Instrumental Methods of Analysis

Atomic Absorption Spectroscopy (AAS)

Lecture 2-5
Prof. Rajakumar, A
Atomic Spectroscopy

• Atomic Absorption Spectroscopy (AAS)

• Atomic Emission Spectroscopy (AES)

• Alan Walsh in 1955 pointed out the potential use of

atomic absorption spectroscopy (AAS) for the analysis
of metallic elements.
• PRINCIPLE
• If light of the resonance wavelength is passed through a flame
containing the atoms in question, then part of the light will be
absorbed, and the extent of absorption will be proportional to the
number of ground-state atoms present in the flame.
Atomic Spectroscopy applicable to?
 Atomic Spectroscopy
air nebulizer

• Flame Atomization liquid

• used for liquid samples
• liquid pulled by action of light beam
nebulizer
• nebulizer produces spray of
sample liquid burner head
• droplets evaporate in spray
chamber leaving particles
spray chamber
• fuel added and ignited in flame oxidant (air or N2O)
• atomization of remaining
particles and spray droplets fuel (HCCH)
occurs in flame
• optical beam through region of
best atomization
nebulizer

sample in
AAS Instrument
Light of certain wavelength (resonance light) is allowed to pass through the flame.

The sample solution is aspirated into the flame, where the liquid salt gets converted into
vapour state, then dissociated into atoms (in ground state).

These g.s. atoms undergo excitation by suitable radiation from an external source.

Finally the unabsorbed radiation from the flame is passed through a monochromator
and then detected.
Schematic: Atomic Absorption Spectrometer
Sample
Compartment

Light Source
Detector
Monochromator bandwidth
(~100x greater than atomic lines The relationship between absorbance and
the concentration of atoms

Integrated absorption

 K d=(e2/mc)N0
K - the absorption coefficient at the frequency 
e – the electronic charge
m – the mass of an electron
c – the velocity of light
f – the oscillator strength of the absorbing line
N0 – the number of metal atoms per milliliter able to
absorb the radiation
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The relationship between absorbance and the
concentration of atoms
This difficulty was overcome by Walsh,
who used a source of sharp emission lines with a
much smaller half-width than the absorption line.
and the radiation frequency of which is centered
on the absorption frequency.

In this way, the absorption coefficient at the centre of the line, K0 , may be measured
instead of measuring the integrated absorption.
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Hollow Cathode Lamp

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Hollow cathode Lamp (HcL)
Noble Anode = Generally tungsten wire
Gas
Cathode = Made up of element whose
spectrum is desired
Cathode Window Window = Made up of glass
Anode
Environment inside the tube = Inert gases like
neon or argon at low pressure.

(i) DC voltage of 300 – 500 V is applied across the electrodes

& filled with neon or argon gas 1 – 5 torr
The atoms of the filler gas undergo ionization at the anode and the fast moving ions strike
the surface of the cathode: displaced the surface metal atoms.
Which gets excited due to collision by charged ions, and emits radiation characteristics of
cathode metal (as sharp line spectrum).
Ex. Cu emits copper spectrum, Zn emits zinc spectrum
Atomic Spectroscopy
Absorption Spectrometers
hollow cathode lamp
• A narrower emission spectrum from emission
hollow cathode lamp (vs. flame
absorption) results in better Beer’s Atomic absorption
law behavior

Intensity or absorbance
spectrum in flame

Additional broadening in
flame from temperature
(Doppler) or pressure

wavelength
Atomic Spectroscopy
Interference in Absorption Measurements
• Spectral Interference
• Very few atom – atom interferences

• Interference from flame (or graphite tube) emissions are reduced by modulating lamp
• no lamp: signal from flame vs. with lamp

• then with lamp: signal from lamp + flame – absorption by atoms

• Interference from molecular species absorbing lamp photons (mostly at shorter

wavelengths and light scattering in EA-AA)

• This interference can be removed by periodically using a deuterium lamp (broad band
light source) or using the Zeeman effect (magnetic splitting of absorption bands)
Hollow Cathode Lamp
• The cathode of the hollow cathode lamp (HCL) contains the
element being analysed.
• Therefore the atomic radiation emitted by the HCL has the
same frequency as that absorbed by the analyte atoms in
the flame or furnace.
• The linewidth from the HCL is relatively narrow (compared
to linewidths of atoms in the flame or furnace) because of
low pressure in lamp and lower temperature in lamp (less
Doppler broadening).
• Thus the linewidth from the HCL is nearly
“monochromatic” (vs sample).
• Different lamp required for each element although some
are mulit-element.

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17
 Electrothermal Atomisation - Graphite
Furnace
• Sample holder consists of a graphite tube.
• Tube is heated electrically
• Beam of light passes through the tube.
• Offers greater sensitivity than flames.
• Uses smaller volume of sample
• typically 5 - 50 l (0.005 - 0.05 ml).
• All of sample is atomised in the graphite tube.
• Atomised sample is confined to the optical path for several
seconds (residence time in flame is very short).
• Uses a number of stages as shown on next slide.

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19
 Stages in a Graphite Furnace

• Typical conditions for Fe:

• Drying stage: 125o for 20 sec
• Ashing stage 1200o for 60 sec
• Atomisation 2700o for 10 sec

• Requires high level of operator skill.

• Method development difficult.

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Schematic Diagram of a Graphite Furnace

h
HCL

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Graphite furnace technique

Processes

drying ashing atomization

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Graphite furnace technique

Advantages
Small sample sizes ( as low as 0.5 uL)

Very little or no sample preparation is needed

Sensitivity is enhanced
( 10 -10 –10-13 g , 100- 1000 folds)

Direct analysis of solid samples

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Graphite furnace technique

Disadvantages
Background absorption effects

Analyte may be lost at the ashing stage

The sample may not be completely atomized

The precision was poor than the flame method

(5%-10% vs 1%)

The analytical range is relatively narrow

(less than two orders of magnitude)
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Cold vapour technique

Hg2+ + Sn2+ = Hg + Sn (IV)

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Hydride generation methods

For arsenic (As), antimony (Te) and selenium (Se)

NaBH4 heat
As (V) AsH3 As0(gas) + H2

[H+] in flame
(sol)

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NaBH4

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Advantages and Disadvantages of Flame AAS
• Advantages
• equipment relatively cheap
• easy to use (training easy compared to furnace)
• good precision
• high sample throughput
• relatively facile method development
• cheap to run
• Disadvantages
• lack of sensitivity (compared to furnace)
• problems with refractory elements
• require large sample size
• sample must be in solution

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 Advantages and Disadvantages of
Electrothermal Atomisation
• Advantages
• very sensitive for many elements
• small sample size
• Disadvantages
• poor precision
• long cycle times means a low sample throughput
• expensive to purchase and run (argon, tubes)
• requires background correction
• method development lengthy and complicated
• requires a high degree of operator skill (compared to flame
AAS)
Sensitivity: [10-10g (flame), 10-14g (non-flame)]
Detection limit: Flame 10-100 ppm (GF AAS = 0.1 to 1 ppm) 29
Applications of AAS
• Lubricating oils
• Agricultural analysis
• Ba, Ca, Mg and Zn additives
• soils
• Greases
• plants
• Li, Na, Ca
• Clinical and biochemistry
• whole blood, plasma and
serum Ca, Mg, Li, Na, K, Cu, • Water and effluents
Zn, Fe etc.
• many elements e.g. Ca, Mg, Fe, Si, Al,
• Metallurgy Ba
• ores, metals and alloys • Food
• wide range of elements
• Animal feedstuffs
• Mn, Fe, Co, Cu, Zn, Cr, Se
• Medicines
• range of elements
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Atomic Spectroscopy

Interference in Absorption Measurements

• Chemical Interference
• Arises from compounds in sample matrix or atomization conditions that affects
element atomization

• Some examples of specific problems (mentioned previously) and solutions:

• Poor volatility due to PO43- – add Ca because it binds strongly to PO43- allowing analyte
metal to volatilize better or use hotter flames

• Formation of metal oxides and hydroxides – use fuel rich flame

• Ionization of analyte atoms – add more readily ionizable metal (e.g Cs)

• Another approach is to use a standard addition calibration procedure (this won’t

improve atomization but it accounts for it so that results are reliable)
2. Chemical Interference
i). Ionization interference: This causes due to the ionization of
some metals at high temperature (Alkali metals)
Thus ionization interference decreases the radiant power of
atomic emission.
+ −
Na  Na + e

This kind of interference can be over come by adding a large

quantity of potassium salt. The addition of potassium prevents
the ionization of sodium salt but it itself undergoes ionization.

ii). Cation – anion interference

Presence of anions such as oxalates, phosphates, sulphates,
aluminates, etc affects the intensity of radiation emitted by an
element.
This is due to the formation of stable compounds, which will not
decompose so easily at low temperatures.
E.g.: 1. Calcium in presence of phosphate ion forms calcium
phosphate, a stable substance.
2. Barium in presence of sulphate ion forms barium sulphate, a
stable substance.
This type of interference can be overcome either by extraction of
anions or using calibration curves.
iii). Oxide formation interference
This type of interference arises due to the formation of stable oxides
with free metal atoms if oxygen is present in the flame. Alkaline earth
metals forms stable oxides.