Transcript Atomic Emission Spectrometry

Flame Emission Spectroscopy
 Measure the intensity of emitted radiation
Excited State
Emits Special
Electromagnetic Radiation
Ground State
Instrumentation
 Consists of:
1.
2.
3.
4.
5.
Nebulizer
Burner
Monochromator
Detector
Readout device / computer
Introduction
 Basic Schematic
Atomizer
Wavelength
Selector
Detector
 Scanning instruments can detect multiple elements.
 Many lines detected so sometimes it is a quantitatively
difficult method.
 Source can be flame, but more commonly plasma because
it is much hotter.
 In flame emission spectroscopy
 Each element emit its own characteristics line spectrum
 Quantitative analysis can be performed here by
observing what  are emitted & comparing these with
various standards.
 Detector permits qualitative as well as quantitative
analysis
 Wavelength of emitted radiation indicates what element
is present and the radiation intensity indicates how
much of the element is present
 In flame emission spectroscopy
 Intensity of the emitted light increase with
concentration
 Relationship between intensity and concentration is
usually linear
I
I = kc
Unknown concentration can be
detected by comparison with
one or a series of standards in
the same manner for the
molecular techniques
c
Types of Atomizer
 Flame
 Plasma
 Arc and spark
Flame Atomization
Slit
Slit
Emitted
Flame
Detector
Lens
Filterer
Process
 Sample is sprayed by the nebulizer into the burner.
 Carried into the flame
 Atomized & excited
 The emission from the excited atoms passes into the
monochromator where the selected wavelength is passed
through for measurement.
 Intensity of the emitted wavelength is measured by the
detection system & indicated on the readout/computer.
Relationship Between Atomic Absorption
and Flame Emission Spectrosopy
 Flame Emission  it measures the radiation emitted by the
excited atoms that is related to concentration.
 Atomic Absorption  it measures the radiation absorbed
by the unexcited atoms that are determined.
 Atomic absorption depends only upon the number of
unexcited atoms, the absorption intensity is not directly
affected by the temperature of the flame.
 The flame emission intensity in contrast, being dependent
upon the number of excited atoms, is greatly influenced by
temperature variations.
Flame Emission Spectroscopy
 Flame Emission Spectroscopy is based upon those particles
that are electronically excited in the medium.
 The Function of Flame
1. To convert the constituents of liquid sample into the
vapor state.
2. To decompose the constituents into atoms or simple
molecules:
M+ + e- (from flame)  M + hv
3.
To electronically excite a fraction of the resulting atomic
or molecular species
M  M*
INTERFERENCES
Spectral
interference
Chemical
interference
NOTE: same interference which occur in AAS
Comparison btw AAS & AES
(Based on Flame)
Flame Atomic
Absorption
Flame Atomic
Emission
Process measured Absorption (light
Emission (light
absorbed by unexcited emitted by excited
atoms in flame)
atoms in a flame)
Use of flame
Atomization
Atomization &
excitation
Instrumentation
Light source
No light source
Beer’s Law
Applicable
Not applicable
Data obtained
A vs c
I vs c
2. Plasma
 Plasma – highly ionized, electrically neutral gaseous
mixture of cations and electrons that approaches
temperature  10, 000 K.
 There are three types of plasma sources:
a) Inductively coupled plasma (ICP)
b) Direct current plasma (DCP)
c) Microwave induced plasma (MIP)
 ICP is the most common plasma source.
Inductively Coupled Plasma (ICP)
 Constructed of three concentric
quartz tube.
 RF current passes through the water-
cooled Cu coil, which induces a
magnetic field.
 A spark generates argon ions which
are held in the magnetic field and
collide with other argon atoms to
produce more ions.
 Argon in outer tube swirls to keep
plasma above the tube.
 The heat is produced due to the
formation of argon ions.
Inductively Coupled Plasma (ICP)
Plasma Appearance
a. Excitation Region
 The bright, white, donut shaped
region at the top of the torch.
 Radiation from this region is a
continuum with the argon line
spectrum superimposed.
 Temperature: 8000 – 10 000 K
b. Observation Region
 The flame shaped region above the
torch with temperatures  1000 –
8000 K.
 The spectrum consists of emission
lines from the analyte along with
many lines from ions in the torch.
Inductively Coupled Plasma (ICP)
1. Sample Introduction
a. Liquid Sample
- Nebulizer similar to FAAS
- Sample nebulized in a
stream of argon with a
flow rate of 0.3 – 1.5 L/min.
- Sample aerosol enters the
plasma at the base through the
central tube.
b. Solid Samples
- Sample atomized by
electrothermal atomization a
carried into the plasma by
of argon gas.
and
a flow
Advantages of ICP-AES over flame AES:
a) Temperature is two to three times higher than in a flame or
furnace, which results in higher atomization and excitation
efficiencies.
b)There is little chemical interference.
c) Atomization in the inert (argon) atmosphere minimizes
oxidation of the analyte.
d)Short optical path length minimizes the probability of selfabsorption by argon atoms in the plasma.
e) Linear calibration curves can cover up to five orders of
magnitude.
ICP-AES over Flame AES
 Much lower detection limit because:
 Higher temperature with the plasma will increase the
population of excited state atoms.
 The plasma environment is relatively chemically inert
due to the higher population of electrons which will
minimize the interference of ionization.
AAS and AES
 Both methods use atomization of a sample and therefore
determine the concentrations of elements.
 For AAS, absorption of radiation of a defined wavelength
is passed through a sample and the absorption of the
radiation is determined. The absorption is defined by
the electronic transition for a given element and is
specific for a given element. The concentration is
proportional to the absorbed radiation.
 In AES, the element is excited. A rapid relaxation is
accompanied by emission of UV or visible radiation is
used to identify the element. The intensity of the
emitted photon is proportional to element
concentration.