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Prepared by Lawrence Kok

Tutorial on Atomic Absorption Spectroscopy and Beer Lambert Law

http://www.astrophys-assist.com/educate/orion/orion02.htm

Continuous Spectrum :Light spectrum with all wavelength/frequency

Emission Line Spectrum :• Spectrum with discrete wavelength/ frequency • Emitted when excited electrons drop from higher to

lower energy level Absorption Line Spectrum :• Spectrum with discrete wavelength/frequency • Absorbed when ground state electrons are excited

Continuous Spectrum Vs Line Spectrum

http://www.astrophys-assist.com/educate/orion/orion02.htm

Continuous Spectrum :Light spectrum with all wavelength/frequency

Emission Line Spectrum :• Spectrum with discrete wavelength/ frequency • Emitted when excited electrons drop from higher to

lower energy level Absorption Line Spectrum :• Spectrum with discrete wavelength/frequency • Absorbed when ground state electrons are excited

Atomic Emission Spectroscopy

Electrons from excited state

Emit radiation when drop to ground state

Radiation emitted

Emission Spectrum

Continuous Spectrum Vs Line Spectrum

http://www.astrophys-assist.com/educate/orion/orion02.htm

Continuous Spectrum :Light spectrum with all wavelength/frequency

Emission Line Spectrum :• Spectrum with discrete wavelength/ frequency • Emitted when excited electrons drop from higher to

lower energy level Absorption Line Spectrum :• Spectrum with discrete wavelength/frequency • Absorbed when ground state electrons are excited

Atomic Emission Spectroscopy Vs Atomic Absorption Spectroscopy

Electrons from excited state

Emit radiation when drop to ground state

Radiation emitted

Emission Spectrum

Electrons from ground state

Absorb radiationto excited state

Electrons in excited state

Radiation absorbed

Continuous Spectrum Vs Line Spectrum

http://www.astrophys-assist.com/educate/orion/orion02.htm

Continuous Spectrum :Light spectrum with all wavelength/frequency

Emission Line Spectrum :• Spectrum with discrete wavelength/ frequency • Emitted when excited electrons drop from higher to

lower energy level Absorption Line Spectrum :• Spectrum with discrete wavelength/frequency • Absorbed when ground state electrons are excited

Atomic Emission Spectroscopy Vs Atomic Absorption Spectroscopy

Ground state

Excited stateElectrons from excited state

Emit radiation when drop to ground state

Radiation emitted

Emission Spectrum

Electrons from ground state

Absorb radiationto excited state

Electrons in excited state

Radiation absorbed

Continuous Spectrum Vs Line Spectrum

Line Emission Spectra for Hydrogen

Energy supplied to atoms • Electrons are excited from ground to excited states• Electrons exist in fixed energy level (quantum)• Electrons drop from higher to lower, emitting energy of particular wavelength/frequency• Higher the energy level, smaller the difference in energy between successive energy level.• Spectrum will converge(get closer) with increasing frequency• Lines in spectrum converge- energy levels also converge• Ionisation energy can be determined (Limit of convergence)

N = 3-2, 656nm

N= 4-2486nm

N= 5-2434nm

N= 6-2410nm

Atomic Emission Spectroscopy

Visible region- Balmer Series

UV regionLyman Series n=∞ → n= 1

Visible regionBalmer Series n=∞ → n= 2

IR regionPaschen Series n=∞ → n= 3

Atomic Emission Spectra• Energy supplied• Electrons surround nucleus in allowed energy states (quantum)• Excited electron returning to lower energy level, photon of light with discrete energy/wavelength(colour) will be given out.• Light pass through a spectroscope, with a prism to separate out different colours• Line emission spectra is produced.

Line emission spectra for different elements can be found here

Atomic Emission Spectroscopy

Principle of Atomic Absorption Spectroscopy:• AAS spectroscopy uses absorption of light to measure the concentration of gas-phase atoms. • Sample in liquids/solids must be vaporized in a flame/graphite furnace to gaseous atomic vapour. • Atoms absorb UV / visible light causing electronic transitions to higher energy levels. • Analyte concentration determined from the amount of absorption.• Amount of light absorbed = (Ratio of light intensity transmitted/incident light) • Amount of light absorbed = directly proportional to concentration of analytes/ions/atoms.

