Inductively Coupled Plasma Emission Spectroscopy, Advantages and Utility, Basic principles, Saha's Equation etc.

1. Dr. P. K. Mani
BCKV

2. E – energy difference between two levels;
h – Plank’s constant, 6.626068 × 10-34 m2kg/s;
c – speed of light, 299 792 458 m/s;
λ – wavelenght, nm
Ion
Emission
Atom
Emission

3. Atomic Emission Spectrometry (AES)
In this method, a sample is normally excited
by the thermal energy of a flame, argon plasma
or an electrical discharge. The atoms in the
sample absorb thermal energy, causing the
excitation of the outer orbital electrons.
As the excited state is short lived, the excited
atoms return back to the ground state after a
very short lifetime (typically10-6 to 10-9 s).
This is accompanied by the emission of EMR,
normally in the form of light in the UV-VIS
region.
The wavelength of the emitted radiation and
its intensity provide the qualitative and
quantitative information about the analyte.
The atomic emission spectroscopy employing
flame as a means of excitation is called ffllaammee
pphhoottoommeettrryy oorr ffllaammee eemmiissssiioonn ssppeeccttrroossccooppyy
((FFEESS))..

4. PRINCIPLE OF FLAME PHOTOMETRY
For a few elements, such as the alkali metals Na and K, an air/
acetylene flame is hot enough not only to produce ground state atoms,
but to raise some of the atoms to an excited electronic state. The
radiant energy emitted when the atoms return to the ground state is
proportional to the concentration and is the basis of flame emission
spectroscopy. i.e., at 589 nm, Na -------> Na* (energy from flame)
Na* -------> Na + hn (at 589 nm)

5. Within the flame, there are many more atoms in the ground state than
in the excited state. For Zn, for instance, in a 2000K flame, there are
7.3 X 1015 atoms in the ground state for every excited atom.
The alkali metals are elements with unocuppied atomic orbitals of
low enough energy to be sufficiently populated by a flame.
For the element sodium, two inner shells are completely filled and
there is one electron in the outer third shell.
This electron is said to be in an s orbital. However, the remaining
orbitals of the third shell and all the orbitals of the fourth, fifth and sixth
shells, etc., are empty. When the outer electron of sodium is in the s
orbital the atom is said to be in the “ground state” or the “unexcited
state”. If the atom absorbs radiation the electron undergoes a transition
to one of the empty orbitals at the higher energy levels.

7. Inductively coupled plasmas
By definition, a plasma is an electrical conducting gaseous mixture
containing a significant concentration of cations and electrons. (The
concentrations of the two are such that the net charge approaches zero.) In the
argon plasma employed for emission analyses, argon ions and electrons are
the principal conducting species, although cations from the sample will also
be present in lesser amounts.
Argon ions, once formed in a plasma, are capable of absorbing
sufficient power from an external source to maintain the temperature at
a level at which further ionization sustains the plasma indefinitely;
temperatures as great as 10,000 K are encountered.
A plasma is an ionized gas that is macroscopically neutral (i.e.
with the same number of positive particles (ions) and negative
particles (electrons)). If a monoatomic gas, X, is used, a plasma can
be described by the following equilibrium:
Ar : 1s22s22p63s23p23p23p2
x
y
z
[Ne]3s23px
23py
23pz
2

8. The gas that is used to generate the plasma (plasma gas) is Argon.
Like any noble gas,
Argon is a mono atomic element with a high ionization energy (15.76
eV), and is chemically inert.
Consequently:
(i)a simple spectrum is emitted by argon in contrast to a flame where
primarily molecular spectra are observed;
(ii) argon has the capability to excite and ionize most of the elements of
the Periodic Table;
(iii) no stable compounds are formed between argon and the analytes.

9. The plasma acts as reservoir of energy provided by the radio frequency
field, and transfers this energy to an analyte, M.
the atomization of a sample is a relatively long process (of the order of a
few ms), while ionization and excitation are very fast processes.
Various ionization and excitation processes have been suggested resulting
from the presence of species that are obtained during the plasma generation.
The major species are not only the argon ions, Ar, and the electrons, e, but
also the excited argon atoms, Ar*,with the special case of the metastable
levels, Arm

10. The hf field is produced by a rf generator and accelerates the electrons.
These electrons will ionize the plasma gas:
the kinetic energy of the projectile
electron must be greater than the
ionization potential of the bound
electron to be ionized
Then, through the process of radiative recombination, the argon ions
are recombined with electrons to lead to excited argon atoms and to a
significant background:
This background is mainly produced by the process of radiative
recombination in the ultraviolet (UV) region, while bremsstrahlung
must also be taken into consideration in the visible part of the spectrum .
Except for the resonance lines that are located at 106.7 nm and 104.8
nm, there are no argon atomic lines below 300 nm.

