Atomic emission spectroscopy uses plasma sources like inductively coupled plasma to excite sample atoms and cause them to emit electromagnetic radiation of characteristic wavelengths. ICP is advantageous over flame sources because its higher temperatures of 6000-10000 K allow for excitation of more elements. The ICP system consists of an argon plasma torch and RF generator. Argon is used because it is inert and maintains high temperatures. ICP-AES provides rapid, multi-element analysis with low detection limits and matrix interferences. However, spectral interferences can still occur from overlapping emission lines.

1. Group D
Analytical and Instrumental
Inorganic Chemistry
By: Dr. Damodar Koirala
1
Atomic Emission Spectroscopy (AES)
Plasma Source

2. Basic principle
The electron of an atom moves from higher energy level to lower energy
level they emit extra amount of energy in the form of light which is consist of
photon
-Small part of sample is vaporized
-Free atom gain energy from plasma source
-It results in transition from lower to higher
energy state
-On returning back a photon of radiation is
emitted.
The plasma based AES in principle, is similar to the flame photometry; the
only difference being that flame is replaced by much more energetic
atomization-excitation processes using plasma.

3. Components of Plasma-AES
Source
• Plasma atomizer/source to create a population of excited
analyte atoms
• A wavelength selector to isolate the specific wavelength
• A light-sensitive detector to measure the light accurately
• Electronic devices to measure response of the detector

4. • Because of the relatively low temperatures (~2000-2500 C) in a flame-
based system, not all of the atoms or elements present in the sample are
excited, particularly if they exist in a polyatomic compound.
• The limitation of flame as an atomization-excitation source led to the
development of high temperature sources for atomic emission spectrometry.
• In plasma-based systems the temperature is considerably hotter (~6000 to
10 000 K) that results in more effective excitation of atoms (generally
greater then 90%) of approximately 60 elements including some nonmetals.
• This intense heat prevents polyatomic species from forming, thus
increasing the detection limits for many elements.
• While plasma-based systems eliminate many problems, they are not free of
interferences
Why plasma ?

5. Plasma is an electrical conducting gaseous mixture that contains the
significant concentration of cations and electrons. In emission
spectroscopy , the frequently used plasma source is Argon plasma. Here,
the Argon ions and electrons are the principle conducting species.
Plasma has 2 characteristics:
I- can conduct electricity
II- affected by magnetic fields
Plasma Source
Types: Depending on the power sources employed
(1) the inductively coupled plasma (ICP): radio frequency
(2) the direct current plasma (DCP): direct current
(3) the microwave induced plasma (MIP): microwave frequency

6. Why argon is preferred ?
i) Lesser chemical interference since it is inert and does not form stable
compound
ii) Argon ions once formed are capable to maintain high temp up to
10,000K in which atomization of any type is possible.
iii)It is optically transparent in the UV-visible region of the spectrum hence
does not offer spectral interference
iv) It has low thermal conductivity, so the heat is retained within the
plasma fireball sustaining stable operation at moderate power inputs.
v) As the abundance of argon is reasonable (1% in air), it is economical as
compared to other noble gases.
vi)As the first ionization energy of argon is quite high (15.75) eV, it has the
capacity to atomize, ionize and excite most of the elements

7. Instrumentation:
• A typical ICP source is called torch which consists of three concentric
quartz tubes through which current of Argon gas flows.
• The diameter of the largest tube is about 2.5 cm.
• The top of this tube is surrounded by a water-cooled induction coil
which is power by radio –frequency(RF) generator
Inductively Coupled Plasma(ICP) source

8. In a plasma torch the sample is injected with argon gas through central tube
(generally 0.3 to 1.5 lit/ min).
A typical arrangement of nebulizer is shown below:
Sample Introduction
Inductively Coupled Plasma(ICP) source
1) Pneumatic Nebulizer
2) Pneumatic
Aerosole Generator
3) Ultrasonic Nebulizer
Solid sample can be introduced via electrothermal vaporization.
Gaseous samples can can be mixed with argon and send to torch.

