We were discussing that both emission and
absorption coefficient must be non-zero, only
if the levels involved are of the opposite
parity and if delta J is equal to plus or
minus 1.
So, using these selection rules, resonance
level of an atom may be defined as that of
the lowest excited energy level that can interact
with the ground state by transition of the
electric dipole type. The corresponding wavelength
is known as the resonance line. Therefore,
it follows that for a particular atom, the
resonance line is the most intense of the
highest oscillator strengths and only this
line would be useful for analysis provided
the wavelengths are in the 200 to 600 nanometer
range, that is were most of the spectrophotometric
as well as atomic absorption wavelength lines,
resonance lines will occur.
So, in practice, it is impossible to get a
truly monochromatic line. And what do we mean
by monochromatic line, truly monochromatic
line means, a electromagnetic radiation of
the exact wavelength, only one wavelength.
So, it is almost impossible to get a truly
monochromatic line, but the energy is distributed
over a narrow waveband.
For example, if you if you look at this spectrum
of a of a monochromatic radiation, the 
frequency or wavelength should be of single
wavelength like this. So, what actually you
get is a small band of radiation, where the
lambda max corresponds to the same 
as 
same as the line resonance resonance line,
but you would also see some amount of other
radiations creeping into the radiation along
with this. Now, you can imagine that, the
energy also is distributed symmetrically over
a small narrow waveband. The width of a spectral
line is defined as the value of delta lambda;
that means, the range of frequencies what
you are getting, where the intensity is 50
percent of the total, this is called as half
width.
The shape and size of an absorption or emission
band is affected by several factors such as
natural broadening, Doppler broadening, pressure
broadening and electric or magnetic field
broadening etcetera, these are the causes
for a line spectrum to be converted into a
band spectrum. And band spectrum is always
associated with half width that is what we
are discussing.
So, a like this, in this figure what you are
seeing is, I 0 is the wavelength what you
need, that is the resonance line, but what
you are getting is, a group of wavelengths
starting from lambda 1 to lambda 2 of which
the midpoint corresponds to lambda 0, this
also happens to be the maximum intensity line
in this case. So, the half width is, half
the height of I 0 corresponds to this I 0
by 2 and the wavelengths corresponding to
this, is known as band pass width, this we
have discussed earlier in molecular absorption
also.
So, the reasons for this to occur as we have
seen in the in this slide is that, there is
a broadening effect taking place, and these
are the causes are due to natural broadening,
Doppler broadening, pressure broadening, electric
or field broadening etcetera.
So, let us discuss about natural broadening
due to the short lifetime of the energy states,
Heisenberg principle uncertainty principle
is applicable for all transitions. Therefore,
a small broadening effect of the order of
few millionth of a nanometer nanometer is
10 rise to minus 9 meters and a few millions
of the nanometer, broadening occurs around
250 nanometers, it rises up to 10 rise to
minus 4 nanometer around 1 micrometer.
This natural width is further influenced by
a variety of factors, chief among them being
the disordered thermal motion of the atoms
and various types of collisions. For example,
if you want to imagine that, the the transition
occurs due to number of atoms present in a
confined space and the atoms being so small,
they are always in a perpetual state of motion.
So, this perpetual state of motion is always
referred as, the disordered thermal motion
of the atoms, it is due to the temperature
as well as, various types of collisions, because
the atoms in a confined space are not stationary,
but they keep on moving here and there and
they can heat also each other and then, there
could be elastic and inelastic collisions
taking place. So, this is due to natural broadening.
And then, we have Doppler broadening, Doppler
broadening is essentially the broadening occurring
due to the movement of the source of the signal
with respect to the observer. Now here, I
have shown you can image simple conditions
like, if you are standing on a railway platform,
if a train is coming towards you, you will
be hearing the sound, increasing sound, until
it reaches you, but after it reaches after
the train passes you, the sound will keep
on diminishing; and this effect can be schematically
shown like this, that train is moving like
this, you are you are standing here, where
photon detector is returned. So, the sound
keeps on increasing and you will be feeling
more sound. When it is going away from you,
the sound intensity decreases, same thing
happens with respect to the radiation also.
