Paper No. : 06 Atomic Spectroscopy
Module :02 Theory of Atomic Absorption spectrometry (AAS)
Principal Investigator:
Dr.NutanKaushik, Senior Fellow
The Energy and Resouurces Institute (TERI), New Delhi
Co-Principal Investigator:
Dr. Mohammad Amir, Professor of Pharm. Chemistry,
JamiaHamdard University, New Delhi
Paper Coordinator:
Dr. MymoonaAkhtar, Associate professor, Dept. of Pharm.
Chemistry, JamiaHamdard, New Delhi.
Content Writer:
Dr. MymoonaAkhtar, Associate professor, Dept. of Pharm.
Chemistry, JamiaHamdard, New Delhi.
Content Reviwer:
Dr. Mohammad Amir, Professor of Pharm. Chemistry,
JamiaHamdard University, New Delhi
Analytical Chemistry
/ Instrumentation
Atomic Spectroscopy
Theory of Atomic Absorption spectrometry (AAS)
Description of Module
Subject Name
Analytical Chemistry / Instrumentation
Paper Name
Atomic Spectroscopy
Module Name/Title
Theory of Atomic Absorption spectrometry (AAS)
Module Id
02
Pre-requisites
Objectives
Keywords
Analytical Chemistry
/ Instrumentation
What is spectra and how are they produced
Importance of Selection of spectral lines
Line broadening concept and what causes line broadening
Relation between laws of absorbance and concentration of substance under
analysis
Atomic Absorption spectrometry (AAS),Atomic spectra, Doppler Broadening,
Pressure Broadening, Beer Lambert law, Zeeman Effect
Atomic Spectroscopy
Theory of Atomic Absorption spectrometry (AAS)
Introduction
Atomic absorption spectroscopy (AAS) is a technique that involves study of absorption of
electromagnetic radiations in relationship to atomic structure. The principle of AAS is
measurement of the concentration of elements present in the sample through their property of
absorption of light. The technique is simple and reliable based on absorption of radiation by
free atoms for their determination. AAS is the oldest instrumental elemental analysis
principle, the origins of which go back to the work of Bunsen and Kirchhoff in the mid-19th
century. AAS is based on absorption by atoms or elementary ions. The components of a
sample are converted into gaseous state (gaseous atoms) or
elementary ions by suitable heat treatment. The absorption is
measured at a selected wavelength, characteristic to individual
element and can serve for qualitative and quantitative
determination of one or more of the elements present in the
sample. The process of converting sample into vapors is called
atomization. The precision and the accuracy of AAS are
critically dependent upon atomization step.
Atomic Structure and Spectra
To understand the concept of atomic absorption process, it is
important to understand the structure of atom first. The basic
processes in this spectrometry involve the outer electrons of the
atom and thus its pros and cons can be well understood from
the theory of atomic structure itself. According to the Niels
Bohr (1913), the structure of the atom consists of central core
Analytical Chemistry
/ Instrumentation
Atomic Spectroscopy
Fig:1 Representation of atomic
structure
Theory of Atomic Absorption spectrometry (AAS)
or nucleus, made up of protons and neutrons surrounded by the electrons in orbits of differing
energy. These orbits were described as energy levels which differ in energy from each other.
Each orbit has a fixed energy associated with it, in general an electron has lowest energy in
its ground level, and higher energy in its excited state, and can be easily removed. When the
associated electron of an atom is in its ground state, the atom is said to be in the ground
state. Normally, electrons try to stay in the lowest energy level open to them, but these
electrons in its ground state can absorb energy in discrete amounts of heat or light at certain
discrete wavelengths, corresponding to the energy requirements of the particular atom. This
absorption of light, heat or collision with another particle results in the increased energy of an
atom which can result in one or more changes.
1) Increase in kinetic energy of the atom or
2) become excited by absorbing energy
The electron tends to remain in its permitted energy levels but may change to another level if
the amount of energy absorbed is equal to the difference between the two levels. When the
electron moves to the higher energy level, such as E1, it is said to be excited. Each atom has
quantized energy levels depending upon the number of protons and electrons present. Each
element has a unique set of energy levels pertaining to the unique set of electrons and
protons. It is these energies which are measured in relation to the ground state, and a
particular excited state above the ground state (Fig2).
