Atomic Absorption Spectrometry1

7/30/2019 Atomic Absorption Spectrometry1

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Atomic Absorption

Spectrometry (AAS)


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Flame test
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Hot hot gas

cold gas

Continuous spectrum

Emission spectrum

Absorption spectrum


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Emission spectra of various elements


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AAS relies on the absorption of light of a specificwavelength by atoms dispersed in a flame.

appropriate wavelength is provided by a hollow

cathode lamp containing the element to be analysed,focused through the flame and onto the detector

when the sample is introduced into the flame, it willdecrease the light detected in direct proportion to

the amount of metal present.


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How AAS works

Metal ions in solution are converted tometal atoms in their ground state byheating

The metal atoms absorb light, the extentof absorption depends on the concentrationof the metal atoms in the solution

Light of a specific wavelength is made to passthrough these ground state atoms


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The amount of reduction of the lightintensity due to absorption by the metalatoms can be directly related to the amount

of the element in the sample.


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How it works


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Components of an atomic absorption spectrometer

Radiationsource

atomizationcell

sampleintroduction

system

wavelengthselection

detector


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Instrumentation

This instrument is designed to operate either with a flame

or with a graphite furnace


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Hollow cathode lamp


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A hollow cathode lamp is a light source thatemits the characteristic spectrum of the element to bedetermined. The cathode material is the sameelement as the one being determined. A different

cathode lamp is needed for each element, althoughthere are some lamps that can be used to determinethree or four different elements if the cathode containsall of them.


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The burner assembly has two major functions:

conversion of sample solution into a fine aerosolsolution,and dissociation of the analyte elementsinto free gaseous ground state form.Not all the analyte goes through the flame, part ofit is disposed as shown in the figure.


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Diagram of the burner and the nebuliser


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As the sample passes through the flame,the beam of light passes through it intothe monochromator .The monochromator isolates thespecific spectrum line emitted by thelight source through spectral dispersion,

and focuses it upon a photomultiplierdetector, whose function is to convertthe light signal into an electrical signal.


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The processing of electrical signal is doneby a signal amplifier

The signal could be displayed

for readout or further fed into a datastation for printout by the requestedformat.


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Features of the technique

Quantification of nearly 70 metals in virtually any sample type

Concentrations as low as parts per billion to as high as

weight percent with minimal or no sample preparation

Microlitre volumes or microgram masses required

C li ti l i f


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Common applications: analysis ofbiological, medical and clinical sample

environmental samples ( water, solidssediments, biota)

samples from steel and metal industry

pharmaceutical industry samples

food industry samples

metal particulates in air

pollution studies


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Preparation of samples

Almost any solid, liquid or gaseous sample can be analysed.most samples are converted to homogeneous solutionbefore analysis.

If level of substances is below the detection limit,then pre-concentration techniques may be applied .


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Analysis time:

The time to perform analysis is linked to samplepreparation.

Preparing the sample can range from 0 sec to 24hrs.

The actual determination ( after calibration set-

up) ranges from approximately 10 seconds to 2min for furnace AAS.


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LIMITATIONS

Provides no information on the chemical form ofthe metal

Sample preparation can be tedious and timeconsuming

Limited to metals and metalloids

Destructive technique


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Accuracy

Accuracy depends on the the complexity of the matrix.A homogeneous solution at an analyte level 5-10 timesabove the detection limit will give an accuracy better than 1%.


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Techniques for quantification

AAS, in common with many analytical

techniques is not an absolute method of analysis.

Solutions of known concentrations are introduced to thesystem and the absorbance of each is determined

The variation of the absorbance with concentrationis plotted (a calibration curve is constructed)

the unknown solution is passed through the systemand the absorbance measured

the concentration of the unknown is read off thecalibration curve


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How to prepare a 10.00 ppm (mg L-1 ) of a metal ion froma metal salt:

For example 10.00 ppm Pb from Pb(NO3 )2

11

2

20.33119.207

/1000.1

molg

saltofmass

molg

Lgx

mass of salt = 1.5985 x 10 -2 g Pb(NO3)2 weighed,

dissolved and diluted to 1.000 L

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Atomic Absorption Spectrometry1