If an atom emitting a radiation lambda 0 moves
with a velocity v relative to the observer,
the observed wavelength is given by a simple
expression like this, lambda is equal to lambda
0 plus lambda 0 into velocity divided by the
velocity of the light in the vacuum. Further,
if the atoms are in thermal equilibrium at
temperature T, their velocity will also have
a Maxwellian distribution that is, they will
be distributed around the central frequency.
So, the monochromatic absorption coefficient
K as a function of lambda may be expressed
as, K lambda is equal to K 0 exponential of
minus lambda minus lambda 0 that is the difference
in frequency divided by delta lambda into
2 ln into rise to 1 by 2 whole square, where
delta lambda d, this one is the Doppler half
width related to the temperature related to
T and the atomic mass M by the equation like
this, 7.16 into 10 rise to 7 lambda 0 into
T by M whole rise to half. The line is thus
shown to have a Gaussian profile. Now, what
you have understand by Gaussian profile?
The Gaussian profile is essentially a curve
like this, which shows a lambda max and equal
distribution on the other side I am I have
not been able to draw it properly, but you
should imagine that, both sides are both sides
in this on the left side of this line and
right side of this line are uniform, so this
is Gaussian profile.
And this Gaussian the line is thus shown to
have a Gaussian profile, it is possible to
calculate the value of delta lambda d that
is the distribution at different temperature
2000, 2500, 3000 degree Kelvin and the line
widths for these temperatures are of the order
of 30 to 50 milliangstroms, that is about
1000 th of angstrom unit.
So, other types of broadening occurs in the,
when the atoms are in the vapour state, they
are in because they are in a perpetual state
of motion and collision among the atoms is
inevitable. Therefore, it causes radiation
quanta of slightly differing frequencies to
be absorbed or emitted; that means, again
the absorption does not occur at the exact
wavelength, but slightly to the left or right
of the frequency graph.
And several types of particles may be involved
in the collisions for example, interaction
of electrically charged particles causes line
broadening known as stark effect; that means,
the atoms are not neutral, but electrically
charged particles. So, you would know that,
positively charged particles would get attracted
to negatively charged particles and start
collisions etcetera, and this kind of transition
collisions are known as stark effect. Then,
sometimes collisions between uncharged atoms
also can, because of their thermal motion,
but no chemical reaction, they lead to van
Der waal's effect, these are very weak collisions
and van Der waal's forces are also very weak,
but collisions nevertheless.
So, collisions between atoms of the same type
lead to leading to a resonance broadening
is known as Holtsmark. Suppose, you have a
matrix of number of elements, then cadmium,
titanium, thorium, cerium etcetera, if you
take a sample of water sea water then, you
will have calcium, magnesium and many other
elements. So, the collision between calcium
and calcium atoms, that is of the same type
leading to resonance broadening is known as
Holtsmark effect.
Since, it is difficult to differentiate between
these three effects, they are collectively
refer to as Lorentz broadening. So, we have
Doppler effect and then, natural broadening
and then, Lorentz effect occurring all the
time whenever you have produce a group of
atoms in the ground state.
Now, the broadening of spectral lines actually
reduces the lifetime of the excited state
of the atoms. It is also it also increases
the line profile of the radiation, because
of the Gaussian distribution. The monochromatic
absorption coefficient of the electromagnetic
radiation at a wavelength lambda; that means,
if this radiation is to be absorbed by a group
of atoms, the absorption coefficient is given
by this equation, that is K lambda is equal
to K 0 divided by 1 plus 2 into lambda minus
lambda 0 into delta L whole square. Now here,
K 0 is the maximum absorption coefficient
and delta L is the half width.
The profile of this distribution is slightly
flatter than Doppler broadening, but both
are almost of the same order. Therefore, the
half width delta L in this equation is thus
a fraction of the frequency of collision that
is numbers Z, number of collisions which in
turn is again a function of the temperature.
