Atomic Emission Spectroscopy
Objectives
After this session students will be able to
• Differentiate
between Flame emission spectroscopy and atomic emission spectroscopy
• Explain
the instrumentation of atomic absorption spectrophotometers
Atomic
Emission Spectroscopy Using Non-Flame excitation sources
- There
is no single excitation source can
excite all elements - The
emitted radiation usually consists of sharp well defined lines, which fall
in UV or visible region - Identification
of the l
of these lines permits qualitative analysis of these elements, whereas
measurements of their intensities permits quantitative analysis
Advantages
- Excellent
method for trace element analysis at ppm level - Used
nearly for all elements in periodic table - Used
for very small samples, even less than 1 mg - There
is no need for prior separation - Relatively
rapid technique
Disadvantages
- Expensive
- Low
precision and accuracy - Destroying
the sample - Used
mainly for metals
High energy
excitation sources
Plasma excitation sources
Laser
Arc and spark emission spectrometry (Spectrography)
Microwave and x-ray
Plasma
excitation sources
- A
plasma is a cloud of
highly ionized gas containing significant numbers of positive and negative
ions, free electrons and neutral particles. - Plasma
sources operate at high temperatures between 7000 and 15000 K. Thus, it
produces a greater number of excited emitted atoms, especially in the UV
region, than that produced by flame. - Using
this technique, excitation operates through a plasma produced electrically
in a carrier gas such as nitrogen or argon.
The main types of
argon plasma sources
- Inductively
coupled plasma; ICP - Direct
current plasma; DCP - A
microwave-induced plasma is recently introduced to spectro-chemical
analysis methods.
Inductively coupled
plasma; ICP
Argon gas flows upward through a quartz tube, around which
is wrapped with a copper or selenoid induction coil.
The coil is energized by a radio frequency AC generator
creating a changeable magnetic field on the flowing gas inside. This induces a
circulating current in the gas, which in turn heats it.
Argon is not a conductor at low temperatures, but becomes
electrically conducting by heating it. The induction is initiated by arc or a
heated graphite rod.
It is used for multi-element determination
Direct current, DCP
It consists of a high-voltage discharge between two graphite
electrodes. The recent design employs a third electrode arranged in an inverted
Y-shaped which improves the stability of discharge.
The sample is nebulised at a flow rate of 1 ml/min. Argon is
used as carrier gas. The argon ionized by the high-voltage is able to sustain a
current.
DCP generally has lower detection limits than ICP. However, DCP is less expensive than ICP.
Advantages
of plasma excitation source:
1- The sample could be introduced in solution form through a
nebulizer (easy for quantitative analysis).
2- It is suitable for quantitative multielement
determinations
3- The high temperature of plasma eleminates many chemical
interferences present in a flame
4- It is well suited for refractory (oxide forming) elements
e.g. P, Ur and tungeston and for difficult-to-excite elements such as Zn and
Cd.
5- The emission intensity-versus-cencentration range is
linear over a very wide dynamic ranges of analytes.
Laser
excitation source
Laser beam is used to vaporize the sample, which is then
excited electrically.
The sample is loaded just beneath the two electrodes that
will be used to generate the electrical discharge.
A ruby laser is then focused through a microscope onto the
surface. The energy from the laser causes an intense local hot spot which
vaporizes a small quantity of sample.
The vapor circuits the electrodes and electrical discharge
occurs which excites the metals in vapor. The excited metals emit typical
emission spectra which are collected and measured as usual.
Advantages of laser
excitation source
- Laser
excitation produces a high density plasma and is used for the
spectrochemical analysis of solid materials. - The
localization effect permits examination of areas as small as 50 µm in
diameter, providing the biological researcher with a tool capable of
examining the insides of individual cells without destruction of organic
materials. - In
laser excitation, the sample needs not to be electrically conducting.
Quantitative
analysis
Use of an
internal standard
If the composition of sample and matrix is unknown. The
internal standard is added to both unknown and calibration standards.
The internal standard
should
1. Resemble the element to be determined in rate of
volatilization and chemical reactivity.
2. Have a measurable emission line in the same spectral
vicinity as the sample emission line.
3. It must not also present in the original sample.
Then, by plotting the ratio of intensities of the element to
the internal-standard element vs. concentration of the element, any
fluctuations should be compensated for.
Standard Addition
Method
In order to partially or wholly counteract the chemical and
spectral interferences introduced by the sample matrix.
Applications
of AES (using non-flame excitation sources)
AES is rapid method for qualitative and quantitative
determination of most metals.
It is superior than flame and atomic absorption methods.
Flame emission spectroscopy has the limitations of being only good for few
elements while atomic absorption techniques need a separate source lamp for
each element. AES methods; being very sensitive, have numerious applications in
analysis of biological samples.
For examples:
- evaluation
of platinum in body fluids and tissues after administration of platinum
containing anticancer drugs - determination
of organic and inorganic Se compounds in biological fluids and
environmental samples - determination
of trace elements such as Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni and Pb - Silicon
is recogonized as an essential trace element participating in normal body
metabolism.
SUMMARY
• AAS
makes use of non-flame energy sources
• The
energy sources are Inductively Coupled Plasma, Direct Current Plasma and laser excitation sources
• The
emitted radiation usually consists of sharp well defined lines, which fall in
UV or visible region
• Identification
of the l of
these lines permits qualitative analysis of these elements, whereas
measurements of their intensities permits quantitative analysis
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