INTERFERENCES IN FLAME PHOTOMETRY

1. INTERFERENCES IN FLAME
PHOTOMETRY
MAHENDRA G S
NGSMIPS

2. Types of Interferences
1. Spectral interference
2. Background emission
3. Chemical interferences
4. Cation interference
5. Anion interference
6. Oxide formation interference
7. Interference due to foreign elements
8. Interference due to salts and acids
9. Matrix effects
10. Interference due to molecular absorption

3. Spectral Interference
 It is encountered while isolating the desired radiant energy.
 The line of emission of the element to be determined and
those due to interfering substances are of similar wavelength.
 For eg., iron line at 324.728 nm overlaps the copper line at
324.754 nm and other iron line at 285.213 nm overlaps the
magnesium line at 285.212 nm.
 By using calibration curves the effect of interfering element is
removed.
 It is also removed by improving the resolution of the
instrument.

4. Background emission
 Interference may arise from the emission band spectra
produced by molecules or molecular fragments present in the
flame gases.
 For eg., band spectra due to hydroxyl and cyanogen radicals
arise in many flames.
 Background effects can also be caused by light scatter.
 It is eliminated by using blank solution.

5. Chemical interferences
 The production of ground state gaseous atoms which is the
basis of flame spectroscopy may be inhibited by two main
forms of chemical interferences.
a) Stable compound formation
b) Ionization
a) Stable compound formation: This leads to incomplete
dissociation of the substance to be analyzed when placed in
the flame or fail to dissociate into the constituent atoms.
For example, the determination of calcium in the presence of
sulphate or phosphate and formation of stable refractory
oxides of aluminium, titanium and vanadium.

6. Chemical interferences
Chemical interferences can be overcome by one of the
following ways:
I. Increase in the flame temperature: often leads to the
formation of free gaseous atoms. For eg., aluminium oxide
is more readily dissociated in an acetylene-nitrous oxide
flame than it is in an acetylene-air flame.

7. Chemical interferences
II. By the use of releasing agents: For eg., determination of
calcium in presence of phosphate, the addition of an excess
of strontium chloride to the test solution will lead to the
formation of strontium phosphate. Calcium can then be
determined in an acetylene-air flame without any
interference due to phosphate.
III. Extraction of an analyte or of the interfering element is an
obvious method of overcoming the effect of interferences.

8. Cation interference
 It is observed with high temperature flame that the intensity of
a line for one cation is enhanced due to the presence of
another.
 This enhancement results from the decrease in ionization of
the cation being studied due to the presence of second cation.
 For eg., the doubling of line intensity of rubidium due to the
presence of potassium ion.
 The addition of radiation buffer to both the sample and the
standard minimizes cation inteferences.
 The buffer consists of high concentration of potentially
interfering ion.

9. Anion interference
 The presence of certain anions in a solution may affect the
intensity of radiation emitted by an element and results in
serious analytical error.
 Anions such as sulphate, phosphate, oxalate, and aluminate
may affect the intensity of radiation emitted by an element.
 For eg., a concentration of barium sulphate produces low
emission intensity than the same concentration of barium
chloride.
 The anion interference is minimized by using radiation buffer
or releasing agents.

10. Oxide formation interference
 It arises due to the formation of stable oxide with free metal
atoms if oxygen is present in the flame.
 Thus, the emission intensity is lowered because a large
percentage of free metal atoms have been removed from the
flame.
 It is minimized by using very high temperature flames to
dissociate the oxides producing free atoms for excitation or
using oxygen deficient environment to produce excited atoms.

11. Interference due to foreign elements
 This interference depends upon the quality of monochromator,
source temperature and the concentration ratio between the
contaminant and the element sought.
 Thus the use of filter will minimize this interference.

12. Interference due to Salts & Acids
 Large amount of salts and acids lower the metallic emission
intensity.
 Use of a releasing agent or protective chelating agents
circumvents this type of interference.

13. Matrix effects
 Matrix effects are physical factors which influence the amount
of sample reaching the flame and are related to viscosity,
surface tension, density, vapor pressure and volatility of the
solvent used to prepare the test solution.
 If a series of standards is to be compared with a test solution, it
is essential that the same solvent should be used for each and
the solutions should not differ widely in their bulk
composition.

14. Interference due to molecular absorption
 In an acetylene-air flame a high concentration of sodium
chloride will absorb radiation at wavelengths 213.9nm, which
is the wavelength of the major zinc resonance line.
 Hence sodium chloride would interfere in the determination of
zinc under these conditions.
 It can be avoided by choosing a different resonance line or by
using a hotter flame resulting in an increase in the operating
temperature, thus leading to dissociation of the interfering
molecules.

15. Conclusion:
• Flame photometry is a simple rapid
method for the routine determination of
elements that are easily excited.
• Using equipment’s with high optical
resolution other metallic elements may
also be determined