Atomic Emission
Spectroscopy (AES, OES)
Introduction:
• Atomic emission spectroscopy (AES or OES) uses quantitative
measurement of the optical emission from excited atoms to
determine analyte concentration.
• Analyte atoms in solution are aspirated into the excitation region
where they are desolvated, vaporized, and atomized by a flame,
discharge, or plasma.
• These high-temperature atomization sources provide sufficient
energy to promote the atoms into high energy levels.
• The atoms decay back to lower levels by emitting light. Since the
transitions are between distinct atomic energy levels, the
emission lines in the spectra are narrow.
• The spectra of multi-elemental samples can be very congested,
and spectral separation of nearby atomic transitions requires a
high-resolution spectrometer.
• Since all atoms in a sample are excited simultaneously, they can
be detected simultaneously, and is the major advantage of AES
compared to atomic-absorption (AA) spectroscopy.
Instrumentation: As in AA spectroscopy,
the sample must be
converted to free atoms,
usually in a high-
temperature excitation
source
• Sample Introduction:
• Liquid samples are nebulized and carried into the excitation source by a
flowing gas.
• Solid samples can be introduced into the source by a slurry or by laser
ablation of the solid sample in a gas stream. Solids can also be directly
vaporized and excited by a spark between electrodes or by a laser
pulse.
Excitation:
• The excitation source must desolvate, atomize, and
excite the analyte atoms. A variety of excitation sources
are described in separate documents:
– Flame
– Arc / Spark
– Plasma
–Inductively-coupled plasma (ICP)
–Direct-current plasma (DCP)
–Microwave-induced plasma (MIP)
–Laser-induced plasma, Laser-induced breakdown
(LIBS)
AES based on Plasma Sources:
Plasma is an electrical conducting gaseous mixture containing significant
amounts of cations and electrons ( net charge approaches zero)
1) increased atomization/excitation
- 2) wider range of elements
A+ e A
3) simultaneous multielement analysis
4) wide dynamic range
1) ICP-OES:
1) ICP-OES: -A typical ICP consists of three concentric
quartz tubes through which streams of argon
gas flow at a rate in the range from 5-20
L/min.
- The outer tube is about 2.5 cm in diameter
and the top of this tube is surrounded by a
radiofrequency powered induction coil
Induction coil
producing a power of about 2 kW at a
frequency in the range from 27-41 MHz. This
coil produces a strong magnetic field as well.
- Ionization of flowing argon is achieved by a
spark where ionized argon interacts with the
strong magnetic field and is thus forced to
move within the vicinity of the induction coil at
a very high speed.
- A very high temperature is obtained as a
result of the very high resistance experienced
by circulating argon (ohmic heating).
-The top of the quartz tube will experience
very high temperatures and should, therefore,
be isolated and cooled. This can be
accomplished by passing argon tangentially
around the walls of the tube
ICP Plasma Structure
The viewing region used in elemental
analysis is usually about 6000 oC, which is
about 1.5-2.5 cm above the top of the tube.
It should also be indicated that argon
A plasma torch looks very much like a flame consumption is relatively high which makes
but with a very intense nontransparent brilliant the running cost of the ICP torch high as well.
white color at the core (less than 1 cm above Argon is a unique inert gas for plasma
the top).
torches since it has few emission lines. This
decreases possibility of interferences with
In the region from 1-3 cm above the top of
the tube, the plasma becomes transparent. other analyte lines.
The temperatures used are at least two to
three orders of magnitude higher than that
achieved by flames which may suggest
efficient atomization and fewer chemical
interferences.
2) Direct Current DC-Plasma
The DCP is composed of three electrodes arranged
in an inverted Y configuration.
A tungsten cathode resides at the top arm of the
inverted Y while the lower two arms are occupied by
two graphite anodes.
Argon flows from the two anode blocks and plasma
is obtained by momentarily bringing the cathode in
contact with the anodes.
Argon ionizes and a high current passes through
the cathode and anodes.
It is this current which ionizes more argon
and sustains the current indefinitely.
Samples are aspirated into the vicinity of
the electrodes (at the center of the inverted
Y) where the temperature is about 5000 oC.
DCP sources usually have fewer lines
than ICP sources, require less argon/hour,
and have lower sensitivities than ICP
sources.
In addition, the graphite electrodes tend
to decay with continuous use and should
thus be frequently exchanged.
Comparison of DCP and ICP
A DCP has the advantage of less argon consumption,
simpler instrumental requirements, and
less spectral line interference.
However,
ICP sources are more convenient to work with,
free from frequent consumables (like the anodes in DCP’s which need
to be frequently changed), and
are more sensitive than DCP sources.
Advantages of Plasma Sources over Flame
and Electrothermal Atomization
1. No oxide formation as a result of two factors including
• Very high temperature
• Inert environment inside the plasma (no oxygen)
2. Minimum chemical interferences
3. Minimum spectral interferences except for higher possibility of
spectral line interference due to exceedingly large number of
emission lines (because of high temperature)
4. Uniform temperature which results in precise determinations
5. No self-absorption is observed which extends the linear dynamic
range to higher concentrations
6. No need for a separate lamp for each element
7. Easily adaptable to multichannel analysis
Plasma Emission Instruments
Three classes of plasma emission instruments can be presented including:
1. Sequential instruments
In this class of instruments a single channel detector is used where the
signal for each element is read using the specific wavelength for each
element sequentially. Two types of sequential instruments are available:
a) Linear sequential scan instruments where the wavelength is linearly
changed with time. Therefore, the grating is driven by a single speed
during an analysis of interest
b) Slew scan instruments where the monochromator is preset to
provide specific wavelengths; moving very fast in between
wavelengths while moving slowly at the specific wavelengths.
Therefore, a two-speed motor driving the grating is thus used.
Sequential MC
2. Multichannel instruments
Since the atomic emission lines are very narrow, a high-resolution
polychromator is needed to selectively monitor each emission line.
