Atomic Absorption Spectroscopy
Topics to be covered
¨Importance of elemental analysis .
¨ Introduction to spectroscopy.
¨ Atomic Absorption Spectroscopy (AAS
¨ Atomic Emission Spectroscopy (AES).
¨ Inductively Coupled Plasma Spectroscopy (ICP).
Importance of elemental analysis
Monitoring levels of certain elements in samples ( eg.
pharmaceutical products and standards) to detect
the concentrations of these elements.
Monitoring the levels of the toxic elements in
samples ( eg. cosmetics, food supplements, entire
plant or part of it) to ensure it’s safety.
With these information, we can take steps to approve
or withdraw products from the markets.
n Widely used in clinical chemistry and environmental
Elements (Heavy Metals)
The term heavy metal refers to any metallic
chemical element that has a relatively high density
and is toxic or poisonous at low concentrations.
They have a specific gravity that is at least 5 times
the specific gravity of water.
Example: Arsenic 5.7,Cadmium 8.65 , Iron 7.9,
Lead 11.34 Mercury 13.546
Trace elements :
Heavy metals that are nutritionally essential for
a healthy life.
Examples are ( Iron, Manganese, Copper and
Commonly encountered toxic
¨ Arsenic ¨ Cadmium
¨ Mercury ¨ Aluminum
Types of samples for analysis
¨ Pharmaceutical supplements
¨ Standards ¨Entire plant
or part of it
¨ Cosmetics ¨ Mixture of
known & unknown
n Spectrometric methods are a large group of
n Spectroscopy is the science that deals with the
interactions of radiation with matter (atomic and
n The most widely used spectrometric methods
are based on electromagnetic radiation (light,
gamma rays, X-rays, UV, microwave, and radio-
¨ consists of discrete packets of energy, which
we call photons
¨ A photon consists of an oscillating electric
field component, E, and an oscillating
magnetic field component, M.
¨ The characteristics of these fields are:
Orthogonal ( perpendicular ) to each other
Orthogonal to the direction of propagation
of the photon
They flip direction as the photon travels
All photons (in a given, non-absorbing
medium) travel at the same
What is Frequency (n)?
The number of flips, or oscillations, that occur in one
What is A Wavelength (l)?
n The physical distance in the direction of
propagation over which the electric and magnetic fields
of a photon make one complete oscillation.
Unit: Angstrom, nm, µm
n Velocity Of Light = 2.99792 x 108 m/s
The electromagnetic nature of all photons is the same,
but photons can have different frequencies
The relationship between the light velocity, wavelength,
and frequency is :
E = hn = hv / l
The energy, E, of one photon depends on its frequency of
where h is Planck's constant (6.62618x10-34 J·s)
The relationship between the speed of light c ,
wavelength, and frequency is :
When light passes through other media, the velocity
of light ¯
Since the energy of a photon is fixed, the frequency of
a photon does not change.
Thus for a given frequency of light, the wavelength
must ¯ as the velocity decreases.
n = c / v
The decrease in velocity is quantitated by the
refractive index, n, which is the ratio of c to the
velocity of light in another medium, v:
1- Absorption of Radiation
n When radiation passes through a layer of
solid, liquid, or gas, certain frequencies may
be absorbed, a process in which
electromagnetic energy is transferred to the
n Absorption promotes these particles
from their ground state to more higher-
energy excited state.
n Tow types of absorption spectra:
– Atomic absorption spectrum.
– Molecular absorption spectrum.
n Many compounds absorb radiation. The
diagram below shows a beam of
monochromatic radiation of radiant power P0
directed at a sample solution.
n Absorption takes place and the beam of
radiation leaving the sample has radiant
n The amount of radiation absorbed may be
measured in a number of ways:
– Transmittance, T = P / P0
% Transmittance, %T = 100 T
n A = log10P0 / P
A = log10 1 / T
A = log10 100 / %T
A = 2 - log10 %T
n The last equation, A = 2 - log10 %T , is
worth remembering because it allows
you to easily calculate absorbance from
percentage transmittance data.
n The relationship between absorbance
and transmittance is illustrated in the
n The equation representing the Beer’s law:
– A is absorbance (no units, A = log10 P0 / P ).
– ε is the molar absorbtivity (is a measure of
the amount of light absorbed per unit -1
concentration) with units of L mol cm .
the the sample that
– b is pathpath length of cuvette in which is,
the length of the the
sample is contained. We will express this
measurement in centimeters.
– c is the concentration of the-1.
solution, expressed in mol L
n Beer’s law tells us that absorbance depends
on the total quantity of the absorbing
compound in the light path through the
cuvette. If we plot absorbance against
concentration, we get a straight line passing
through the origin (0,0).
The working curves are used to
• Determine the concentration of an
• To calibrate the linearity of an
What are the Processes by which a molecule
can absorb radiation?
1- Rotational transition :
The molecule rotate about various axes, the
energy of rotation being at definite energy
levels, so the molecule may absorb radiation
and be raised to a higher rotational energy level .
2- Vibrational transition:
The atoms or group of atoms within a
molecule vibrate relative to
each other. The molecule may then absorb a
discrete amount of energy
and be raised to a higher vibrational energy
3- Electronic transition:
The electrons of molecule may be raised to a
higher electron Energy.
The three types of internal energy are
Rotational transitions: low energy E [long λ (microwave or
Vibrational transitions: takes place at high energy E [ near,
far infrared region]
Electronic transitions: takes place at higher energy E
[visible and U.V region]
Type of Radiation Frequency Range (Hz) Wavelength Range Type of Transition
gamma-rays 1020-1024 <1 pm nuclear
X-rays 1017-1020 1 nm-1 pm inner electron
Ultraviolet 1015-1017 400 nm-1 nm outer electron
Visible 4-7.5x1014 750 nm-400 nm outer electron
outer electron molecular
near-infrared 1x1014-4x1014 2.5 µm-750 nm
Infrared 1013-1014 25 µm-2.5 µm molecular vibrations
Microwaves 3x1011-1013 1 mm-25 µm
electron spin flips*
radio waves <3x1011 >1 mm nuclear spin flips*
Cont. Introduction to
TRANSITION SPECTRUM TECHNIQUE MAIN USE
Electronic UV-vis UV-vis spectroscopy Quantitative
Vibrational IR IR Spectroscopy Functional groups
Spin Orientation Radio NMR Structure
¨ Which molecules or atoms can absorb radiation?
n For absorption to occur there must be change in the dipole
moment (polarity) of the molecule.
i.e polar covalent bond in which a pair of electrons is
eg: of a molecule that can not exhibit a dipole
N º N Can not exhibit a dipole and will not absorb in the I.R region.
eg. of a molecule that can exhibit a dipole moment.
O= C =O Unsymmetrical diatomic molecule, does have a permanent dipole
and so will absorb light.
OÞCÜO Vibration mode ® symmetry and no dipole moment
OÜ CÜ O By induced dipole ® dipole moment and the molecule can absorb I.R radiation.
Incase of atoms only electronic transition occurs.
Atomic absorption spectra Molecular absorption spectra
1- The outer most electrons The outer most electrons occupy s,
occupy one of the atomic orbitals p or n electronic energy in the
and have its energy levels [K, L, ground state.
M, N, ......
s, s,p s,p,d s,p,d,f ]
2- Upon excitation electrons are Upon excitation electrons
promoted to any permissible higher p* or s* energy.
atomic energy levels levels
3-Since there are no bonds there Since there are bonds, there are
are no vibrational or rotational vibrational and rotational energy
energy levels in either the ground or levels in both the ground and
excited state. excited states
4- The analytical wavelength is the The analytical wavelength is the
resonance wavelength of the lmax.
5- The spectra are line form. The spectra are in the form
of bands due to the presence
of very close, superimposed
and unresolved vibrational and
rotational energy levels in the
Atomic Absorption Spectroscopy (AAS)
AAS was employed in the 1950’s
Used for qualitative and quantitative detection.
