Atomic

Emission Spectroscopy
 Atomic Spectra
Ultraviolet
wavelength 1 x10-8 m -400nm
energy 12000-310 kJ mol-1
Visible
wavelength 400-800 nm
energy 310-150 kJ mol-1

νx λ = c= speed of light

Where ν is the frequency in Hz,

λ is the wavelength in m
c is the speed of light (2.998 x 108 m s-1)
 Atomic Spectra Excited
state
ground
state n=1
À Shell structure & energy level of atoms n=2
– In an atom there are a number of shells n = 3,
etc.
and of subshells where e-’s can be found energy
– The energy level of each shell & subshell ∆E
are different and quantised
• The e-’s in the shell closest to the nuclei
has the lowest energy. The higher shell
number is, the higher energy it has.
• The exact energy level of each shell and 4f
subshell varies with substance 4d
n=4
4p
À Ground state and excited state of e-’s 3d
– Under normal situation an e- stays at the 4s

Energy
n=3
lowest possible shell - the e- is said to be 3p
at its ground state 3s
n=2 2p
– Upon absorbing energy (excited), an e- 2s
can change its orbital to a higher one - we n=1 1s
say the e- is at is excited state.
 Atomic Spectra
energy
n=1
∆E
Electron excitation n=2

– The excitation can occur at different n = 3,

etc.
degrees
• low E tends to excite the outmost e-’s first
• when excited with a high E (photon of
high v) an e- can jump more than one
levels
• even higher E can tear inner e-’s away
from nuclei 4f
4d
n=4
– An e- at its excited state is not stable and 4p
tends to return its ground state 3d
4s

Energy
– If an e- jumped more than one energy n=3 3p
levels because of absorption of a high E, 3s
the process of the e- returning to its n=2 2p
ground state may take several steps, - 2s
n=1 1s
i.e. to the nearest low energy level first
then down to next …
 Atomic Spectra
energy
n=1
∆E
À Atomic spectra n=2
– The level and quantities of energy n = 3,
supplied to excite e-’s can be measured & etc.
studied in terms of the frequency and the
intensity of an e.m.r. - the absorption
spectroscopy
– The level and quantities of energy emitted
by excited e-’s, as they return to their
ground state, can be measured & studied 4f
4d
by means of the emission spectroscopy n=4
4p
– The level & quantities of energy absorbed 3d
or emitted (v & intensity of e.m.r.) are 4s

Energy
specific for a substance n=3 3p
3s
n=2 2p
– Atomic spectra are mostly in UV 2s
n=1
(sometime in visible) regions 1s
 Principle of Atomic
Emission Spectroscopy

∆E = hν = h c / λ
Where h = Plank's constant (6.6256 x 10-27 erg sec)
c = Velocity of light in vacuum (2.9979 x 1010 cm sec-1)
 Cu Na Cu

CORRELATION OF
THE EMISSION
INTENSITY OF SUCH
CHARACTERISTIC
RADIATION WITH THE Sr Fe Sr
CONCENTRATION OF
THAT ELEMENT
FORMS THE BASIS OF
THE EMISSION
SPECTROSCOPY.
Frequency of emitted light
– depends on allowable energy transitions
– is characteristic of species
 Energy level diagrams
À The emission does not necessarily occur as radiation of a
single wavelength
À The electron may pass back in one step or in a series of
step corresponding to intermediate orbitals
À The return to the ground state being accompanies by the
emission of several spectral lines.
À The most prominent line results from the transition
between the lowest excite level and the ground state and is
called as the Resonance line
À Next in intensity are the lines ending in the same ground
state but originating in the level immediately above the
preceding lines.
À By controlling the flame temperature, the type of the
energy level transitions may be regulated.
 Lithium Energy level diagram

610
671

An electron can only move between s⇒p or p ⇒d NOT s⇒s

Sodium Energy level diagram

330

819
589

Sodium Energy Level diagram

Potassium Energy level diagram

Excitation potential, eV
 Sequence of events in emission of radiation

When an aerosol is uniformly delivered into a source of

energy, the following sequence of events take place:

™ NaCl (l) ====== NaCl (s)

™ NaCl (s) ====== NaCl (g)

™ NaCl (g) ====== Nao (g) + Clo (g)

™ Nao (g) + Energy ====== Naex ===== Na+ + e-

™ Na+ + OH- or O= ( from flame gases) ===== NaOH or

CaO

™ From the excited state of the atom or molecule or ion, a

reversion takes place to the ground electronic state.
Subsequently, the neutral atoms, ionized atoms and the
molecules of NaOH and NaO will be excited and their
characteristic spectra will appear in emission.

The spectrum of an ionized atom (ion) is quite different

from that of the neutral atoms. In fact, the spectrum of a
singly ionized atom will bear a strong resemblance to that
of the neutral atom of At. No. one less.
 Intensities of Atomic spectral lines

The measured intensity of a given spectral line is determined, in

general, by the following factors:

The fraction of the introduced salt that is evaporated

The fraction of the salt molecules that is evaporated.

The fraction of the atoms formed by dissociation that are not ionized.

The fraction of the non-ionized atoms that are excited.

The probability of a transition from the excited level to a lower level.

The energy of the light quantum.

The self absorption factor that accounts for the fraction of emitted energy
that is lost by impacts with other unexcited atoms due to higher concentration.
 Interferences in the Emission
Spectroscopy
1. Spectral Interferences
a. Band Overlapping:
► Orange band of CaO interferes with the Na lines at 590 nm by
overlapping.
► Orange red band SrOH interferes with the Li line at 670.8 nm.

Fortunately, most of the useful lines lie in the Blue and UV regions
while most of the molecular bands occurs in green and red region.

b. Adjacent lines (Nearly same wavelengths)

Overlapping by the adjacent lines are read together in proportion to the
degree of overlapping.
Remedies
• Increased resolution by prism or diffraction grating.
• Selection of other lines free from overlapping
• Prior removal of interfering element by selective solvent extraction.
 Interferences in the Emission Spectroscopy

2. Background emission
Sources of background emission are:
(a) Flame gases
(b) Sample matrix

Remedy
i) Substracting reading of blank from that of sample
ii) Adjusting zero with blank
iii) Using spectroscopic buffer
 Interferences in the Emission Spectroscopy

3. Self Absorption
Energy emitted by excited atoms can be absorbed by the
atoms of its own kind present in the ground energy
level. The strength of the spectral line is weakened.
It is insignificant at very low concentration because the
vapour density of the absorbing atoms is very low.

Remedy
To work in narrow ranges of low concentration.
 Interferences in the Emission Spectroscopy

4. Ionization
If the energy of the source is too high, the valence electrons
in some of the atoms are knocked out of their energy shells
and atoms change into ions. This results in depletion of the
number of available neutral atoms that can be excited and
therefore, the intensity of the atomic emission spectrum gets
weakened and that of the ionic spectrum is strengthened.
This is more true in case of alkali and alkaline earth metals.

Remedy
This problem can be minimised by the addition of easily
ionizable metal (buffer element) into the standard as well as
sample. The buffer element increases the number of neutral
atoms of the test element due to increase in partial pressure
of electrons of the buffer element.
 Ionization of elements at different temperatures

ELEMENT IONIZATION AIR – HYDROGEN – ACETYLENE

POTENTIAL PROPANE OXYGEN, OXYGEN
eV 20000K 24500K 28000K
Li 5.37 < 0.21 0.9 16.1
Na 5.12 0.3 5.0 26.4
K 4.32 2.5 31.9 82.1
Rb 4.16 13.5 44.4 89.6
Ce 3.87 28.3 69.6 96.4
Ca 6.11 < 0.01 1.0 7.3
Sr 5.69 < 0.1 2.7 17.2

Ba 5.21 1.0 8.6 42.8

 Interferences in the Emission Spectroscopy

5. Influence of anions in the test solution

Large amount of acids and their salts in the test solution
decrease the emission intensity. When the concentration of
acids (particularly sulphuric acid and phosphoric acid) is
greater than 0.1M, the metallic emissions decreases due to
incomplete volatilization as the salts of high melting point
are not converted to gaseous state.