11. In addition to ionization and excitation, beta particles may also transfer
energy by a process called Bremsstrahlung. As negative beta particles
approach the positively charged nucleus of an atom, some are slowed down and
pulled into a new direction (like the gravitational pull of the earth on a
satellite). As the electrons are slowed down some energy is released in the
form of X-rays called Bremsstrahlung. These X-rays are more penetrating
than the beta particles and may require more shielding. Plastic is commonly
used to shield beta particles because low density materials result in less
production of Bremsstrahlung

12. Electron ionization (EI, formerly known as electron impact) is an ionization
method in which energetic electrons interact with gas phase atoms or
molecules to produce ions.
where M is the analyte molecule being ionized, e− is the electron and
M+• is the resulting ion.
Radiative recombination (RR) is a non-resonant, one step
recombination process in which a free electron recombines with an
ion, emitting excess energy in the form of a photon:

13. The Penning ionization process, in which a metastable atom or
molecule A* collides with another atom or molecule B, to leave A
unexcited and B+ as an ion,
is one member of a class of irreversible energy conversion processes
in which irreversibility is due to the loss of an electron into its
continuum.
Penning ionization refers to the interaction between a gas-phase excited-state
atom or molecule G* and a target molecule M resulting in the formation
of a radical molecular cation M+●, an electron e−, and a neutral gas
molecule G
Penning ionization occurs when the target molecule has an ionization
potential lower than the internal energy of the excited-state atom

14. Two laws are used to describe equilibrium in a plasma:
the Boltzmann law to describe the equilibrium between the population of the
various levels within the same ionization state, including both excited and
ground states, and
the Saha law to describe the equilibrium between the population of two
successive ionization states.
If we consider the population nm and nk of the excited level Em
and Ek, respectively, their ratio can be given by the
Boltzmann law:

15. The Saha Equation
In 1920, Meghnad Saha derived an equation for the relative number of
atoms in each ionization state. Here, just present the result:
Note:
3/2
i
This Edepends on the number density of electrons, n. This is
ebecause as the number of free electrons increases, it is more
likely that they can recombine with an atom and lower the
ionization state.
The Boltzmann factor exp(-/kT) means it is more difficult
to ionize atoms with high ionization potentials

16.  For a gas at a high enough temperature, the thermal collisions of the
atoms will ionize some of the atoms. One or more of the electrons that
are normally bound to the atom in orbits around the atomic nucleus will be
ejected from the atom and will form an electron gas that co-exists with
the gas of atomic ions and neutral atoms. This state of matter is called a
plasma.
 The Saha equation describes the degree of ionization of this plasma as
a function of the temperature, density, and ionization energies of the
atoms.
 The Saha equation only holds for weakly ionized plasmas for which
the Debye length is large. This means that the “screening” of the
coulomb charge of ions and electrons by other ions and electrons is
negligible. The subsequent lowering of the ionization potentials and the
“cutoff” of the partition function is therefore also negligible.

17. In general, the ICP is not in true thermodynamic equilibrium
because the various processes that are occurring in the plasma are
individually not in equilibrium. Therefore, the ICP cannot be
characterized by a single equilibrium temperature. The Saha equation
can be used to make a reasonable estimate of the degree of ionization:
3/2
where ni, na, and ne are the number densities of ions, atoms, and free
electrons, respectively; m is the electron mass; Za and Zi are partition
functions; Ei is the ionization potential; and T is the temperature.
Using this equation, Houk (1986) calculated the degree of ionization of
elements for a plasma temperature of 7500 K and an electron density of
10l5 cm-3.

18. PLASMA FORMATION :
Inductively coupled plasmas are formed by coupling energy
produced by a RF generator to the plasma support gas with an
electromagnetic field. The field is produced by applying an RF power
(typically 700-1500 W) to an antenna (load coil) constructed from 3-
mm-diameter copper tubing wrapped in a two- or three-turn 3-cm-diameter
coil, positioned around the quartz torch assembly designed
to configure and confine the plasma.
An alternating current field is created that oscillates at the
frequency of the tuned RF generator.
The plasma is initiated by the addition of a few "seed” electrons,
generated from the spark of a Tesla coil or a piezoelectric starter, to
the flowing support gas in the vicinity of the load coil.
After the plasma is initiated it is sustained by a process known as
inductive coupling.

19. As these seed electrons are accelerated by the electromagnetic RF field,
collisions with neutral gas atoms create the ionized medium of the
plasma. The mean free path of accelerated electrons in atmospheric
pressure argon gas is about1 μm before a collision occurs with an argon
atom. These collisions produce additional electrons. This cascading
effect creates and sustains the plasma.
Once the gas is ionized, it is self-sustaining as long as RF power is
applied to the load coil. The ICP has the appearance of an intensely
bright fireball-shaped discharge.

20. Inductively Coupled Plasma Spectroscopy

21. A photograph of the ICP torch

22. SScchheemmaattiicc vviieeww ooff aann IICCPP..
Induction
coil
Magnetic
field
Manning T.J. and Grow W.P., 1997

23. Plasma Generation
 The ICP is generated by coupling the energy from a radio frequency
generator into a suitable gas via a magnetic field which is induced through
a two- or three-turn, water -cooled copper coil.
 The radiofrequency energy is normally supplied at a frequency of 27.12
MHz, delivering forward power at between 500 and 2000 W.
 Two gas flows, usually argon, flow in a tangential manner through the
outer tubes of a concentric, three-tube quartz torch which is placed axially
in the copper coil.
 Because the outer and intermediate gases flow tangentially (i.e. they
swirl around as they pass through the torch), the plasma is continually
revolving and has a 'weak spot' at the centre of its base, through which the
inner gas flow, containing the sample, can be introduced.