Higher the temperature, more is the number
of collisions, lower the temperature, less
is the number of collisions. And for collisions
to occur and cause broadening we also need
to effective collisions; that means, the the
atoms should not just grace each other, but
they should hit each other squarely and this
is known as effective cross section, that
is defined by delta lambda is equal to effective
number of collisions Z and lambda 0 square
divided by pi into C.
So, it may be noted that, both Lorentz broadening
and Doppler broadening occurs simultaneously
resulting in a similar profile, but broader
profile, both of them put together we get
what is known as a Voigt profile. And it is
not just a Gaussian profile, but it is a slightly
different Voigt profile, K lambda which may
be mathematically expressed as, K lambda is
equal to K 0 into a divided by pi and an integral
of e to the power of minus y square divided
by a square plus w minus y whole square d
y, where a is given by these expressions.
Now, this is slightly complicated expression
derived from several other considerations
we will not go into the details of these derivations,
but we will understand that, the profile of
a resonance line is never a single line, but
it is a broad profile something similar to
dop Gaussian distribution, but slightly different
which is known as Voigt profile.
Now, the curves are actually symmetrical with
a maximum at lambda 0. Apart from Doppler
and Lorentz effects, line broadening also
occurs due to nuclear spin we have not considered
nuclear spin so far, because mostly the movement
of the electrons is much more compare to nuclear
movement. So, because the nucleus also is
spinning, many resonance lines occur due to
nuclear spin and this nuclear spin results
in what is known as very fine splitting of
the lines known as hyperfine splitting.
So suppose, there are atoms of the same element,
but different atomic weight, but same atomic
number then we have what is known as isotopes.
And these isotopes also shift the resonance
lines contributing additionally to the line
broadening. So, these effects are also significant,
but not so prominent as Doppler broadening,
natural broadening and collisional broadening
etcetera.
So, in essence what we are trying to say is
that, the sum total of all these broadening
effects is of the order of about 0.0005 to
0.005 nanometer; that means, it is a very
small one, but still significant enough, which
increases the broadening the range increase
this with increasing temperature and pressure.
So, if the atom population is subjected to
very high temperature, the broadening increases
more; that means, you will your chances of
getting a monochromatic line are so much less.
And then, if the atomic population is also
under pressure then also, this broadening
increases because of the increase in the number
of collisions taking place. So, the significance
of the peak width at half the peak height
as a profound effect on the emission characteristics
of radiation sources, especially hollow cathode
lamps, which will be will be discussing later.
Now, you can imagine how do we go about effecting
atomic absorption? So, what do you need is,
based on the quantum mechanical physical description
given earlier, rigorous mathematical expressions
have been derived to determine the absorption
coefficients, its variation with N f and l,
effect of monochromator bandwidth and also
of optical density we can derive all these
equations.
However, for practical purposes, it is not
necessary to quantitatively define each and
every term, what we have discussed earlier,
because for practical purposes only a physical
understanding of these phenomena is more relevant
which maybe we can interpret as follows.
Now, the discussion what we had earlier about
natural broadening and all these things can
be summarized like this, what we need is,
a very narrow frequency interval is essential
for the atomic absorption of the resonance
radiation. However, it is impossible to isolate
and obtain high intensity of illumination
especially corresponding to 0.0005 to 0.005
nanometer single wavelength essentially, it
is almost impossible.
If the source of radiation emits all kinds
of radiation, starting from X rays, ultraviolet,
visible, I R etcetera. So, it would be even
if you are successfully in obtaining a single
wavelength, it would be too weak to be of
any practical use, because the intensity would
be low, strength would be low and detector
would be difficult and isolation would be
difficult and absorption will be difficult,
so that it is of practical practically it
is of no use.
So, how to overcome this difficulty? For this,
Walsh has recommended that, suppose you use
the radiation source, made of the same element
that you want to analyze, then it is necessary
because, if you are able to excite a radiation
from a given atom which you want to analyze,
only the elements specific emission lines
would be obtained from the element; and you
have to just filter the resonance line and
use it for atomic absorption. So, the whole
process has become very simple, if I can make
an element source as a source; that means,
if you want to determine copper you must have
a copper filament as a source and a bulb made
of copper.