9. Fig: Typical ICP source (Torch).
Emission
region
• Meaning that the plasma is generated due
to differences in the magnetic field and
currents.
• Ionization of the flowing argon is initiated
by a spark from a Tesla coil.
• The resulting ions and electrons, then
interact with the fluctuating magnetic field
produced by the induction coil. This
interaction causes the ions and electrons
within the coil to flow in the closed annular
paths.
Inductively Coupled Plasma(ICP) source

10. Inductively Coupled Plasma(ICP) structure
Brilliant white core: Ar continuum and lines
Flame-like tail : up to 2 cm
Transparent region : Analytical region
(Cone shaped 5000-6000 K)

11. Advantages of ICP-AES
• Rapid and simultaneous multi-element analysis. 40-50 elements in 5 min
• Lack of chemical interferences.
• High temperature of the excitation source : 5000 to 10000K.
• Low sample requirements.
• Absence of self absorption, the cause of non-linear calibration plots in FES.
• Validity of calibration curves over 4 to 6 orders of magnitude.
• Low detection limits: 1 to 100 ng/g or µg/l (part per billion).
• Good accuracy and precision: relative standard deviation of about 1 %
• Applicability to elements that are difficult to be determined by AAS: B, C,
Ce, La, Nb, Pr, S, P, Ti, Ta, V and Zr can also be measured.

12. ICP systems greatly reduce the number of interferences over those created
in flame-based systems.
Spectral, physical and chemical interference may present, however
spectral are most prominent.
Chemical interferences are common in FAAS and FAES but are less
common or practically nonexistent in ICP-AES due to the relatively high
temperature of the plasma, long residence time in the plasma, and inert
atmosphere of the Ar plasma.
Interference in ICP-AES
These interferences are associated with the processes of sample
nebulisation and transport.
The chemical interferences include molecular compound formation,
ionisation effects, and solute vaporisation effects.
If present, these can be minimised by carefully controlling the operating
conditions such as incident power, observation position, matrix matching,
etc.

13. Interference in ICP-AES
Spectral Interference (common):
Involves the overlap of the spectral lines:
eg: Cu emission at 515.323 nm and Ar at 515.139 nm.
*more than one analytical line of a single element can be used.
Another common spectral interference involves the formation of undesired
species (e.g., ions, metal oxides). e.g., Fe along with Fe+, Fe+2, etc.)
Similarly, the formation of metal oxides or metal carbides, are also potential
source of interference and need to be evaluated on an individual basis.
Background emission and stray light can interfere and usually be compensated
for by subtracting the background emission

14. Direct current plasma (DCP)
vs ICP

15. Direct current plasma (DCP)
The plasma jet is formed by bringing the cathode into momentary contact with the
anodes.
Ionization of the argon occurs and a current develops (~14 A) that generates additional
ions to sustain the current indefinitely.
The temperature at the arc core is more than 8000 K and at the viewing region about
5000 K.
The sample is aspirated into the area between the two arms of the Y, where it is
atomized, excited, and detected.
• DCP is an order of magnitude less sensitive than ICP
• Similar reproducibilities
• DCP requires less Argon gas
• Auxiliary power less expensive in DCP
• Graphite electrode must be replaced every couple of hrs
vs ICP

16. Application of Plasma Sources
Agricultural: Analysis of agricultural products
Health sciences : Determination of Al in blood, Cu in brain tissue, Se in
liver, Na in breast milk.
Geological sciences: Presence of lanthanides and other elements in rock
Forensic Sciences : Crime scene soil analysis.
Metallurgy : Analysis of trace elements in stainless steel.
Environmental science: Waste water analysis, determination of pollutant
Industry : Presence of metals like Cu, Fe, Ni, and Si in gasoline at tracer
concentration.
Plasma sources are rich in characteristic emission lines, which makes them
useful for both qualitative and quantitative elemental analysis.

17. Comparison of Spectroscopic Techniques
Performance Criteria Flame AA
Electrothermal
Sampling
Flame
AES
DCP ICP
Matrix interfernces high high high low low
Spectral Interferences low low low moderate high
Precision & Accuracy
Better for
unskilled
Worse for
unskilled
Costs $$ $$$ $ $$$ $$$$
Instrumentation low low low moderate
high
Maintenance
low low low moderate high
Sample Preparation moderate moderate
low
moderate moderate
Operator skill lower higher higher higher higher