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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.