionization of atoms or molecules is comparatively very small in combustion mixtures that involve air as the oxidant.

The substitution of air occurs with either oxygen or nitrous oxide. However, gives rise to temperatures that are high enough to cause ionization.

M                                       M  +  e-

M= Neutral atom or molecule

M= its corresponding ion

e= electron

Example: The intensity of atomic absorption lines for the alkali metals, such as K, Rb, and Cs is found to be affected by temperature and there is a noticeable decrease 

How to overcome it?

The resulting effects of ionization equilibrium shifts may be eliminated by the addition of an ionization Suppressor that promptly gives a comparatively high concentration of electrons to the flame and results in the suppression of analyte ionization.