Analytical Chemistry
/ Instrumentation
Atomic Spectroscopy
Theory of Atomic Absorption spectrometry (AAS)
Fig 2 Energy level diagram illustrating the excitation, ionization and emission processes for an
atom.The energy levels within the atom are represented by the horizontal lines, and the vertical
arrows signify energy transitions–a and b represent excitation (Adopted from Varian Australia
Ptv Ltd (A.C.N. 004 559 540))
On the other hand every element has characteristic spacing between the energy levels that is
proportional to wavelength of the absorbed light. For example the shorter wavelength of light
energy will be absorbed if the spacing between the energy levels is wider
Atomic spectra
Each orbital in an atom is characterized by principle and azimuthal quantum numbers n and l,
respectively corresponding to the electron's energy and angular momentum. When an electron
undergoes a transition from a higher energy level (E2) to a lower energy level (E1), light of
frequency
ν = (E1 - E2)/h = ∆E/h
is given off. In terms of wavelength,
E1
λ = C/ν =hc/∆E
Where c is the velocity of light and h is the
ΔE = E1-E
planck’s constant. ν is the frequency, E energy
E
Analytical Chemistry
/ Instrumentation
Atomic Spectroscopy
Energy Level Diagram
Theory of Atomic Absorption spectrometry (AAS)
and λ is the wavelength and E1, E2 are the energy levels of excited and ground states.
The latter is most frequently used in atomic absorption spectroscopy. For a given electronic
transition the parameters ∆E, ν and λ have unique values. An element can go many electronic
transitions and the transition between electronic energy states is characterized by a their
different in energy and hence different atoms will absorb at different set of wavelengths.
These are the characteristic wavelengths which will emit an element in the process of being at
a higher energy level and relaxing to the ground state. When a range of wavelength is
surveyed a sharp energy maximum is shown by each wavelength (a spectroscopic 'line') and
these lines characteristic to atom and are basis of distinguished atomic spectra. The lines
originating from the ground state of the atomare called resonance lines and are the most
sensitive and useful analytical lines for atomic absorption spectroscopy. Non-resonance lines
are from the transitions of one excited state to another excited state when empty orbitals are
available. These non-resonance lines are not
generally
useful
for
atomic
absorption
analysis.
Excited state
This technique can be used for qualitative
analysis of a sample as every atom is
different and absorbs energy of distinct
pattern of wavelengths. For example Sodium,
the energy level diagram of sodium metal
study shows number of transitions all of
which arise from 3s. The dark line represent
the most probable transitions and they would
Ground state
appear more intense than any other transition
in AAS. Transitions at 589.0 and 589.6 nm
Figure 3: Energy level (term or Grotrian diagram)
diagram of Sodium metal
Analytical Chemistry
/ Instrumentation
Atomic Spectroscopy
Theory of Atomic Absorption spectrometry (AAS)
are comparable transitions in terms of energy. This indicates splitting of p orbital into two
levels which are slightly different in their energies. This is called splitting which results due
to interaction of magnetic field generated from the spin and orbital moment. The same can
occur in d and f orbital’s but the energy difference between the levels is too small to be
observed. The splitting of higher energy p, d, f orbitals into two states is characteristic of all
species containing a single valance/external electron
That is why energy level diagram of Mg+ is similar to that of Na, but the energy difference
between 3s and 3p state in Mg is roughly twice than that of Na which may be because of
large nuclear charge of the Mg. Dipositive atoms have similar kind of energy level diagram.
However, as the size of atom increases that means the number of electrons in the valance
shell also increases, lead to complicated atomic spectra and difficult to understand
Selection of spectral lines:
The Grotrian (term) diagram is usually used to illustrate the spectral transitions and emission
of an atom. The term diagram of sodium is given in Fig 4, and it is clear from it that the
common atomizers used in AAS do not generate sufficient thermal energy to cause excitation
a larger number of atoms. According to the selection rules all lines occurring in the
absorption spectrum corresponds to the valence electron of 3 2S1/2 term which arise from
transitions from 3S to 3p.
3 2S1/2
----→ 3
2
P1/2;3/2
(589.593nm/588.966nm)
3 2S1/2
----→ 4
2
P1/2;3/2
(330.294/nm330.234nm)
3 2S1/2
----→ n
2
P1/2;3/2
These lines are called also resonance lines which emerge from the ground state of the neutral
atom and in absorption measurements are the most commonly used analytical line.
