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Dr. N. T. Dhokale (M. Sc., SET, Ph.D.)Assistant Professor, Dept. of Chemistry

K. J. Somaiya College, Kopargaon, Dist-Ahmednagar

Atomic Absorption Spectroscopy

To understand the relationship of these techniques to each other, It is

important to understand the atom itself and the atomic process involved

in each technique.

Excited states

Ground state

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3

3

Light energy

Ground state atom (stable or

normal orbital configuration)

Excited states

Ground state

Spectral resonance line (The strongest line)

Degree of absorption:

Total amount of light absorbed = (πe2/mc)NfWhere:

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

Detector

Monochromator

Instrument components

Hollow cathode lamp

Source

Lampslow-pressure inert gas

Inert carrier gasNe or Ar

Hollow Cathodlamp

Atomic Absorption spectroscopy involves the study of the absorption of

radiant energy by neutral (ground state) atoms in the gaseous state.

PP0

Sample

< TD>

< TD>

2-The chopper

its function is to fluctuate the source output.

It is a circular disc divided into four quarters two are mirrored and

two are opened.

The disc rotates at high constant speed, when the mirrored

quarter in front of the lamp, it reflects the radiation

the second moment the open in front of the lamp and the radiation

Passes to the sample being absorbed by it and reaches the

detector in pulses.

The detector converts the radiation to alternating current signal

and amplified it.

The radiation coming from the flame itself and from atoms excited

by the flame will reach the detector continuously and converted to

direct current signal which can be suppressed and eliminated.

This process is known by modulation of the source output.

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A B

Nebulization: sample solution is introduced through an orifice into a

high velocity gas jet, usually the oxidant, in either parallel or

perpendicular manner

• sample stream is converted into a cloud of droplet in the aerosol

modifier or spray chamber, combined with the oxidizer/fuel and

carried to the burner

Electrodless Discharge Lamps, EDL

For easily evaporized elements as Hg or As

Used for AAS and AES

Give much greater radiation intensities than hollow cathod

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

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.

Interferences

Chemical Interferencesoccurs 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

Overcome1. 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.

Chemical Interference: Formation of Stable or Refractory

Compounds

• Elements that form very stable compounds are said to be refractory because

they are not completely atomised at the temperature of the flame or furnace.

• Solution

– Use a higher flame temperature (nitrous oxide/acetylene)

– Use a release agent

– Use protective chelation

Examples

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Determination of calcium in the presence of sulfate

or phosphate (e.g. in natural waters)

3Ca2+ + 2PO43- = Ca3(PO4)2

(stable compound)

Release agent

Add 1000 ppm of LaCl3

2LaCl3 + Ca3(PO4)2 = 3CaCl2 + 2La(PO4)

CaCl2 readily dissociates

Protective chelation

Ca3(PO4)2+3EDTA = 3Ca(EDTA) + 2PO43-

Ca(EDTA) dissociates readily.

Ionisation Interference

• M(g) M+(g) + e-

• A problem in the analysis of alkali metal ions at low flame temperatures and

other elements at higher temperatures.

• Because alkali metals have the lowest ionisation potentials, they are most

extensively ionised in flames.

• At 2450 K and a pressure of 0.1 Pa, sodium is 5% ionised.

• Potassium is 33% ionised under the same conditions.

• Ionised atoms have energy levels which are different to the parent atoms

– therefore the analytical signal is reduced.

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Solution

Add an ionization suppressor

Add an easily ionised element such as Cs.

Add 1000 ppm of CsCl when analysing Na or K.

Cs is more readily ionised than either Na or K.

This produces a high concentration of electrons in the flame.

Matrix effects

• The amount of sample reaching the flame is dependent on the physical

properties of the solution:

– viscosity

– surface tension

– density

– solvent vapour pressure.

• To avoid differences in the amount of sample and standard reaching

the flame, it is necessary that the physical properties of both be

matched as closely as possible.

