Atomic absorption spectroscopy

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IIntroductionntroduction

Elementary TheoryElementary Theory

InstrumentationInstrumentation

InterferencesInterferences

Experimental preliminariesExperimental preliminaries

AApplicationspplications

Atomic Absorption SpectroscopyAtomic Absorption Spectroscopy（（AASAAS））

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Atomic absorption spectroscopy is a quantitative method of analysis that is applicable to many metals and a few nonmetals.

IIntroductionntroduction

I ntroductionI ntroduction

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The technique was introduced in 1955 by Walsh in Australia (A.Walsh, Spectrochim. Acta, 1955, 7, 108)

Alan Walsh 1916-1998

The application of atomic absorption spectra to chemical analysis

AASAAS

The first commercial atomic absorption spectrometer was introduced in 1959


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AASAAS

DetectorDetector

SampleSampleCompartmentCompartment

Light SourceLight Source

An atomic absorption spectrophotometer An atomic absorption spectrophotometer consists of a light source, a sample compartment consists of a light source, a sample compartment and a detector.and a detector.


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AAS

A much larger number of the gaseous metal atoms will normally remain in the ground state.

These ground state atoms are capable of absorbing radiant energy of their own specific resonance wavelength.

If light of the resonance wavelength is passed through a flame containing the atoms in question, then part of the light will be absorbed.

The extend of absorption will be proportional to the number of ground state atoms present in the flame.


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the gaseous metal atoms

specific resonance wavelength

the extend of absorption vs the number of ground state atoms present in the flame.

extend of absorption

AAS


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Characteristic wavelength

Characters of the atomic absorption spectrum

ΔΔ EE = = EE11 –– EE00 = = hchc / /

E1 - excited state

E0 – ground state

h – Planck’s constant

c – velocity of light

- wavelength

Elementary Theory Elementary Theory


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K0 - maximal absorption coeffi cient

Δ - hal f width

0 -central wavelength


Profile of the absorption line


1010

Natural bNatural broadeningroadeningDDeterminedetermined by the lifetime of the excited stateby the lifetime of the excited state andandHeisenbergHeisenberg’’s uncertainty principles uncertainty principle（（1010--5 5 nmnm））

Doppler BroadeningDoppler Broadening（10-3 nm）Results from the rapid motion of atoms as they emit or absorb radiation

CollisionalCollisional BroadeningBroadeningcollisions between atoms and molecules in the gas phase lead to deactivation of the excited state and thus broadening the spectral lines



1111

Doppler BroadeningDoppler Broadening（10-3 nm）Results from the rapid motion of atoms as they emit or absorb radiation



1212

I t = I0ν e -Kν l

The relationship between absorbance and The relationship between absorbance and the concentration of atomsthe concentration of atoms

A = log （ II00νν / / I t）= 0.4343 K l

Beer’s law

I t - intensity of the transmitted light

Io – intensity of the incident light signal

l – the path length through the flame (cm)


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K d=(e2/mc)N0

Integrated absorption


K =the absorption coefficient at the frequency e = the electronic chargem = the mass of an electronc = the velocity of lightf = the oscillator strength of the absorbing lineN0 = the number of metal atoms per milliliter able to

absorb the radiation


1414

The measurement of the integrated absorption coefficient should furnish an ideal method of quantitative analysis


K d=(e2/mc)N0


1515

The line width of an atomic spectral line is about 0.002 nm.


To measure the absorption coefficient of a line would require a spectrometer with a resolving power of 500 000.

The absolute measurement of the absorption coefficient of an atomic spectral line is extremely difficult.


1616

This difficulty was overcome by


who used a source of sharp emission lines with a much smaller half-width than the absorption line. and the radiation frequency of which is centredon the absorption frequency.

Walsh,


1717


In this way, the absorption coefficient at the centre of the line, K0 , may be measured instead of measuring the integrated absorption.


1818

2

2ln)(2

log0

00

v

vvKKv

ov

D

fNmc

eK

2.2ln20

A = 0.4343 K0 l = K1N0v A = KCA = KC



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InstrumentationInstrumentation

Line source Monochromator Detector

Read-outNebulizer

Schematic diagram of a flame spectrophotomer

Atomization

I nstrumentationI nstrumentation

2020

Resonance line sources

•Provide the sharp emission lines with a much smaller half-width than the absorption line

Emit the specific resonance lines of the atoms in question

• Intensity

•Purity

•Background

•Stability

•Life-time


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Hollow cathode lamp Hollow cathode lamp (HCL)(HCL)

Cathode--- in the form of a cylinder, made of the element being studied in the flame