If you want to determine iron, you need to
have a filament made of the iron as a cathode
and it will emit only the iron lines and they
because they contain emission lines as well
as, a continuum, what you need is? Only an
emission line, not the continuum. So, it is
very simple to filter out, all other radiations
and permit only the resonance line to be taken
out of the source and make it fall on the
sample, containing the number of atoms you
want to analyze.
So, if you are able to do this, prepare a
lamp radiation source of each element then,
we should be able to analyze all the elements
that we can make the radiation sources off.
Therefore, only the resonance line needs to
be separated from other sources by using a
filter, and these spectral lines can be separated
we have discussed earlier, we can use a filter,
we can use a monochromator, and we can use
grating and all those things are possible.
And it is required only to separate the resonance
line and that will lead to this thing separation.
Now, assuming that a monochromator isolates
a spectral band delta S covering the absorption
line lambda 0 that is your resonance line,
the total spectral energy received by the
detector also should be much more, should
be significant. So, you can evaluate that
by determining the intensity over a lambda
over a range of wavelengths, where delta S
represents the negative and positive side
of the of the Gaussial distribution that is
lambda 0 minus delta S by 2 to lambda 0 plus
delta S by 2; and intensity is integrated
with respect to lambda. Now, if you evaluate
this expression, you would end up with an
expression like this, I 0 into delta lambda
S, which is essentially a area of the rectangle
A B C D.
This is the area, where we can where intensity
can be integrated here is lambda 0 and here
is lambda 0 minus delta S by 2 this side and
then, the right side we have lambda 0 plus
delta S by 2. Now, if a homogenous gas having
an absorption K lambda, that is you have the
atom source which can absorb this radiation
monochromatic radiation is interposed I am
going to interpose it here, if the in the
absence of this I have the intensity represented
by A B C D, this is A B C D this rectangle.
Now, if I am introducing certain amount of
atoms in the light path, it is going to absorb
the radiation and the profile of the radiation
would be reduced by the amount of radiation
that is absorbed by the number of atoms. So,
you would see a profile like this, that the
energy within this band will decrease by the
same amount of the spectral profile, but same
amount of the number of atoms that absorb
the radiation. And the spectral profile will
have the same shape that is like this, this
is atomic absorption, this is total emission
from the source A B C D and the distribution
represents the atomic absorption.
So, instead of considering the radiation flux
per unit volume, if the total radiant flux
is considered like that, triangle in that
A B C D. Triangle in the A B C D, if we consider
that, the total radiant flux then, it may
be proved that, the absorption factor and
hence optical density is proportional to the
concentration of the free atoms present in
the path length. And the path length also
is important because, if the path length is
more, the number of atoms present in the radiation
optical path would be more; and provided that,
the concentration is low and the spectral
bandwidth is narrow.
Then, we can have an expression something
like this, phi transmitted is equal to radiation
original radiation intensity multiplied by
e to the power of x v N into l. So here, phi
0 is the original intensity, phi transmitted
is the radiation transmitted; and e to the
power of minus is the spectral absorption
coefficient, l is the absorption in the path
length, x v is the spectral absorption coefficient
and N is the total number of atoms, this is
essentially a essentially nothing but, Beer-Lambert's
law.
Because, you can exchange this expression,
rearrange in the familiar form instead of
transmitted radiation I am going to put absorbance
here, and I get log of transmitted radiation
phi 0 divided by phi trans incident radiation
divided by transmitted radiation, which should
be a product of 2.303 into x v into N into
l; that means, number of atoms cross-section
absorption coefficient, l is the number of
path length centimeters.
So, the total number of free atoms in the
optical path cannot be determined, because
I prefer that or rather, not to determine
the total number of atoms because, it leads
to lot of confusion, and it requires different
kind of technology to calculate and also lot
of calculations are involved, but it is not
necessary for routine applications. So, what
do we do is, just like in any other spectroscopic
technique, we measure the atomic absorption
as a relative technique; that means, I make
my standard find out what is the absorbance
I and then, I put my sample, find out what
is the absorbance and if the system follows
Beer Lambert's law over a particular absorbance
range, then I can simply say the sample contain
so much of the analyte in the in the atomic
form.