• Example:

– Analysis of blood

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Interference:Same interferences occur in FES and Flame AAS but to different extents, 4 general

classes

A) Background absorption: is caused by the large number of species present in

the flame (metal oxide, OH radical, H2, fragments of solvent mol.etc)

• incident radiation is absorbed by these species as well as by the analyte atoms

Correction:

1) “blank” solution (solution devoid of analyte) can be used to correct the

measurement from sample solution (in practice it is difficult to prepare exact

blank)

2) use of a continuous source of radiation in conjunction with hollow-cathode line

source

• resonance line of the hollow cathode is absorbed by both free analyte atoms

and interfering species in the flame.

• radiation from the continuous source is absorbed over the entire wavelength

(due to the component of the background) and is a measure of the background

• background correction is then made at the same wavelength as the resonance

line used for AAS determination

3) Pulsed hollow cathode lamp background correction.

• two absorption measurements are made one with the lamp run at a normal low

current and a second with the lamp pulsed to a large current

• the first measurement indicates the absorbance due to both analyte atoms and

background, where as the second measurement indicates primarily background,

because large current eliminates the resonance line

• subtraction of these two absorption measurements yields a corrected value for

atomic absorption

B) Spectral Line Interference:

• arise when the absorption or emission of an interfering species either overlaps or

lies so close to the analyte band that resolution by the monochromator become

impossible

• in AAS amplitude modulation of the radiation source can minimize this interference

C) Vaporization Interference:

• arise when some component of the sample alters the rate of vaporization of salt

particle that contain the analyte

• hotter flame minimizes vaporization interference (acetylene/nitrous oxide flame

as compare to cooler acetylene/air flame for refractory phosphate, sulphate etc)

• pretreatment of sample also helps

D) Ionization Interference:

• atoms with low ionization potential become ionized reducing the population of

both the ground state and excited state free atoms

• by adding an excess of easily ionized element (viz. K, Cs or Sr), ionization in

the sample and calibration solution can be suppressed

• more easily ionized atoms produces a large concentration of electrons in the

vapor and, by mass action, suppresses the ionization of analyte atoms

Relationship Between Atomic Absorption and Flame Emission

Spectroscopy

Atomic Absorption Flame Emission

1. Measures the radiation absorbed by

the unexcited atoms

1. Measures the radiation emitted by

the excited atoms

2. Depends only on the number of

unexcited 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

High energy excitation sources

Plasma excitation sources

Laser

Arc and spark emission spectrometry (Spectrography)

Microwave and x-ray

3. Used for very small samples, even less than 1 mg

4. There is no need for prior separation

5. Relatively rapid technique

Disadvantages 1. Expensive

2. Low precision and accuracy

3. Destroying the sample

4. Used mainly for metals

If the composition of sample and matrix is unknown. The internal standard is added

to both unknown and calibration standards.

The internal standard should

1. resemble the element to be determined in rate of volatilization and chemical

reactivity.

2. have a measurable emission line in the same spectral vicinity as the sample

emission line.

3. It must not also present in the original sample.

Use of an internal standard

Quantitative analysis

Then, by plotting the ratio of intensities of the element to the internal-standard

element vs. concentration of the element, any fluctuations should be compensated

for.

Applications of AAS

Applications of AAS• Agricultural analysis

– soils

– plants

• Clinical and biochemistry

– whole blood, plasma and serum Ca, Mg, Li, Na, K, Cu,

Zn, Fe etc.

• Metallurgy

– ores, metals and alloys

• Lubricating oils

– Ba, Ca, Mg and Zn additives

• Greases

– Li, Na, Ca26

• Water and effluents

– many elements e.g. Ca, Mg, Fe, Si, Al, Ba

• Food

– wide range of elements

• Animal feedstuffs

– Mn, Fe, Co, Cu, Zn, Cr, Se

• Medicines

– range of elements

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Applications of AAS