Analytical Chemistry
/ Instrumentation
Atomic Spectroscopy
Theory of Atomic Absorption spectrometry (AAS)
Fig 4.Grotrian(Term) diagram for sodium. The continuous lines with double arrows represent
transitions for the main series and occur in both absorption and emission. The broken lines are
for the transitions of secondary series and at the temperatures of interest occur only in emission.
Thick transition lines indicate strong spectral lines. (Adopted from Hyperphysics by R Nave)
In case of sodium atoms, a strong absorption at 589.0 nm is because this wavelength of light
possesses the exact amount of energy to excite the sodium atom to another higher state.
Therefore this transition of electrons is very specific for sodium and different atoms have
different energy requirements hence do not absorb light at this wavelength. The sodium ion
which is in the excited state by absorbing energy is still sodium atom but with more energy.
Depending upon the number of electrons and protons each atom has quantized energy levels.
That is each element has a set of energy levels which is unique pertaining to the unique set of
electrons and protons? The energies are usually measured in relation to the ground state of
an atom to its particular excited state, for example, sodium atom has 2.2 eV more energy in
its excited state than in a ground state which, by convention is arbitrarily set at zero.
Analytical Chemistry
/ Instrumentation
Atomic Spectroscopy
Theory of Atomic Absorption spectrometry (AAS)
Figure 5 depicts the simplified form of absorption spectrum of sodium. The intensity
decreases regularly towards the short wavelength of the main series, till the convergence limit
is reached. Beyond this value no more lines are observed, only the continuous spectrum is
observed. The total stripping of the valence electrons from the shell of the sodium atom
corresponds to this convergence limit in the spectrum which represents the energy of
ionization for this atom. A different characteristic spectrum of sodium ion is resulted from the
changed electronic structure. From the term diagram, the relatively simple relationships that
can be deduced which indicate the absorption spectra have comparatively few lines and are
thus clear.
Fig 5: Spectra of sodium (a) absorption (b) emission (Adopted from Atomic Absorption
Spectrometry by Welz and Sperling)
Line width and line profile
Line width is important in AAS, as narrow lines are highly desirable because they reduce the
possibility of interference due to overlapping spectra. The atomic absorption spectra arises
from transition between two quantum states. Since the energy difference between the two
states is definite, the atomic absorption spectra should have thin lines with small or zero line
width. But on careful observation and examination the lines were found to have finite width
(which is also in accordance to Heisenberg principle of uncertainty). The lines are generally
made up of asymmetric distribution of wavelengths (λs) that center about a mean wavelength
Analytical Chemistry
/ Instrumentation
Atomic Spectroscopy
Theory of Atomic Absorption spectrometry (AAS)
(λ0) which is the wavelength (λ) of maximum intensity for emitted radiation. The energy
association with λ0 is equal to energy difference between quantum state responsible for
absorption or emission.
Line width of an atomic absorption is defined as its with is λ units when measured at half the
maxima.For example the spectral lines discussed up to now do not have an exactly defined
frequency but has line profile as presented as in figure 6
Analytical Chemistry
/ Instrumentation
Atomic Spectroscopy
Theory of Atomic Absorption spectrometry (AAS)
Fig 6: Line profile and half- width of spectral line. The parameters central frequency, ν0 the peak
amplitude lp and the frequency distribution (line profile) with a width of ∆ νcw are characteristics
of this profile. The frequency distribution (line profile) with a width of ∆ νcwis generally termed
as the width of the profile at half maximum (FWHM+= Full Width at Half Maximum), lP/2. The
spectral range within the half- width is the line core and ranges to either side are the wings. The
reason for this broadening of spectral line is explained in following section.
Line broadening and its types
There are number of mechanisms which can broaden the absorption or emission line in its
frequency distribution, because of the fact that an emitting or absorbing atom are not two
isolated atom, but a single atom in interaction with its environment.