So, the physical conditions for the highest
sensitivity we can summarize now. Now, what
happens? We need an absorption line and the
resonance line should have the lowest energy
state and highest population of the atoms
in the ground state, because absorption can
occur from the ground state to excited state
therefore, highest number of atom should be
in the ground state, rather than the excited
state; that means, I must be able to produce
atoms in the ground state.
If several resonance lines are there, the
one with the highest oscillator strength has
to be chosen; that means, most intense atom
absorption line we need to choose then, I
can employ a source of radiation that emits
a line of the same wavelength, but lower half
width. Now, path length I can increase I can
have 2 mm, 2 centimeters, 3 centimeters, 5
centimeters upto 10 centimeters, I can take
a tube introduce the atoms in the tube. So,
make the optical path to go through, then
I should be able to obtain Beer Lambert's
law, that is linearity of the absorbance with
respect to the concentration, and the absorption
also can be used for determining the unknown
same entity.
Now, employing these conditions we can construct
an atomic absorption spectrometer. How do
we do that? We employ these conditions, what
do we need, we need to construct an atomic
absorption spectrometer using a hollow cathode
lamp made of the same element as the analyte.
We have to construct one lamp of the element,
what you want to analyze? Then, you have to
have an atomizer; that means, the sample must
be broken down into small particles, which
can produce atoms.
So, the sample has to be broken down into
produce a population of the ground state atoms
and then, we need a monochromator with an
entrance slit and an exit slit, you do not
need a monochromator initially, because basically
you are taking the hollow hollow cathode lamp
output, which corresponds to only one wavelength.
And the detector, subsequently you are going
to put your sample in the path of the monochromator
radiation and then, you need a monochromator
to remove all other radiations, except the
hollow cathode lamp resonance line output.
And then, once you have that, you can have
a detector for the measurement of radiation
intensity followed by an amplifier and read
out device etcetera.
So, the schematic diagram of a system atomic
absorbance spectrometer is shown here, what
you need is, a source then you need your sample
in the form of atoms, which we call auto absorption
cell and the radiation must pass through that
and here, the atomic absorption will occur
and the radi intensity of the radiation will
decrease, but there will be other radiations
coming from the absorption cell, which need
to be filtered out. Therefore, you need a
monochromator again to remove the unwanted
radiation, except the source hollow cathode
lamp resonance line; and then detector you
need, amplifier and display, so this is how
the architecture of an atomic absorption should
be defined.
Now, depending upon the choice of the components
and method of operations you can have a you
can have a variety of atomic absorption spectrometers
and these are all systems what you know already,
we can have a single beam D C instrument,
that is simplest arrangement. The earliest
A A S instruments were of this type; that
means, you take a single beam and put your
put your sample and measure the put your standards
measure the absorbance, constructed calibration
curve and then, determine the unknown by putting
it in the same cell. Then you can have a single
beam A C instrument by applying the pulsed
current to the radiation source are by mechanically
chopping by putting a fan and then, radiation
passes through etcetera, before it enters
the absorption cell you can have a single
beam A C instrument atomic absorption.
Then you can have double beam atomic absorption
A C instrument, here what we do is, we use
a rotating mirror or a chopper and then, the
radiation is passed alternately between the
sample and non sample, that is flame and non
flame. Because in general, an atomic absorption
we use flames to produce the atoms. So then,
it is possible to construct a double beam
instrument. So here, what do we do? We take
50 percent of the beam, away from the optical
path and allow only 50 percent of the radiation
to pass through the sample; collect the unabsorbed
beam back through another optical circuit
and then, combined both and then, the differences
would be simple double beam atomic absorption
just like a spectrophotometer; then both beams
are to be recombined by using a semitransparent
mirror placed behind the flame.
The electronics of the system can be designed
to yield directly the ratio of the transmitted
radiation flux to that of the incident radiation
flux, and the stability is also better because,
whatever changes occur during the operation
will be reflected in the double beam all the
time. So, any changes, change in the intensity
would always be compensated by the reference
beam.