Electrons in excited state

Electrons from ground state

Absorb radiationto excited state

Radiation absorbed

http://csep10.phys.utk.edu/astr162/lect/light/absorption.html

1) Detection of metal ions in small quantities (ppm/ppb)2) Detection of Al, Ca, Cr, Fe in various sample• Al - Blood serum• Ca – Blood serum• Cu – Copper based alloys• Cr – Sea water• Fe - Plants

Uses of Atomic Absorption Spectroscopy (AAS)

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Parts of Atomic Absorption Spectroscopy

Fuel• Burning mixture of ethyne with O2

(combustion mix)• Sample dehydrated/solvent removed

Atomiser• Atomise sample to atomic

state • Atom vapour (gas phase)

produced

Monochromator• Select light with specific frequency/wavelength of interest• Provide the specific light which will be absorb by metal

Detector• Measure the decrease in intensity of light absorb by analyte• Convert the decrease in light intensity into electrical signal using photomultiplier

Hollow Cathode Lamp• Emits light of specific wavelength to be absorbed by element

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Hollow Cathode Copper Lamp• Emits light of specific wavelength to be absorbed by copper element

How AAS works to measure the Copper concentration in a sample ?

1

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Hollow Cathode Copper Lamp• Emits light of specific wavelength to be absorbed by copper element

How AAS works to measure the Copper concentration in a sample ?

Hollow cathode lamp (Cu) • Made of element (Cu) and use to determine Cu conc in sample• Light source emits light of specific wavelength/frequency which will be absorb by

the sample/element

• Light source of 325nm used, Cu atoms in sample will absorb that particular wavelength to be excited.

Light , λ = 325nm emitted

Atomic Cu vapour absorb light, λ = 325nm

1

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How AAS works to measure the Copper concentration in a sample ?

Fuel/ Atomizer• Fuel – Air/acetylene flame, Temp of 2300°C• Desolvation (drying) – Solvent evaporated and dry vapour sample produced• Atomization – Sample molecules dissociate into free atoms /ions and converted finally to atomic vapour• Copper atoms vapour state produced .

Light , λ = 325nm emitted

Atomic Cu vapour absorb light, λ = 325nm

2

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How AAS works to measure the Copper concentration in a sample ?

Detector• Atoms absorb UV/visible light, exciting its electrons (electronic transition) to

higher energy levels. • Measure the decrease in light intensity (emitted by Cu lamp) after being absorbed

by copper atoms• Amount light absorbed is directly proportional to the concentration of the atoms.

3

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How AAS works to measure the Copper concentration in a sample ?

Light , λ = 325nm emitted

Atomic Cu vapour absorb light, λ = 325nm Decease in Light Intensity due to Absorption

Detector• Atoms absorb UV/visible light, exciting its electrons (electronic transition) to

higher energy levels. • Measure the decrease in light intensity (emitted by Cu lamp) after being absorbed

by copper atoms• Amount light absorbed is directly proportional to the concentration of the atoms.

3

Transmitted Light Intensity

Initial Light Intensity, Io

Beer’s Lambert Law• Apply for diluted solution• Absorbance, A α Concentration• Absorbance, A = log10 (Io/I) = έlc• Transmittance, T = Fraction of incident radiation (I) transmitted by the solution, T= I/Io

Beer’s- Lambert Law and Standard Calibration Curve

Beer’s Lambert Law• Apply for diluted solution• Absorbance, A α Concentration• Absorbance, A = log10 (Io/I) = έlc• Transmittance, T = Fraction of incident radiation (I) transmitted by the solution, T= I/Io

Beer’s- Lambert Law and Standard Calibration Curve

How Absorbance equation is derived ?