24.  When the gas is seeded with electrons, usually by means of a spark, the
electrons accelerate in the magnetic field and reach energies
sufficient to ionize gaseous atoms in the field.
 Subsequent collisions with other gaseous atoms causes further
ionization and so on, so that the plasma becomes self-sustaining. This
occurs almost instantaneously.
 The magnetic field causes the ions and electrons to flow in the
horizontal plane of the coil, thereby heating the neutral argon by
collisional energy exchange, and a hot fireball is produced.
 The hottest part of the ICP has a temperature between 8000 and 10000
K, which is the temperature of the surface of the Sun, though the
analytically useful region is in the tail-flame with a temperature between
5000 and 6000 K.

25. ICP-AES: Plasma
Inductively Coupled Plasma Source

26. Advantages of an ICP source
1. The analytes are confined to a narrow region.
2. The plasma provides simultaneous excitation of many elements.
3. The analyst is not limited to analytical lines involving ground state
transitions but can select from first or even second ionization state lines.
For the elements Ba, Be, Mg, Sr, Ti, and V, the ion lines provide the best
detection limits.
4. The high temperature of the plasma ensures the complete breakdown of
chemical compounds (even refractory compounds) and impedes the formation
of other interfering compounds, thus virtually eliminating matrix effects.
5. The ICP torch provides a chemically inert atmosphere and an optically thin
emission source.
6. Excitation and emission zones are spatially separated : this results in a low
background.
7. Low background, combined with a high S/N ratio of analyte emission,
results in low detection limits, typically in the parts-per-billion range.

27. list of some of the most beneficial characteristics of the ICP source.

28. Additional information

29. Inductively Coupled Plasma Source
A plasma is a hot, partially ionized
gas. It contains relatively high
concentrations of ions and electrons.
Argon ions, once formed in a plasma, are
capable of absorbing sufficient power from
an external source to maintain the
temperature at a level at which further
ionization sustains the plasma indefinitely.
The plasma temperature is about 10 000 K.
After Manning T.J. and Grow W.P., 1997
ICP-AES: Plasma

30. The main ionization processes are:
the charge-transfer ionization,
the electron-impact ionization
the Penning ionization,
while the main excitation processes for the analyte atom are:
the electron impact excitation,
And the
ion–electron radiative recombination,

31. Ar 1s22s22p63s23px
23py
23pz
2 [Ne]3s23px
23py
23pz
2
Ionization states
Consider an atom in an ionization state r, for example a carbon ion
with only 3 electrons (C IV) when normally it would have six in its
neutral state. In its ground state, 2 electrons would occupy its n = 1
state and 1 would occupy its n = 2 state.
This atom can be raised to the next higher ionization state (C V)
with the ejection of the latter electron if a collision or photon
provides the appropriate energy, the ionization potential,
Such a transition is indicated as Transition 1 in Fig. 2. It takes the
atom from the ground state of the r ionization state to the ground
state of the r+1 ionization state. The ejected electron will take on any
excess energy p the exciting photon may have had beyond r. The
photon energy would be

32. In Transition 2 the energy of the
exciting photon would be
if the final momentum of
the ejected electron is
p2/2m.
In Transition 1 the photon
energy would be
Energy states of an atom in two ionizations states. The r+1 state has one less
bound electron less than the r state. In the r+1 state, that electron is free.

33. More generally, the transition can initiate in any energy state of
the original ion and terminate in any energy state of the final
ion as illustrated by Transition 2 of Fig. 2. The
energy of the exciting photon would be
if the final momentum of the ejected electron is p2/2m.

34. d5 means
d8 means
Mn 1s22s22p63s23p63d54s2
Fe 1s22s22p63s23p63d64s2
Co 1s22s22p63s23p63d74s2
Ni 1s22s22p63s23p63d84s2
Cu 1s22s22p63s23p63d104s1
Zn 1s22s22p63s23p63d104s2
A transition element is defined as one which has partially filled d orbitals
either in the element or any of its compounds. Zinc (at the right-hand end
of the d-block) always has a completely full 3d level (3d10) and so doesn't
count as a transition element.

35. 3 power sources have been employed in argon plasma spectroscopy.
(i)dc electrical source capable of maintaining a current of several
amperes between electrodes immersed in a stream of argon. The
second and third utilize powerful
(ii)radio-frequency and
(iii)microwave-frequency fields through which the argon flows.
Of the three, the radio-frequency, or inductively coupled plasma
(ICP), source appears to offer the greatest advantage in terms of
sensitivity and freedom from interference.
On the other hand, the dc plasma source (DCP) has the virtue of
simplicity and lower cost.
The microwave induced plasma source (MIP) is not widely used
for analysis

36. • If we confine ourselves to a discussion of an argon ICP then we can say
that it consists mainly of the following species: