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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