So here, I am showing you this is the radiations
electronic part a, is the electronics part
and this is known as number 1, it is common
in all the three that is hollow cathode lamp.
First one is, single beam a c sorry single
beam d c, second is single beam a c, third
is double beam a c atomic absorptions spectrometer
designs schematics. So here, I have a hollow
cathode lamp, supported by the electronics
here and then, it is I have a flame here,
the green part represents a flame and here,
I am introducing my sample to produce a population
of the atoms in the optical path. And then,
I have a slit and monochromator and then,
wavelength that is to be taken to the detector
d I have a detector here and associated electronics
here.
Essentially, same thing here happens here
in single beam a c we need a chopper here
and then, the same thing that is hollow cathode
lamp and then, sample introducer absorption
cell and then, slit monochromator grating
and the detector. And in the third case, what
we have is double beam a c, the incoming radiation
is split into 50 50, before it reaches the
absorption cell. It is recombined after the
absorption cell and then, again the radiation
can be separated from other unwanted wavelengths
and then, it has to be collected through the
monochromator and onto the detector. So, these
are the three variants of atomic absorption
in general, we can construct.
Now, we can also have instead of instead of
one element, made of the same material as
the analyte I can use an alloy as a source
of source of radiation. Alloy will have number
of metals and these metals when excited through
a filament as a radiation source, can give
multiple lines, resonance line corresponding
to each element present in the alloy. Now,
if I can have a collection mechanism for each
wavelength, then I can do the multiple analysis
simultaneously.
So, it is something something like multi element
analysis using a radiation sources containing
number of resonance lines. And how do we generate
that? I make a filament out of an alloy and
all the composition, all the elemental elements
in the alloy would be emitting resonance line.
Only thing is electronics gets complicated
because, simultaneously you should be able
to analyze the radiation flux corresponding
to each wavelength and analysis can be done
or you need to have separate electronics for
each element.
Now, such instruments are available in the
market now, radiation has to be separated
using number of filters and funny thing is,
we can do the we can use multi multi element
lamp and analyze any element you want present
in present in the sample sequentially also
not simultaneously, but sequentially, so that,
we will discuss in the, when we are discussing
these sources.
So, the multi element simultaneous analysis,
it requires the use of radiation sources containing
resonance lines of several elements focused
in to the absorption cell, which permits simultaneous
determination of several elements. The optics
and electronics need to be suitably modified
to handle various signals and readouts and
printouts.
Then, I can substitute the absorption cell
that is, instead of the flame I can use a
heated graphite furnace tubes and generate
the same subject graphite is a conducting
material, I can subject it to very high temperature,
if I put my sample in the graphite tube I
can generate the atoms electrically by heating
heating with the current and that, would produce
number of atoms in a very efficient manner,
compare to flame because as we all know, flame
is a a transient phenomena. It is not always
reproducible to the same extend and electrically
heated graphite tube can be heated to the
same temperature by good electrical electrical
control and it is a very efficient means of
producing a atomic vapour.
So, this technique has gained wide popularities
since last 15 years permitting the quantitative
determination in p p b levels, parts per billion
levels, this is known as electrothermal atomic
absorption spectroscopy.
So, the other possibilities are, hydride generation
atomic absorption. What happens is, I have
a sample containing arsenic, antimony, bismuth,
selenium, tellurium, germanium and lead, all
these elements if I reduce if I introduce
hydrogen, they all form hydrides; and these
hydrides can be taken to the flame absorbance
cell, where they decompose giving the element
vapour directly. So, this is known as hydride
generation atomic absorption; and the hydrides
of these respective elements combined easily
and then, the dissociate also very easily
into that metallic and nonmetallic components
which, when introduced into the flame, permit
not only their separation from the matrix,
but also estimation in parts per billion levels,
p p b that is 10 rise to minus 9 grams.
Then another variant is mercury cold vapour
atomic absorption here what happens is, mercury
has a unique property of being present in
the vapour form even at room temperature.
So, it can form amalgams with number of elements.
So, it is from the combined state, it is possible
to determine the mercury just by subject to
slightly high temperature or even, if mercury
is present in the ambient air, the vapour
pressure is enough to permit determination
of mercury in the sample.