Remedy
Use of releasing agents which combines with anion or
protective chelating agent which combine with metal and
is completely decomposed the source of energy can be
used to alleviate this problem
 Interferences in the Emission Spectroscopy

6. Solution Properties

Solution properties such as vapour pressure, surface

tension and viscosity also affect the emission intensity
of the test element. Vapour pressure and surface tension
determine the droplet size, the viscosity determines the
rate at which the aerosol reaches the source of flame.
Instruments Based on
Emission Spectroscopy
1. Flame Photometer
2. Electric Spark/Arc Emission
Spectrophotometer
3. Inductively Coupled Argon Plasma
Atomic Emission Spectrophotometer
(ICAP-AES)
 Atomic Emission Spectrometer
(AES)
P
Wavelength Signal Processor
Source Detector
Selector Readout

Type Method of Atomization Radiation

Source

argon plasma sample heated in an argon plasma sample

Sample arc sample heated in an electric arc sample

spark sample excited in a high voltage

spark sample

flame sample solution aspirated into

a flame sample

Flame photometry is a simple form of atomic emission spectrometry

 Flame Emission Spectroscopy
Slit Slit

Sample Emits Light Detector

Lens
Filter

The Functions of Flame

1. To convert the constituents of liquid sample into the vapor state.
2. To convert the constituents into atoms :
M+ + e- (from flame) −> M + hν
3. To electronically excite a fraction of the resulting atomic species
M −> M*
 Flame photometer
The gas flame as a source of excitation for atomic
emission has been used extensively under the name of
Flame photometry. It is used principally for the
estimation of alkali and alkaline earth metals. The
intensity of radiation from the flame at a wavelength of
a given element is closely proportional to its
concentration.

The flame photometer consists of:

i) The pressure regulators and flow meters
ii) The Atomizer
iii) The burner
iv) The optical system
v) The photosensitive detector
vi) Recorders
 Flames and flame temperature

A flame is used for :

■ Transforming the sample from liquid or solid state into the gaseous state
■ Decomposing the molecular compounds into simple molecules or atoms
■ For exciting the simpler molecules or atoms to light emission

Gases utilize for obtaining the range of temperature are

Gas Flame temperature oC
In Air In Oxygen
Propane 1925 2800
Butane 1900 2900
Hydrogen 2100 2780
Acetylene 2200 3050
Normally air – gas flame is used because of its low
temperature (about 1700 0C) but enough to excite about
a dozen elements, chiefly alkali and alkaline earth metals.

Mixture of hydrogen and acetylene with oxygen produce

much hotter flame and correspondingly greater
excitation. Compared with an arc / spark or plasma
source, the flames excite fewer lines of each element.
Schematic diagram of a flame
photometer
Electric spark /Arc atomic emission
spectrophotometer

For the estimation of heavy metals high temperature

sources are required.

As many elements are excited simultaneously,

therefore, the instrument are fixed with better
dispersing system such as diffraction grating,
photodetectors like photomultiplier tubes and
recording devices such as typewriters to measure the
intensity of multi elements simultaneously. Such
instruments are called Atomic emission
spectrophotometers.
 Excitation methods
The function of excitation methods is to introduce the sample into the source
in the vapourised form and to excite electron to the higher energy levels.

These methods are

d-c arc: is produced by a voltage of 50 to 300 volts, struck between two
carbon or graphite electrodes. Arc temperature ranges from 4000 to 80000K.
The emission lines produced are from neutral atoms. Current across the
discharge gap ranges from 1 to 30 amperes. It is a very sensitive source with a
good line to background ratio. It is used for elements present in very small
amounts. Its disadvantage are due to the tendency of flickering.

a-c arc: The high voltage of a-c arc employs potential difference of 1000 volts
or more. A-c arc is more steadier and more reproducible.

a-c spark: is produced by high voltage transformer across the two electrodes.
It gives much higher excitation energies than the arc with less heating effect.
The spark is preferred source whenever high precision rather than extreme
sensitivity is required. Since the heating effect is less than that of arc it is
suitable for analysis of low melting materials
Electrodes: Generally graphite electrodes are used for the production of a
spark or an arc. The lower electrode may be used as rotating porous graphite
disc, the upper electrode is a rod shaped
 Summary

À Atomic spectrometry involves the interaction of UV-

visible radiation with valence electrons.

À Atomic emission involves the excitation of the atoms

by heat to a higher energy level, followed by
spontaneous emission of a characteristic wavelength
of light Ej-Ei = hc/λ.

À Flame photometry is a simple form of atomic

emission spectrometry.