Types of line broadening
Lorentzian Homogeneous (affects 1. Natural broadening
all molecules equally)
Result of finite radiative
lifetime
2. Collisional/pressure broadening
Finite lifetime in
quantum state owing to collisions
Gaussian Inhomogeneous (affects 1. Doppler broadening Thermal motion
certain class of molecule)
4. Voigt profile Convolution of 1-3
Lorentzian + Gaussian
The most important broadening mechanisms are discussed below in detail:
Natural Line Width:
The natural line width is the minimum possible half-width. An excited atom can typically
stay for about 10-9 to 10 -8s in an excited state before it loses the excess energy as a photon to
reach the ground state.
Analytical Chemistry
/ Instrumentation
Atomic Spectroscopy
Theory of Atomic Absorption spectrometry (AAS)
According to the principle of uncertainty, the energy levels of the transition can be determined
with an uncertainty of ∆E over the observation time ∆t.
∆E ∆t ≥ h/2π
From this uncertainty of energy levels of a radiative transition we obtain via:
∆E =hν
An uncertainty of frequency ν of the respective spectral line;
𝛿ν = ∆E /ν =1/2π τ
The relaxation time of each level also enters the uncertainty relationship with transition from one
energy level to another transition level, as depicted schematically in Fig 7 and the expression for
the natural half width ∆νN is obtained
Fig.7: Natural broadening line as a result of the broadened energy levels of the transition.
(Adopted from Atomic Absorption Spectrometry by Welz and Sperling)
∆νN = ν20 (
1
𝜏𝑘
+
1
𝜏𝑓
)
2π
Where ν0 is the frequency of the line, τk relaxation time of the excited state, and τf relaxation
time of the lower state
Analytical Chemistry
/ Instrumentation
Atomic Spectroscopy
Theory of Atomic Absorption spectrometry (AAS)
However it is observed only the relaxation time of the excited state enters in the expression
in case of transition from excited state to a stable ground state (resonance line), as the ground
state is stable. The half width for the alkaline-earth metals from strontium to beryllium is in
the order of 0.01pm to 0.14
which is insignificant compared to other mechanisms of
broadening in AAS. Doppler Broadening
The Doppler Effect was named after the Austrian physicist Christian Doppler who proposed
this effect in 1842. According to this effect when a source of wave approaches, passes and
recedes from the observer, the observed frequency is higher as compared to the emitted
frequency when source approaches observer, identical when passing by and lower when
recedes from observer. Doppler broadening of spectral lines is an important effect in the line
broadening in AAS due to the Doppler Effect. Similarly higher frequency radiation is
encountered by the atoms moving towards the light source than atoms moving away from the
source. The rapid motion of atoms results in Doppler broadening as they absorb or emit
radiation. The wavelengths of radiations from atoms moving at right angles to the detector
are slightly larger than the wavelengths of radiations from atoms moving towards the
detector. This difference is a manifestation of the well-known Doppler shift; whereas the
effect is reversed in case of atoms moving away from the detector. The net effect is an
increase in the width of the emission line. The broadening of absorption lines is also caused
by Doppler Effect for precisely the same reason. Since the effect is related to motion of atoms
the effect of flame temperature is pronounced due to increased rate of motion of the atoms.
Pressure Broadening:
The collision between the atoms causes the pressure broadening that result in slight variation
in ground state energies of atoms and in the energy differences between ground and excited
states. With increases in temperature the pressure broadening becomes greater as a result;
broader peaks of absorption and emission are always encountered at elevated temperatures.
Analytical Chemistry
/ Instrumentation
Atomic Spectroscopy
Theory of Atomic Absorption spectrometry (AAS)
Impact pressure broadening or collisional broadening
The collision of the emitting particles with other particle interrupts the emission process, and by
shortening the characteristic time for the process, increases the uncertainty in the energy emitted
(as occurs in natural broadening). The effect depends on both temperature and density of the gas.
The shapes of the peak are often approximately Lorentzian, i.e.
I(λ) ∝ {1 + [(λ - λ0)/Δλ1/2]2}-1.
The wavelength are expressed in Å
Self-absorption and self-reversal
A photon can be absorbed by an atom of the same species in a ground state which is emitted
by an atom in a radiation source, if the emitted radiation emerges from a resonance line.
This kind of absorption is seen in between identical species and is limited to lines with the
upper or lower level having an electric dipole transition (resonance line) to the ground state.