So, this permits its determination at room
temperature, only requirement is it must be
transported into the absorption cell; that
means, you must have an arrangement to introduce
air itself into the absorption cell. And if
there is mercury in the air, it can be determined
using atomic absorption, because we are not
using any flame or any high temperature. Therefore,
it is known as mercury cold vapour atomic
absorption spectrometry.
So in general, the variants of atomic absorption
are single beam D C, single beam A C, double
beam A C and then, multi element 
multi element analysis, simultaneous atomic
absorption and then, electro thermal atomic
absorption and hydride generation and mercury
vapour cold vapour atomic absorption. Among
these, the hydride generation atomic absorption
spectrometry is possible to use it only as
an accessory to existing flame as well as,
mercury cold vapour A A S can also be either
a dedicated instrument or as an you can buy
an accessory for the determination of mercury,
that is mercury cold vapour attachment.
So, over the years the atomic absorption spectrometry
as an analytical tactic has been accepted
as a standard method of analysis all over
the world. And an enormous amount of literature
exists on the instrumentation, radiation,
sources, atomization, optics, signal handling
and data presentation is there and combining
the advances in electronics and computer,
beautiful instruments are available for atomic
absorption for the determination of p p m
levels.
The advent of computers of course, has made
it possible for maximum use of automation
and then, instrument control and statistical
data evaluation. On an average, more than
500 research papers are being published every
year on the application of atomic absorption
spectrometry to various matrix matrices every
year. So, you can appreciate that, the atomic
absorptions spectrometry is a very very versatile
technique. Now, we shall discuss the details
of the detailed aspects of atomic absorption
spectrometry.
So, let us try to understand what we have
discussed so far? Atomic absorption spectrometer
is the measurement of the absorption of electromagnetic
radiation by the atoms in the gaseous state.
Free atoms if they are there, they do not
undergo vibrational and rotational transitions,
but they undergo only the electronic transitions.
And such excited electron may return to the
ground state by atomic emission, atomic fluorescence
or atomic absorption, we are discussing only
the atomic absorption for the time being.
So, the various energy states of the atom
are described by n, l and inner quantum number
J and magnetic quantum number m and spin quantum
number are also there. So, selection rules
usually permit the orbital angular momentum
to vary by plus or minus 1. If it varies by
plus or minus 2, atomic absorption does not
occur and n can be any number, that is principle
quantum number n can be any number.
So, for sodium atom the most loosely bound
electron is designated by, 3 S 2 and S half
and this can go over to 3 P 2 P half and 3
by 2, 4 P 2 P half P 3 by 2 or it can go to
any of the P orbitals represented by other
quantum numbers as 5, 6 etcetera. Here, I
have tried to represent some of these transitions,
this is the ground state 3 S and here, it
is 3 P, 4 P, 5 P etcetera. So, all these transitions
are possible permit and permitted also.
Since, all elements can be excited to their
next higher energy level, in theory any element
can be determined by atomic absorption spectrometry,
any element in theory. But, there is a small
rider below 200 nanometers, analysis that
is in the vacuum ultraviolet region, arsenic,
selenium, iodine, sulfur, phosphorus etcetera
present difficulties, because their resonance
lines are in the vacuum ultraviolet region,
somewhere between 190 and 200.
So, these things do not even though they are
being determined, but they present difficulties,
because of the optics. And cerium, thorium
and many other refractive elements, it is
very difficult to produce atoms using the
flame. So, these elements present difficulty
in atomic absorption using flames. And then
obviously, artificial and radioactive elements
we cannot analyze, because of the obvious
reasons, because radioactive elements are
difficult to handle and there are other ways
of determining the such elements.
So, we need to find the way to produce number
of atoms in the ground state and this we will
be doing using a flame, and to produce high
temperature and into the flame, if we are
able to introduce the sample the water the
content or water content will evaporate and
then, it will produce the compounds; and these
compounds may decompose and produce atoms.
And once these atoms are produced, they can
be taken for atomic absorption. How we can
do that, we will discuss in the next class.
