The spectroscopy of atoms

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• This chapter deals with THE SPECTROSCOPYOF ATOMS.
• Flame Emission Spectrometry
• Atomic Absorptionspectrophotometry.
Summary:
1.Why it is called atomic spectrometry?
2.Flame photometry or Emission Spectrometry
Processes in Flame, Basic principle and instrumentation
3. Atomic Absorption Spectrophotometry
Principle
Instrumentation: sources, Burners, Flames
Atomic spectrometry:
Atoms are the simplest and purest form of matter and cannot rotate or vibrate as
a molecule does, only electronic transition within the atom can take place when
energy is absorbed.
Free atoms are obtained (atomic vapour) and the absorption and emission of
radiation by these is measured in the form of line spectra.
Flame Emission Spectrometry:
 Formerly Flame photometry
 Source of excitation energy: Flame
 Emission spectrum is simple as Flame is low energy source
 Sample in the form of solution

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MECHANISM:
Step 1:
The solution is aspirated into the flame as fine spray
Step 2:
Solvent evaporates leaving the dehydrated salt (CaCl2)
Step 3:
The salt (CaCl2) dissociates into free gaseous atoms in the ground state (Cao)
A small fraction of these atoms (Cao) absorb energy from flame and be raised to
an excited electronic state. The excited levels have short life time and drop back
to the ground state emitting photons of characteristic wavelength with energy
hv. These can be detected to give emission spectrum
Main Phenomenon(in flame):
To analyze the substance:it must be present in the form of atomic or molecular
vapour form.
The sample solution thus passes from the three following stages in Flame.
(1)Transport: sample transport must be constant. on atomization small
droplets which are carried directly or indirectly to the flame
Figure 1.1: Processes occurring in Flame during FES and AAS technique

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(2)Vaporisation: this stage involves vapourization, dissociation (free
atoms) and ionization.
Example is NaCl, the sodium chloride is sprayed in the flame and the
solvent in the mist evaporates first.The resulting salt particles (solid)
grow hot and evaporate i.e. NaClbecomes gaseous state. Theresulting
NaCldissociates into atomic vapour state. These then absorbs energy and
get excited. Thespectrum obtained when the molecules come back to the
ground state.
(3) Excitation : the elemental vapour must be excited in order to have
emission spectrum.
Distributionbetween Ground state and Excited
state: (Principleof emissionspectroscopy)
The relative population of ground state (No) and excited state (Ne) population at
a given flame temperature is measured by Maxwell-Boltzmann expression:
ge and go= statistical weights of the excited states
(represents the probability that an electron will reside in a given energy level)
Ee and Eo = energies of two states
k= Boltzmann constant 1.3805 x 10 -16 ergK-1
T= absolute Temperature

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Atomic AbsorptionSpectrophotometry
Closely related to Flame Emission Spectrometry?
As both uses flame as the atomizer
Introduction:
Relatively new technique developed in the late 1950’s
And it is the method for detecting and measuring elements, particularly metallic
elements.
BasicOperations:
Sample solution is aspirated into a flame as in FES and the sample element is
converted to the atomic vapor. The flame that contains atoms of that element.
Some are thermally excited , but most remain in ground state . The ground state
atoms can absorb radiation of a particular wavelength by a special source made
of that element (under investigation). The wavelength of radiation given by the
sourceis same as those absorbed bythe atoms in flame.
Principle:
Identical in principle to UV, IR.
Atomic AbsorptionSpectrophotometer
Instrumentation:
Components
1. Source (HCL)
2. A cell burner and flame
3. Monochromator
4. Detector

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Figure 1.2: The atomicabsorption spectrophotometerinvolving HollowCathode Lamp(source), Premix burner and
otheraccessories similarto UV-Vis and IR.
1. Source:
Single element or multielement Hollow Cthode Lamp are employed as
sourcein Atomic absorption. The cathode of the HCL comprises an element
under investigation
Constituents:
• Cathode of element under investigation
• Anode ring
• Argon or neon gas
• Quartz window
Working:
A potential of 400 volts is placed between the electrodes. When the lamp
is switched on, the inert gas ionises and converts some of the atoms into
positive ions. they strike the metal surface of cathodeand cause some metal to
vapourize. The metal atom in vapour state become excited and the dexcitation
of this metal electrons emits the quantum of light required for exciting the
ground state atoms.

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Figure 1.3: The Hollow Cathode Lamp usedas light source for exciting ground state elements.
Figure 1.4: Working of Hollow Cathode Lamp (HCL) i.e. the emission of photons.
2. Burner (Flame):
Two basic burners used in AAS are:
 Total consumption Burner
 Premix Burner (Laminar Flow Burner)
1. Total Consumption burner:
The fuel, sample and oxidant passed through separate chambers to a single
opening to the flame. Noisy and Hard to use

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Figure 1.5: Total consumption Burner with compartments different for fuel, oxidant and sample.
2. Premix burner (Laminar Flow burner):
In this burner the fuel and oxidant are mixed into the separate chamber in
which the sample is also introduces as fine droplets the process called
nebulization. The large droplets of aerosol condenseand drain out. The
remaining fine droplets mix with the combustion gases and enter the flame/
Figure 1.6: Premix burner with a single compartments for fuel, oxidant and sample. The sample is introduced
through nebulizera process of fine droplets formation calledas nebulization.

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3. Flame:
Chief flames used are:
• Air-Acetylene flame
• Nitrous-Oxide- Acetylene flame with premix burner
Other flames are
1. Hydrogen-oxygen (2677oC)
2. Hydrogen-air (2045oC)
3. Propane-air (1725oC)
4. Propane-oxygen (2900oC)
5. Acetylene-air (2250oC)
6. Acetylene-oxygen (3060oC)
::::::NOTE:::::
In FES a hot flame is required for the analysis of large number of
elements and hence nitrous oxide-acetylene flame is used.The emission
bears proportional relation to the flame temperature i.e. with the rise in
flame temperature more an more elements get excited and starts emitting
radiation .

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Comparison between the bunsen burner and the premix
burner Flame
Figure 1.7: the temperature ranges of premix and Bunsen burner. The profiles shows that the premix burner ranges
the highest 3000 o
C temperature whereas Bunsen burnerranges to 1500o
C.
Flame consistof an external mantle and the inner core. The maximum
temperature reachedin the flame is just above the inner core.
4. Monochromator,Phototubeand Amplifier:
The same as in UV-Vis spectroscopy

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Single Beam and DoubleBeam instruments: