atomic absorption spectroscopy (aas)€¦ · atomic absorption spectroscopy (aas) atomization: it...

ATOMIC ABSORPTION

SPECTROSCOPY (AAS)

Prepared by:

Dr.Elsadig H.KH.Adam

Pharmaceutical Instrumental Analysis Dr.Elsadig H.kh.Adam

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What is Atomic Absorption Spectrophotometer ?

A spectrophotometer which analysis atomic absorption characteristics of atoms is called Atomic Absorption Spectrophotometer.


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How does energy absorption happen in an atom ?

When sufficient energy is supplied to an atom it can excite electrons present in the atom, and electron will absorb this light energy and will transmit to higher energy level.

“ This phenomenon is called Atomic Absorption. ”

Each element absorbs specific light whose wavelength is unique.


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Operation of a Spectrophotometer

Light source emits specific light.

Sample will absorp some of the light, then remaining will be transmitted.

Detector will measure the remaining portion of the light.


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Basic Structure of AAS

ATOMIC ABSORPTION SPECTROSCOPY (AAS)

Atomization: It is the conversion of molecules to theircomponent atoms in gaseous state using a source of heat(flame).

Atoms in vapor state absorb ultraviolet or visible light andmake transitions to higher electronic energy levels.

The intensity of the absorbed light is proportional to theconcentration of the element in the flame.

In AAS, atoms in the vapor state are subjected to externalsource of radiation which produces one line or beam ofmonochromatic light with single wavelength. Thiswavelength is the resonance wavelength that will beabsorbed by the atoms.


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The absorbance takes place following Beer’s law

A = log I0 / I = 0.434 k C b

where K is a coefficient that depends on the nature ofatoms and the radiation.

C is the concentration of atoms which depends on thesample concentration.

b is the light path length which depends on thedimensions of the flame (standard and constant for eachinstruments).


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INSTRUMENT FOR ATOMIC ABSORPTION

• Atomic absorption spectrometers (AAS) consist of 6main parts:

1- A light source (usually a hollow cathode lamp)

2- Chopper

3– An atom cell (atomizer)

4- A monochromator

4- A detector

5- A read out device.


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1-Source of radiation (Hollow cathode lamp):

Analysis of each element requires the use of its own unique lamp.


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2-The chopper

In front of the lamp there is the light chopperwhich is a fan-shaped object.

Its function is to fluctuate the source output.


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3- Atomizer:

There are two types of atomizers

a- Flame atomizer:

It serves two functions

- Atomization of the sample.

- Source of thermal energy to excite the atoms.


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b- Flameless atomizer:Furance method (Electric Thermal Atomization )

a graphite furnace heated electrically up to 6000oC .

Upon heating the furnace, sample is dried, ashen, and then vaporized by the

action of heat.


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Concentration Level


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Consumables (running cost)

• AA Flame；– gas

•Acetylene (C2H2)； 2-7 L/min.•Dinitrogen monoxide(N2O)； 6-7 L/min.

• AA Furnace；– Argon (Ar); 2 L/min.– Graphite tube

Advantages of non flame atomizer: Solutions, slurries and solid samples can be analyzed. High sensitivity. Smaller quantities of sample (typically 5 – 50 μL). Heat distribution is uniform and temperature is steady. No flame noise.

Disadvantages Expensive. Low precision. Requires high level of operator skill.4-A monochromator :The main purpose of the monochromator isto isolate the absorption line from background light due tointerferences

5- Detector:Photomultiplier:determines the intensity of photons of the analytical

line exiting the monochromator.

6- Readout meter:Absorbance or transmission output.


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Quantitative ApplicationCalibration curve method:1- Choose a suitable lamp.2- Prepare a series of standard solutions of the metal to

be determined, in the solvent to be used in thesample. The concentration range has to be in ppm.

3-Adjust zero absorbance while spraying a blanksample (solvent).

4- Measure the absorbance while spraying each of thestandard solutions.

5- Plot a graph representing the concentration vsabsorbance.

6- Spray the sample and measure the absorbance anddetermine the concentration from the graph.


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Standard addition method:1-Prepare a standard solution of exact and known

concentration e.g. 100 ppm.2- Prepare a spiked sample by addition of a volume of

the standard solution to the sample solution(unknown concentration). For example: add a volumeof the standard equivalent to 5 ppm of the standard.

3- Measure the absorbance while spraying the sample A1and the spiked sample solution A2.A1 = K CX

A2 = K (CX + CS)where Cx is the sample concentration and CS is the

concentration of standard added (spiked sample).A1 CS

CX = __________

(A2 - A1)


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Comparison between Flame & Furnace Method

Flame Method Graphite Method

Principal of Atomization Heat of Fire Heat of electric current

Atomization Efficiency About 10% About 90%

Sample Volume About 1-2 ml 10-50 micro-liters

Shape of Signal Steady Signal Peak

Sensitivity Low (ppm level) High (ppb level)

Interference Small Big

Analysis Time Short1-30 sec / 1 sample

Long1-5 min/ 1 sample

Applications for atomic absorption spectrophotometry:

Clinical analysis: blood samples: whole blood, plasma and serum;

Ca, Mg, Li, Na, K, Fe.

Environmental analysis: finding out the levels of various

elements in rivers, seawater, soil, air and petrol.

Water analysis: (e.g. Ca, Mg, Fe, Si, Al, Ba content).

Food analysis and animal feed (e.g.Mn, Fe, Cu, Cr, Zn).

Analysis of additives in lubricating oils and greases (Ba, Ca, Na, Li,

Zn, Mg).

Pharmaceuticals: In some pharmaceutical manufacturing

processes, minute quantities of a catalyst used in the process

(usually a metal) are sometimes present in the final product. By

using AAS the amount of catalyst present can be determined.


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