aas 4
TRANSCRIPT
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Atomic absorption spectrometry
(AAS)
Done by: Samyah Alanazi
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History
Atomic absorption was discovered in 1814by Joseph von Fraunhofer (German optician)who observed dark lines Fraunhofer lines
in the spectrum of the sun. These lines weresubsequently found to be due to the elementsin outer core of the sun absorbing continuouslight emitted from inner zones. Fraunhofer
lines were originally given labels from A to Zbut these labels are now obsolete inchemistry.
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Fraunhofer lines
The atomic absorption spectrometer was
invented in 1950 by Sir Alan Walsh.
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Principle
When a solution containing a metallic salt is
sprayed into flame a small fraction of atoms
become thermally excited and emit light.
However, most atoms remain in the ground
state. These ground state atoms can absorb
at the same wavelengths that thermally
excited atoms of the same metal emit.
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The processes of atomic emission
and absorption
Emission Absorption
E1
E0
E = E1 E0
Photon Photon
E1
E0
E = E1 E0
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So, in atomic absorption it is necessary to
supply photons of precisely the right
energy to excite the atoms, so that their
absorbance by the sample can bedetermined. The way this is done is
described in the next slide.
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Instrumentation for atomic
absorption spectrometry
AAS similar to flame emission
spectrometer except that there is a source
od radiation of exactly the right wavelengthto excite the element being measured. The
key components of an atomic absorption
spectrometer are shown in the next figure.
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Components of an atomic
absorption spectrometerModulator
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Source
The source is almost always a Hollowcathode lamp (HCL) . A hollow cathodelamp contains elements that are made
with the same metal as being tested. I.e. ifzinc is being analysed a zinc cathode willbe used in the lamp. The lamp is filled withHelium or Argon at low pressure and
several hundred applied between theelectrodes. Energetic electrons from thecathode ionise the gas by collisions.
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E.g.
Ar + e-
The positively charged ions areaccelerated towards the cathode (-vecharged) and sputter metal atoms from thesurface. The gaseous metal ions are then
excited by collisions with electrons andgas ions:
M (g)
Ar + + 2e-
M* (g)e- , Ar +
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The excited metal atoms then emit the
characteristic emission lines :
M * (g) M (g) + energy
Different cathode lamp will be used to
analyse another element .
It is possible to use multi- element lamps(e.g. Ca/Mg) .
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Electrical modulator
In AA there will not only be absorption of
light from HCL but there will be also atomic
emission. i.e. if the concentration of
sodium atoms is being measured in anAAS, a small proportion of a sodium atoms
will be excited in the flame and will emit
light . This will give rise to the backgroundemission which will effect the recorded
absorption value.
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To overcome this, is to use modulation of
the source . The light from HCL is
modulated ( the intensity is rhythmically
changed ) by either :
1- putting a chopper in the light path.
2- modulating the power delivered to the
HCL lamp to alter its output.
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Mechanical modulation in AAS
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In effect the signal from the lamp in thedetector is now an alternating current (AC)signal. In contrast the background
emission is a direct current (DC) signal asit does not change with time. So, themodulation is synchronised to the signalamplifier. This means that the signal
amplifier is (tuned) so that it will onlyamplify signals of the same frequency bythe modulator.
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Flame
The sample is usually introduced into the
flame using a nebuliser. The purpose is to
get the sample in the form of aerosol of
fine droplets. As in FES the venturi effect
is used.
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Schematic of an AAS nebuliser
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Efficiency of the nebuliser depends upon:
1- surface tension of liquid (organic liquids
give finer aerosols).
2- flow rates of gases (these can be
accurately controlled).
3- viscosity.
4- Density.
5- Vapour pressure.
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It is possible to enhance absorbance by includinga suitable organic solvent in the calibration/samplesolution, e.g. 80% ethanol or methanol. Thesereduce the surface tension of the aspirated
solution, reducing aerosol size and enhancingatomization.
Once nebulised, the sample is passed into theburner/atomiser. This is usually long and thin toincrease absorption in accordance with Beer-
lamberts law. It is narrow to reduce scattered lightentering monochromator (scattered light will tendto disperse away from the line of the flame ratherin line).
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The important characteristics of the flame are:
1- Its temperature.
2- whether it is oxidizing/ reducing.
Commonly used fuels include:
1- Alkanes. 2- Hydrogen.
3- Acetylene.
Commonly used oxidants:
1- Air. 2- Oxygen.
3- Nitrous oxide (NO).
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Monochromator
Monochromator functions to ensure only
the light of a selected wavelength passes
through the flame. Slits on either sides of
the monochromator will reduce the amountof a scattered light from the flame and
reduce the scattered light energy from the
dispersion device.
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Detector
The detector in an AAS is usually a photomultiplier.A photomultiplier contains a photo emissive cathodecoated with an easily ionised material (e.g.Caesium- antimony alloy). A photon of a suitableenergy hitting this material causes the release of an
electron- the surface converts a light beam intoelectrical signal. The electron is accelerated towardsa positively charged anode with sufficient energysuch that when it hits the anode 2 to 10 electronsare released. These are then attracted to
subsequent anodes and the process repeat for moreand more electrons each time. Typicalphotomultiplier have 10 anodes, giving very highamplification.
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Recorder
The recorder is usually a digital readout of
the absorption.
Modern instruments have full computer
control and data interfacing.
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Procedure
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Terms used in AAS
1- Sensitivity:
Is a concentration of the element under
investigation in ppm that reduces the light
intensity by 1%, i.e. :
Io = 100 Flame It =99
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Thus:
A = log Io/It = log 100/99 = 0.0044
Therefore, sensitivity is the concentration
of the element in ppm giving anabsorbance of 0.0044.
2- limit of detection:
in AAS is the minimum amount of theelement that can be detected with 95%certainty.
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3- It is usually taken as twice the sample
standard deviation of the results at one or
near the absorbance of the blank.
4- Expressed as concentration of an
element in ppm that can be distinguished
from zero concentration with 95%
certainty.
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Some applications of AAS
Biological Samples:
1- Determination of calcium in serum.
2- Determination of cadmium.
3- Determination of lead.
4- Zinc in plant leaves. Environmental Samples:
1- Analysis of airborne particulate matter.
2- Mercury in air/water.
3- Trace elements contamination in soil Industrial Samples:
1- Determination of molybdenum in steel.
2- Tin in canned fruit juice.
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Advantages of AAS over FP
1- Sensitivity.
2- Applicability.
3- Smaller flame effect.
4- Less interference from other cations.
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Similarities of AAS and FP
AAS suffer the same level of interferences
as FP from:
1- Anion due to complexation.
2- Alcohols due to surface tension effects.
3- Sugars due to viscosity effects.
4- Ionisation (overcome by use ofreleasing agent).
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Disadvantages of AAS
1- Many metals absorb in the very far UV
region and thus can not be determined.
2- HCL to be changed for different
elements in the sample.
3- flame problems .Such as high rate of
the flame, loses of samples in the
nebuliser, sample amount and absorptionof emission lines near 200 nm.
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To overcome these issues Electro-thermal
ionisation is used via graphite furnace
technique (GF-AAS). The sample is
electrically heated in a pyrolytic graphitetube which replace flame.
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Advantages of GF-AAS
1- the atom kept in the light beam for long
time
2- 100- 1000 fold improvement in
detection limits.
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Disadvantages of GF-AAS
1- lack of precision.
2- Background absorption.
3- Analyte can be lost during ashing ifvolatile salts are present.
4- Analysis time is slow.
5- Expensive.