It’s used for the determination of the presence and
concentrations of metals in liquid samples.
Metals that can be detected include Fe, Cu, Al, Pb,
Ca, Zn, Cd and many more.
Concentrations range is in the low mg/L (ppm)
Elements that are highlighted in
pink are detectable by AAS
the AAS instrument
The simple diagram for the AAS
4. The element in the sample
will absorb some of the light,
thus reducing its intensity
3. A beam of UV light monochromator
will be focused on the isolates the line of
1. We set the
instrument at certain
wavelength suitable 2. The element
for a certain element in the sample
6. The detector
will be atomized
measures the change
7. A computer data
system converts the
change in intensity into
The disadvantage of both the HCL and laser is that they
have narrow-band light sources and so only one element is
measurable at a time.
They are intense enough to excite atoms to higher
energy levels. This allows AA and atomic fluorescence
measurements in a single instrument.
Simple dedicated AA instruments often replace the
monochromator with a bandpass interference filter.
A. Flame Atomic Absorption Spectroscopy:
n The technique requires a liquid sample to be
aspirated, aerosolized, and mixed with
combustible gases, such as acetylene and air
or acetylene and nitrous oxide.
n The mixture is ignited in a flame whose
temperature ranges from 2100 to 2800 ºC.
4. The mixture flows
immediately into the
5. It burns as a smooth,
laminar flame evenly
distributed along a narrow
1. mixes acetylene (the fuel)
(air or nitrous oxide). 3. The result is a heterogeneous
mixture of gases (fuel + oxidant) and
suspended aerosol (finely dispersed
2. A negative pressure is formed
at the end of the small diameter,
plastic nebulizer tube®
6. Liquid sample not flowing Note:
into the flame collects in the When do we use NO2 ?
The process of lighting the AAS flame involves:
turning on first the fuel then the oxidant and then
lighting the flame with the instrument's auto
The flame breaks down the analyte's matrix ® create
the elemental form of the analyte atom.
During combustion, atoms of the element of
Interest in the sample are reduced to free,
unexcited ground state atoms, which absorb
light at characteristic wavelengths,
as shown in the figure.
Optimization is accomplished by :
• Aspirating a solution containing the element
• Adjusting the fuel/oxidant mix until the
maximum light absorbance is achieved.
• Careful control of the fuel/air mixture is
important because each element's response
depends on that mix in the burning flame.
3. Lamp (Hollow Cathode Lamb):
Consists of a cathode and an anode. The cathode is
made of the element of interest
1. A large voltage across the anode and cathode will
cause the inert gas to ionize.
2. The inert gas ions will then be accelerated into the
cathode, sputtering off atoms from the cathode.
3. Both the inert gas and the sputtered cathode
atoms will in turn be excited by collisions with each
4- When these excited atoms decay to lower
energy levels they emit a few spectral
lines characteristic of the element of
5- The light is emitted directionally through
the lamp's window, a window made of a
glass transparent in the UV and visible
6- The light can then be detected and a
spectrum can be determined.
: 4. Monochromator
The light passes from the HCL through the element in
the sample to the monochromator.
It’s function is:
It isolates the specific light of the element of
interest from the other background lights and
transfers it to the photomultiplier tube (detector).
5. Photomultiplier Tube (PMT)
Before an analyte is aspirated, a measured signal is
generated by the PMT as light from the HCL passes
through the flame. When analyte atoms are present in
the flame--while the sample is aspirated--some of that
light is absorbed by those atoms. This causes a
decrease in PMT signal that is proportional to the
amount of analyte
The PMT detects the amount of reduction of
the light intensity due to absorption by the
analyte, and this can be directly related to
the amount of the element in the sample.
The PMT converts the light signal into an
signal and a computer system translates
Convert light energy to electrical energy
Gases used in the FAAS:
Different flames can be achieved using different
mixtures of gases, depending on the desired
temperature and burning velocity.