Abs = έlc

Beer’s Lambert Law• Apply for diluted solution• Absorbance, A α Concentration• Absorbance, A = log10 (Io/I) = έlc• Transmittance, T = Fraction of incident radiation (I) transmitted by the solution, T= I/Io

Beer’s- Lambert Law and Standard Calibration Curve

Amount light absorb depend on • έ = Molar extinction of

compound• c= Concentration• l = path length of absorbing

solution

How Absorbance equation is derived ?

Molar extinction of compound, έ :• Measure the strength of absorption of

compound• Higher έ ↑ = Higher ↑ Absorbance• Compound with high έ = effective at absorbing light even when low conc is used.Path Length, l:• Longer path length ↑ – Higher ↑ AbsorbanceConcentration, c:• Higher conc of analyte – Higher ↑ Absorbance

If έ and l = constant• Abs α Conc analyte• Abs ↑= Conc ↑

Abs α Conc, c

Abs = έlc

Preparing a Standard Calibration Curve for Lead ions concentration

Determine the unknown concentration of Pb2+ using Beer-Lambert Law

1

Preparing a Standard Calibration Curve for Lead ions concentration

Determine the unknown concentration of Pb2+ using Beer-Lambert Law

Plot a Standard calibration curve of Abs vs Conc

Conc

Abs

1

2

Preparing a Standard Calibration Curve for Lead ions concentration

Determine the unknown concentration of Pb2+ using Beer-Lambert Law

Plot a Standard calibration curve of Abs vs Conc

Measure Absorbance of unknown (Pb2+) sample

Conc

Conc

Abs

Abs

Abs = 0.340

Conc = 0.310

1

2

3

Preparing a Standard Calibration Curve for Lead ions concentration

Determine the unknown concentration of Pb2+ using Beer-Lambert Law

Plot a Standard calibration curve of Abs vs Conc

Measure Absorbance of unknown (Pb2+) sample

Determine Conc of unknown (Pb2+) by interpolation

Conc

Conc

Abs

Abs

Abs = 0.340

Conc = 0.310

1

2

3 4

Amount of light absorb α Conc of Pb vapour in flameAbsorbance α Conc of Pb atomsAbsorbance, A = έlc έ = molar absorptivity

l = path length constantc = conc of sample

Since έ and l are constantA = c ( Absorbance directly proportional to Conc)

Determine the concentration of unknown using Beer-Lambert Law

7.25 x 10-5M solution of X has a transmittance of 44.1% when measured in a 2.10cm cell at wavelength of 525nm. Calculate the a) Absorbance, Ab) Molar absorptivity, έ

100dm3 of contaminated water was reduced by boiling to 7.50dm3. The reduced volume was tested, it had absorbance of 0.55. Calculate the conc of Pb2+ ions (mgdm3) in original sample.

Question, Q1

Question, Q2

a) Absorbance = -log10

T or Absorbance = έlc b) Molar absorptivity,

έ Abs = - log

10 T έ = A/bc =

0.3554 / ((2.10 x 7.25 x 10-5) = - log10 0.441 = 2.33 x 103 L mol-1cm-1

= -(-0.3554) = 0.355

Determine the concentration of unknown using Beer-Lambert Law

7.25 x 10-5M solution of X has a transmittance of 44.1% when measured in a 2.10cm cell at wavelength of 525nm. Calculate the a) Absorbance, Ab) Molar absorptivity, έ

100dm3 of contaminated water was reduced by boiling to 7.50dm3. The reduced volume was tested, it had absorbance of 0.55. Calculate the conc of Pb2+ ions (mgdm3) in original sample.

V= 100dm3

M= ?V= 7.50dm3

M= 1.15mg/dm3

Amount /moles before heating = Amount/moles after heatingMoles before = Moles afterM x V = M x VM x 100 = 1.15 x 7.50M = (1.15 x 7.50)/100M = 8.63 x 10-2 mgdm-3

Question, Q1

Answer to Q1

Question, Q2

Answer to Q2

Conc of Pb2+ from graph – 1.15mgdm-3

Acknowledgements

Thanks to source of pictures and video used in this presentation

Thanks to Creative Commons for excellent contribution on licenseshttp://creativecommons.org/licenses/

Prepared by Lawrence Kok

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