The FWHM may be estimated as
Where λ is the wavelength of the observed line, fr and λr are the oscillator strength and
wavelength of the resonance line; gk and gi are the statistical weights of its upper and lower
levels. Ni is the ground state number density.
This also holds for atoms with sufficiently populated low lying excited states.
This
phenomenon is termed as self-absorption, which leads to the reduction of the emitted
radiation. However the region of an optically dense medium will hardly be reached in the
absorption volume as we deal with trace quantities of the analyte in analytical AAS.
Nevertheless the effects cannot be avoided in line sources.
Convolution of the various broadening mechanisms
A convolution product of all broadening mechanisms is the total observed line broadening of
any spectral line. The half widths can be calculated as simple addition products if the various
Analytical Chemistry
/ Instrumentation
Atomic Spectroscopy
Theory of Atomic Absorption spectrometry (AAS)
broadening processes (Doppler and Lorentz) will be considered statistically independent of
each other. However in reality these processes are correlated and cannot be considered
statistically independent of each other.
In general the overall broadening is a mixture of Lorentz and Doppler broadenings. This is
known as the Voigt Profile. TheDoppler-like behavior in Voigt profile is shown by the line
core and a Lorentz-like behavior in the line wings (Fig8).
Fig.8: A comparison of Doppler and Lorentz line shapes. (Adopted from Absorption Line
Physics)
In conclusion:
 Doppler broadening are most significant at:
Low pressure, high temperature, small wavelength (λ)
 Collision broadening most significant at:
High pressure, low temperature, large wavelength (λ)
 Many conditions require consideration of both effects (Fig 9)
Voigt profile→Together
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Atomic Spectroscopy
Theory of Atomic Absorption spectrometry (AAS)
Fig 9: Line profiles: A is Lorentz, B is Doppler, and C is Voigt profile resulting from
convolution of A and B. (Adopted from Absorption Line Physics)
Relation between Absorption and Concentration
The concentration of an element in a solution can be measured with the help of absorbance
values and this relation between absorption of light and concentration of analyte is given by
Beer-Lambert law:
Lambert's Law states that the quantity of light absorbed is independent of the intensity of
the incident light by a transparent medium and each successive unit thickness of the medium
absorbs an equal fraction of the light passing through it i.e.
Light absorbed is proportional to the length of the path
A Beer’s Law state that the amount of light absorbed is proportional to the number of
absorbing species present in the sample. Thus in the atomizer of Atomic absorption, it means
the amount of light absorbed is directly proportional
produced by the concentration of atoms 'c', then the
absorbance
'2a'
would
be
produced
by
the
concentration '2c’
Absorbance
to the analyte concentration. Thus if absorbance 'a' is
The combined Beer-Lambert law can be expressed as:
Concentration
Analytical Chemistry
/ Instrumentation
Fig. 10: Plot of absorbance versus
concentration (limited range)resulting
in a straight line
Theory of Atomic Absorption spectrometry (AAS)
Atomic Spectroscopy
log10Io/It = absorbance = a * b * c
where:
Io = incident light intensity
It = transmitted light intensity
a = absorption coefficient (absorptivity)
b = length of absorption path
c = concentration of absorbing atoms
For a given set of conditions, a and b are constants. The path length changes depending on
the type of burners used for example if an air/acetylene burner is used the path length will be
of 100 mm compared to 60 mm for the nitrous oxide/acetylene burner. If a curve of
concentration versus absorbance is drawn Beer's Law predicts that a straight line will result
(fig. 10). But the same is not observed in practice, deviations from the linear calibration, have
been reported especially at higher concentrations due to several factors like spectral effects or
instrumental design.
In atomic absorption another significant issue is the time of residence of atoms in the path of
light in the instrument. Usually greater absorbances are associated with longer residence
times.
The Beer-Lambert law cannot directly be applied to AAS due to:
i.
Due to effect of sample matrix and concentration non-uniformity on efficiency of
atomization
ii.
Path length of analyte atoms (in graphite furnace AA).
Deviations from the linearity of the calibration function:
The magnitude of concentration working range for an element is generally 4 to 5 orders by
atomic absorption spectroscopy. At higher end of the concentration range, the graph generally
bends towards the concentration axis. However curved calibration graph can be obtained
throughout the whole working range but the frequency is very less. The Beer- Lambert law is
valid for monochromatic radiation.
