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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