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Types of Interferences in Atomic absorption Spectroscopy

Classification, causes and remedies.

An interference is a phenomenon that affects the measurement or the population of ground state atoms of an analyte element.


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Spectral Interferences


Spectral interference is caused by radiation overlap of absorption line due to emissions from another element or compound.

If an absorbing wavelength of an element , not being determined but present in the sample falls within the measuring line of the element of interest, the absorbance of the element will be measured together with the analyte of interest and give a higher absorbance value.

Example 1: Vanadium line is 3082.11 A and Aluminium is at 3082.15 A. Choose a different Aluminium line at 3092.7 A.

Note: Interference due to overlapping lines is rare in AAS.

Spectral interference is also effected by the radiation of an emission line of another element or compound, or background radiation from flame, solvent or sample.

Example2: Radiation interference in the determination of Na with Mg present, and in the determination of Iron with Cu or Ni present.

Use alternate wavelength.

Use smaller slit width.

Use blank.

Scattering

This is caused by scattering of the radiation source due to matrix impurities.

Scattering as a spectral interference arise when particulate matter from flame atomization scatter the incident radiation from the source.

Incomplete combustion of organic materials which give rise to molecular species that exhibit broad band spectra.

Example: Refractory oxides formed by Ti, Zr, and W due to atomization of high concentration solutions.

The absorption and scattering of radiation by flame atomization can be corrected by analyzing a blank.

Use Blank.

Background Absorption

Background absorption extends over a broad wavelength band. It is also referred to as Molecular absorption, Broadband absorption or Non specific absorption.

Background absorption is caused by light absorption due to unvaporised solvent droplets in flame.

It is also caused by absorption of unknown molecular species in flame.

The absorbance and scattering of radiation due to matrix interference give rise to sample background which becomes a problem at wavelength below 350nm.

Use Background correction methods.

A deuterium source lamp is used for background correction which comes already fitted into the Atomic absorption instrument by Manufacturer.

Note : Background Absorption as an interferent cannot be corrected by Standard Addition method.

Chemical interferences.


Chemical interference occurs when an analyte is not totally decomposed in flame. There is less atoms present, and therefore a reduced absorbance of the analyte.

Compound Formation.

Compounds cannot be broken down in flame. Anions SO4 and PO4 combines with metal to form stable compounds and decrease the absorption of the analyte.

Example 1:Calcium signal is depressed due to formation of CaSO4 or CaPO4.

Example2: Magnesium signal is depressed in the presence of Aluminium. Aluminium forms heat stable compound with Mg.

Use Hotter flame

Use Releasing agent :

These agents reacts with the interferent. It is a cation that reacts with the interfering anion and frees up the element to be analysed.

La an Sr are good releasing agents for Ca in the presence of SO4 and PO4.
La an Sr are also good releasing agents for Mg in the presence of Al.

Use Protective agent :

These agents form stable but volatile compounds. EDTA and 8-Hydroxy quinolone are protective agents. They can complex Ca in the presence of SO4, PO4 or Al.

Dissociation

Dissociation occurs when metal oxides and hydroxides dissociate in flame to release the metal atom.

Note :
Alkaline earth metal oxides and hydroxides are stable and will produce broad band spectra. Very high temperature is required to break them up.

However, Alkali metal oxides and hydroxides are unstable even at low temperature and will produce line spectra.

Metal oxide and Fuel rich flame signal increase, less oxygen in flame.
Metal oxide and Lean flame signal is decreased, more oxygen for reaction with metal.

Example : Determination of Vanadium in presence of Al and Ti.

Fuel rich flame gives higher absorbance for V because the little oxygen present in flame will be captured by the Al and Ti, and more V atoms available.

Lean flame Xs oxygen is present and V will form the oxide. Addition of extra Al or Ti will not affect the signal decreased absorbance.

Ionization Interferences


Ionization interference affects Gp 1 and 2 only. These include Ba, Ca, Sr, Na, K

This occurs only in hot flames. The energy of the flame excite the ground state atoms to ionic state by loss of electron resulting in a depletion of ground state atoms, and the element does not absorb at the correct wavelength.

Use Low Temperature Flame.

Use Ionization Buffer
(also known as ionization suppressor or ionization suppressant) which prevents analyte ionization

An ionization buffer is a salt of an alkali metal which ionizes to give a mass of electrons that shift the ionization equilibrium of analyte to form atoms.

Add a 0.1% KCl soln to blank, standard, and sample.

This gives an increase in the concentration of electrons in the flame, and drives back the ionic form to ground state where they absorb at the right wavelength.

Matrix interferences


Matrix interference is a physical interference, and can either suppress or enhance absorbance signal of analyte.

It occurs when components of sample matrix other than the analyte react to form molecular species and sample background.

The detector picks up unspecified signals from sample matrix that do not match the absorbance line of the analyte. This results in spurious readings that can affect quantitative and qualitative analysis.

Causes:

1. Characteristics of sample and standards differ in viscosity and surface tension.
2. Sample and standards are prepared in different solvents.
3. Sample and standards are measured at different temperatures.
4. Sample contains a high concentration of dissolved salts or acid.
5. Organics are present in sample matrix.
6. Sample and standards differ in aspiration and atomization rate in flame.

These are physical interferences and can be minimized by the user carrying out some of the following operations:

Use a Blank to zero instrument.

The blank should be representative of the sample matrix. If the sample was digested, the blank should also be digested. It must be prepared in the same way as the analyte except it does not contain the analyte.

Viscosity and solution problems can be solved by adding methanol to sample, standard and blank. This will enhance nebulization and increase the amount of sample entering the flame to give a higher absorbance value.

Incomplete combustion of organics in matrix that produce broadband signal can be reduced or eliminated by increasing the temperature of the flame to ensure complete combustion.

Components in matrix react to form molecular species such as oxides and hydroxides which can be prevented by switching to a higher flame atomization temperature.

Use Standard Addition.

Standard addition is a means to determine the analyte without eliminating the matrix interference.
Add a known component of matrix to both standards and samples so that the interferent becomes insignificant.

Only if there is no other way to compensate for matrix interference, should the method of standard addition be used.

Note
The method of standard addition will not compensate for background absorption or any other type of atomic absorption interferences, not spectral, chemical, or ionization.

It is better to eliminate interferences by proper choice of analytical conditions and chemical treatment of sample.

The absorbance an scattering of radiation due to matrix interference give rise to sample background which becomes a problem at wavelength below 350nm.

Use Background correction.

A deuterium source lamp is used for background correction which comes already fitted into the Atomic absorption instrument by Manufacturer.

Operator actions that can be used to resolve matrix interference problems:

Change instrument settings.
Adjust fuel-to-oxidant ratio in flame.
Use different oxidants for flame.
Increase temperature of flame.

Guidelines for dealing with matrix effects

Match matrix components as closely as posible in sample, standards and blank. Any reagents added to sample should be added to standards and blank.

If an organic solvent is used, sample and standard must be prepared with the same solvent.

All solutions, blank, samples and standards must be allowed to come to the same temperature before measurements are made.



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Signature: Dhanlal De Lloyd, Chem. Dept, The University of The West Indies, St. Augustine campus
The Republic of Trinidad and Tobago.
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