Anode---tungsten


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A hollow cathode lamp for Aluminum (Al)A hollow cathode lamp for Aluminum (Al)


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SpectrAASpectrAA -- AASAAS

motorizedMirror

HCL


2424


2525

Sample atomization techniquesSample atomization techniques

Flame atomization

Electrothermal atomization

Hydride atomization

Cold-Vapor atomization


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Processes occurring during atomization

Flame atomization


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Nebulizer - burner

A typical premix burnerA typical premix burner

Flame atomization


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Nebuliser - burner

To convert the test solution to gaseous atoms

Nebuliserto produce a mist or aerosol of the test solution

Burner head

The flame path is about 10 –12 cm

Vaporising chamber

Fine mist is mixed with the fuel gas and the carrier gas.

Larger droplets of liquid fall out from the gas stream and discharged to waste.


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Fuel and oxidant

flame

Air – acetylene

Air- propane

Air- hydrogen

Nitrous oxide – acetylene

Auxiliary oxidant

Fuel


3030

Common fuels and oxidants used in flame spectroscopy


3131

Disadvantages of flame atomization

Only 5 – 15 % of the nebulized sample reaches the flame

A minimum sample volume of 0.5 – 1.0 mLis needed to give a reliable reading

Samples which are viscous require dilution with a solvent


3232

Graphite furnace technique

Electrothermal atomization


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Plateau Graphite Plateau Graphite TubeTube

3434


Process

drying ashing atomization


3535


Advantages

Small sample sizes ( as low as 0.5 uL)

Very little or no sample preparation is needed

Sensitivity is enhanced

( 10 -10 –10-13 g , 100- 1000 folds)

Direct analysis of solid samples


3636


Disadvantages

Background absorption effects

Analyte may be lost at the ashing stage

The sample may not be completely atomized

The precision was poor than the flame method

(5% -10% vs 1% )

The analytical range is relatively narrow

(less than two orders of magnitude)I nstrumentationI nstrumentation

3737

Cold vapour technique

Hg2+ + Sn2+ = Hg + Sn (IV)


3838

Hydride generation methods

For arsenic (As), antimony (Te) and selenium (Se)

As (V) AsH3As0

(gas) + H2

NaBH4

(sol)

heat

in flame[H+]


3939


4040

•Diffraction grating

Monochromator


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Detector

•Photomultiplier


4242

Read-out system

•meter

•chart recorder

• digital display


4343

Atomic absorption spectrophotometer


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Interferences

Spectral interferences

Chemical interferences

Physical interferences

I nterferencesI nterferences

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• Spectral overlap

（+, positive analytical error）

Cu 324.754 nm, Eu 324.753 nm

Al 308.215 nm , V 308.211nm,Al 309.27 nm

Avoid the interference by observing the aluminum line at 309.27 nm


4646


•non-absorption line

•molecular absorption （+）

Combustion products (the fuel and oxidant mixture)

Correct by making absorption measurements while a blank is aspirated into the flame


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• light scatter （+）

Metal oxide particles with diameters greater than the wavelength of light

When sample contains organic species or when organic solvents are used to dissolve the sample, incomplete combustion of the organic matrix leaves carbonaceous particles that are capable of scattering light


4848


• light scatter （+）

The interference can be avoided by variation in analytical variables, such as flame temperature and fuel-to –oxidant ratio

Standard addition method

Zeeman background correction


4949


----- Formation of compound of low volatility

Increase in flame temperature

Use of releasing agents (La 3+ )

Separation

Ca 2+ ， PO43- Mg2+, Al3+

Use of protective agents (EDTA)


5050


----- Ionization

Adding an excess of an ionization suppressant (K)


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Physical interferences

•Viscosity

•Density

•Surface tension

•volatility

Matrix matching


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Experimental Preliminaries

Preparation of sample solutions

Optimization of the operating conditions

• resonance line

•slit width

•current of HCL•atomization condition

Calibration curve procedure

Experimental P reliminariesExperimental P reliminaries

5353

The standard addition technique


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Sensitivity and detection limit

Sensitivity

• the concentration of an aqueous solution of the elements which absorbs 1% of the incident resonance radiation

• the concentration which gives an absorbance of 0.0044


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Detection limit

Sensitivity and detection limit

• the lowest concentration of an analytethat can be distinguished with reasonable confidence from a field blank

D = c × 3σ / A

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Sensitivity and detection limit (ng/ mL)

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Advantages and disadvantages

High sensitivity

[10-10g (flame), 10-14g (non-flame)]Good accuracy

(Relative error 0.1 ~ 0.5 % )

High selectivity

Widely used

A resonance line source is required for each element to be determined

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Atomic absorption spectroscopy

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