Atomic Absorption spectroscopy – Instrumental Methods of Analysis B. Pharma 7th Semester

Atomic Absorption spectroscopy

Objectives

After this session students will be able to

       Explain
the principle  and  instrumentation of atomic absorption
spectrophotometry

       Distinguish
between the atomic absorption spectroscopy and flame photometry

Atomic Absorption
Spectroscopy

Atomic Absorption spectroscopy involves the study of the
absorption of radiant energy by neutral (ground state) atoms in the gaseous
state.

Hollow
Cathode Lamp

Emission is form elements in cathode that have been
sputtered off into gas phase

Electrodless
Discharge Lamps, EDL

For easily evaporized elements as Hg or As

Used for AAS and AES

Give much greater radiation intensities than hollow cathode

There is no electrode, but instead , the inert carrier gas
is energized by an intense field of radiofrequency or microwave radiation →
plasma formation which cause excitation of the metal inside

Degree of
absorption:

Total amount of light absorbed = (πe2/mc2)Nf

Where:

e = electronic charge,  
m = mass of electron

c = speed of light,    
     N = total No. of atoms that
can absorb light

f = Ability of each atom to absorb light

π, e, m,
and c are constants, therefore

Total amount of light absorbed = constant x Nf

Since f is also constant for the same substance

A & C

Interferences

Spectral
Interferences

1. They arise when the absorption line of an interfering
species either overlaps or lies so close to the analyte absorption line that
resolution by the monochromator becomes impossible. Ex. Mg in presence of Ca.

2. They occur from band or continuous spectra which are due
to absorption of molecules or complex ions remaining in the flame

3. They arise from flame background spectrum.

Correction:

1. It may be useful to shift to another spectral line

2. Two line correction method: (Instrumental correction)

    It employs a line
from the source as a reference.  The line
should lie as close as possible to the analyte line but must not be absorbed by
the analyte.  If the conditions are met,
any decrease in the reference line from that observed during calibration arises
from absorption by the matrix of the sample.

Chemical
Interferences

occurs during atomization that prevent the gaseous atoms
production of the analyte.  They are more
common than spectral ones. 

Types of chemical
interferences

  1. Formation
    of stable compounds: → incomplete dissociation of the sample in flame

  2. Formation
    of refractory oxides: → which fail to dissociate into the constituent
    atoms

Examples

  1. Detn.
    of Ca in presence of sulphate or phosphate

  2. Formation
    of stable refractory oxides of TiO2, V2O5
    or Al2O3 by reaction with O2 and OH
    species in the flame

Overcome

1. Increase in the flame temp. → Formation of free gaseous
atoms

 e.g. Al2O3
is readily dissociated in acetylene-nitrous oxide flame

2. Use of releasing agents:  
M-X  + R  → RX  +
M  ex. Detn of Ca in presence of  phosphate   

  (Ca – phosphate +
SrCl2 → Sr-phosphate + Ca atoms) or (Ca – phosphate + EDTA → Ca-EDTA
easily dissociated complex).

3. Solvent extraction of the sample or of the interferring
elements

Ionization
Interferences

Ionization of atoms in the flame → decrease the absorption
or emission

Overcome :  1. Use of lowest possible temp which is
satisfactory for the sample ex. Acetylene –air must not be used for easily
ionised elements as Na, K, Ca, Ba

2. Addition of an ionisation supressant ( soln of cation has
a lower ionisation potential than that of the sample, e.g. addition of K-soln
to Ca or Ba soln.  Ca → Ca2+ +
2e     K → K+  + e

Physical
Interferences

  1. Variation
    in gas flow rate

  2. Variation
    in sample viscosity

  3. Change
    in flame temp.

Overcome:  1. by continuous calibration

                             
2. Use of internal standard

Advantages of AAS:            
Very sensitive.

                                                      Fast.

Disadvantages of
AAS:
  Hollow cathode lamp for
each element.

                                                     Expensive element.

 

Relationship
between Atomic Absorption and Flame Emission Spectroscopy

Atomic Absorption

Flame Emission

1.  Measures the
radiation absorbed by the excited atoms

1.  Measures the
radiation emitted by the excited atoms

2.  Depends only
on the number of excited atoms

2.  Depends only
on the  number of excited atoms

3.  Absorption
intensity is NOT affected by the temperature of the flame

3.  Emission
intensity is greatly affected by the temperature variation of the flame

Summary

       Atomic
Absorption spectroscopy involves the study of the absorption of radiant energy
by neutral (ground state) atoms in the gaseous state.

       Hollow
cathode lamp is the radiation source.

       Total
amount of light absorbed = (πe2/mc2)Nf

Where:

e = electronic charge,  
m = mass of electron

c = speed of light,         
N = total No. of atoms that can absorb light

f = Ability of each atom to absorb light

π, e, m,
and c are constants,

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