Analytical Chemistry
/ Instrumentation
Atomic Spectroscopy
Theory of Atomic Absorption spectrometry (AAS)
Absor
Abso
Concentration
Absorbance
Absorbance
Absorbance
Concentration
A
Concentration
Concentration
B
C
Concentration
Absorbance
Figure A: shows "upward curvature" the graph curves towards the absorbance axis. This
Absorbance
behavior is seen only for a relatively small concentration range.
Figure B: Shows the graph curving towards the concentration axis. As the concentration
increases the graph tends to become parallel to the concentration axis. This "asymptotic” graph
means that above a certain concentration value the absorbance does not increase at all.
Figure C is a special variation of Figure B,
above a certain concentration the graph does not
Concentration
Concentration
become parallel but curves back towards the concentration axis. This "roll-over" behavior means
Absorbance
that two different concentrations can give the same absorbance.
Concentration
Fig 11 Calibration graphs showing devaition from the ideality.
A number of researchers have the postulated the reasons for the bending of curves. However
the two common causes of this deviation were given by de Galan and Samaey. One from the
failure of slit and monochromator system to prevent multiple, closed-spaced lines from
reaching the detector. If the absorption coefficient lines falling on the detector are different
Analytical Chemistry
/ Instrumentation
Atomic Spectroscopy
Theory of Atomic Absorption spectrometry (AAS)
from each other, a nonlinear relationship will be obtained between absorbance and analyte
concentration (Fig 11).
The emission source must have line width narrower than the absorption line width. If this is
not the case, a nonlinear relation between absorbance and concentration will occur. The use
of excessive lamp currents is the greatest cause of line broadening, in good hollow-cathode
lamps.
Often multiples in absorption or emission lines have been resolved by the monochromator of
an AA spectrometer. But splitting of the emission line due to interactions between the
electrons and the nucleus cannot be resolved. This interaction generates a number of
transitions with very similar but distinct energy differences so that an emission line consists
of a number of component lines very close together a structure called as hyperfine structure
(HFS). The difference in wavelengths of the radiations is so small that a routine
monochromator cannot resolve them. This in effect results in broadening of emission line and
contributes to curvature. Also, each HFS component may have different absorption
coefficient and thereby gives rise to more curvature. Example of such effect is curved
calibration graph of sharp Cd 228.8 nm line. In absence of hyperfine components in an
emission lines the lines are called "ordinary lines". For example Te line 225.9 nm. Whereas if
the HFS components so close together that they cannot be resolved in the emission lines,
these lines are termed as "quasi-ordinary lines". An example is the 213.9 nm Zn line.
The Zeeman Effect
Some Spectrophotometers are fitted with Zeeman Effect background correction systems in
such case curvature of calibration graph and reflex curvature. The Zeeman Effect is given by
splitting of atomic spectral lines in the presence of a magnetic field. In simple Zeeman Effect
2 sigma and one pi component is observed in the splitting of emission line. Around the
Analytical Chemistry
/ Instrumentation
Atomic Spectroscopy
Theory of Atomic Absorption spectrometry (AAS)
original wavelength, the sigma components are symmetrically displaced by a few picometers
while as pi component remains at its original wavelength. These components are polarized–
the pi component is linearly parallel to the applied magnetic field while as the sigma
components are circularly polarized perpendicular to the applied magnetic field. To remove
the pi components of the transmitted radiation a polarizer is positioned in the optical system.
The Zeeman effect is used to reduce the background interference which is either caused by
non- specific absorption arising from light scattering caused by solid particles or liquid
droplets in the atomizing cell, or by the presence of undissociated molecules of matrix that
have broadband absorption spectra. This back ground absorption is compensated usually by
measuring background absorption of a separate experiment and subtracting it from the
absorption of sample solution.
Summary:
The topic discusses about the concept of atomic spectra and how spectra’s are generated from
molecules, what type of spectra are produced. It also helps to understand how important is
selection of spectral lines and how to do that. What affects the spectral lines and how
broadening of lines happen. It also describes the relationship between the absorption by an
atom and its concentration in sample under analysis.
Analytical Chemistry
/ Instrumentation
Atomic Spectroscopy
Theory of Atomic Absorption spectrometry (AAS)