Some elements can only be converted to atoms at high
Some metals form oxides that do not re-dissociate into
atoms. To inhibit their formation, conditions of the
flame may be modified to achieve a reducing, non-
Table of the characteristics of various
Max. flame speed (cm/s) Max. temp. (oC)
Air-Coal gas 55 1840
Air-propane 82 1925
Air-hydrogen 320 2050
Air-50% oxygen-acetylene 160 2300
Oxygen-nitrogen-acetylene 640 2815
Oxygen-acetylene 1130 3060
Oxygen-cyanogen 140 4640
Nitrous oxide-acetylene 180 2955
Nitric oxide-acetylene 90 3095
Nitrogen dioxyde-hydrogen 150 2660
Nitrous oxide-hydrogen 390 2650
B. Graphite Furnace Atom Absorption
(GFAAS) Graphite Furnace Atomic Absorption
(ETAAS) Electro thermal Atomic Absorption
5. The monochromator isolates the light of
the element of interest from the
4. The graphite tube is
permanently flushed with 1. The source of light (lamp)
background lights to the PMT. The PMT argon while it is in operation emits light with a wavelength
tube measures the change intensity. specific to the element of
2. A controlled voltage is applied at
the ends of the graphite tube, which 3. Samples are deposited in the graphite tube
is heated rapidly to high ® heated to vaporize and atomize the analyte
temperatures (up to 2600°C). ® atoms absorb ultraviolet or visible light and
make transitions to higher electronic energy
vFormation of stable Thermal oxide (Al, Mo, Ti)
vSample stay for Long time in graphite tube>>>hig
vSuitable for solid samples
vIonic interferase rather than chemical or physical
due to high temp.
Mercury Cold Vapor
-Free mercury atoms can exist at room temperature
can be analysed using atomic absorption without
employing flame and graphite furnace techniques at
-Mercury is reduced in solution using stannous
chloride or sodium borohydride in a closed
-The reaction quantitatively releases mercury
(from the sample solution) and is carried by a
stream of air or argon through a quartz
sample sell placed in the light path of an AA
instrument for analysis.
The detection limit for mercury by this cold
vapor technique is approximately 0.02 mg/L.
Hydride Generation Atomic Absorption
It’s used for elements that are forming volatile
hydrides (e.g. As, Se, pb, Sb, Te, Sn, Bi)
(HAAS) is very useful in case of interferences, poor
reproducibility, and poor detection limits.
4. The PMT detects the amount of
reduction of the light intensity due to
absorption by the analyte and can be 1. The HCL emits the light
directly related to the amount of the with a wavelength
element in the sample characteristic to the element
3. The monochromator isolates 2. The Light passes from the HCL
analytical lines' photons passing through the optical cell to the
through the optical cell monochromator then to the
and removes the scattered light of detector
other wavelengths from the optical
sample flow in the (HGAAS)
4. In the optical cell the flame is ignited automatically by 3. The liquid mixture flows
the air/C2 H 4 and the gaseous into a gas/liquid separator where
metal hydride form decomposes into the elemental form the hydride and some gaseous hydrogen are
which can absorb the purged (via a high purity inert gas) into the optical
HCL's beam. The light passes to the Mon. and then to cell via a gas transfer line
Gas liquid separator
1. The metal oxyanions reacts with 2. The peristaltic pump and the flowing reagents along
Sodium Borohydride and HCl and with the tubing of specific lengths controls the time
produces a volatile hydride: H2Te, from reagent mixing and separation of
H2Se, H3As, H3Sb, etc. the volatile hydride from the
liquid and sending the sample to the optical cell.
Any factor that affects the ground state population
of the analyte element.
Factors that may affect the ability of the
instrument to read this parameter.
A) Spectral interferences: due to radiation
overlapping than of the light source.
B) Formation of compounds that do not dissociate in
The most common example is the formation of
calcium and strontium phosphates.
C) Ionization of the analyte reduces the
This is commonly happens to barium, calcium,
strontium, sodium and potassium.
D) Matrix interferences: due to differences
between surface tension and viscosity of test
solutions and standards
E) Broadening of a spectral line
1. Doppler effect
2. Lorentz effect
3. Quenching effect
4. Self absorption or self-reversal effect