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A brief history of Atomic Absorption Spectrometry 1955 - Alan Walsh

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									   Spectroscop1c Analysis
Part 6 – Spectroscopic Analysis using Fluorescence and
         Atomic Absorption Spectrophotometry

Chulalongkorn University, Bangkok, Thailand January 2012

                     Dr Ron Beckett

        Water Studies Centre & School of Chemistry
         Monash University, Melbourne, Australia


             Email: Ron.Beckett@monash.edu
         Water
         Studies
         Centre
                                                      1
2
                Fluorescence Analysis
Fluorescence                                                            Vibrational
                                                                        relaxation

1.   Excitation to higher              Singlet Excited States
     electronic state by                                                              Triplet Excited State

     absorption
                                                       S                                          T1
                                                        1
2.   Loss of vibrational
     energy as heat

                                    ENERGY
3.   Emission of
     fluorescence EMR                                      Absorption

                                                                     Fluorescence
4.   Results in Stokes shift
     to longer wavelength
     (lower energy).               Ground
                                             S0
                                   state

5.   Lifetime of excited                          l2            l1        l3                 l4

     singlet state 10-5 - 10-8 s                             Stokes Shift

                                                                                                   3
Fluorescence Analysis
                         l
Scan Incidence
EMR            PI       C     P Absorbance
                                Signal

                    F
               Fluorescence
               Signal




                                             4
    Factors Affecting Fluorescence
Molecular Structure
  – Most intense fluorescence from aromatic groups, esp.
    multi-ring compounds or highly conjugated molecules,
    e.g. PAH's (polycyclic aromatic hydrocarbons).




              Phenanthrene       Anthracene



  – Halogen or carboxyl substitution inhibits fluorescence
                                                             5
  Factors Affecting Fluorescence
Relationship between Fluorescence and
 Concentration of an analyte in solution
                   l

          PI        C      P

                F


               F = Kc
                                           6
    Atomic Absorption Spectrometry
   • AAS was invented in 1955 by Sir Alan
   Walsh at CSIRO in Melbourne

   • It is now used extensively around the
   world for elemental analysis of
   environmental, industrial and biological
   samples
                        Sample
Atomic emission         atoms in
produces light of the   gas phase   Detector
right frequency         absorb
                        light
                                               7
 Origin of an Atomic Emission Peak

                                 E2
Excitation to a higher
electronic state by                         DE = hn
                           Excitation
heat, EMR, etc.
                                 E1



Emission Spectrum        Intensity




                                        n      Frequency
                                                           8
Atomic Emission




                  9
Atomic Emission After Excitation in a Flame
Different atoms give rise to characteristic colours that
can be used to identify the elements present




  Ba                             Na



                                                     10
   Atomic Emission Spectra

Mercury (Hg)          Neon (Ne)




                                  11
 Origin of an Atomic Absorption Peak

Energy Transition                        E2
                    DE = hn

                                         E1


Absorption Spectrum
                      Intensity




                                  n   Frequency
                                                  12
  Atomic Absorption for a Hydrogen Atom

Electronic energy levels
in a H atom

1s 2s 2p 3s 3p 3d


The lowest energy state
has the single electron in
the 1s orbital

1s1


                                      13
          A brief history of
   Atomic Absorption Spectrometry
• 1802 Wollastone - discovered black lines in Sun's
  spectrum which were subsequently investigated by
  Fraunhoffer (1823).




                                                 14
                    Solar spectrum
          A brief history of
   Atomic Absorption Spectrometry
– 1820 Brewster - suggested black lines in solar
  spectrum due to absorption processes in Sun's
  atmosphere.
– 1859 Kirchhoff and Bunsen - demonstrated Na D
  line absorption in visible spectra.




                                               15
            A brief history of AAS
Interpretation of Fraunhoffer lines in terms of atomic
absorption by elements in the suns atmosphere

1859-1955 Astronomers use atomic absorption to
estimate metal concentrations in atmospheres of stars.




                      Solar spectrum




                    Hydrogen spectrum




                                                         16
            A brief history of
     Atomic Absorption Spectrometry

• 1955 - Alan Walsh at CSIRO in
  Melbourne proposed atomic
  absorption spectrometry for
  chemical analysis.


• Mid 1960’s - First commercial
  AAS manufactured in
  Melbourne by Varian-Techtron.
                                  Sir Alan Walsh (d 1998)
                                                     17
Atomic Absorption Spectrometer




                                 18
Flame Atomic Absorption Instrumentation




                                      19
           Principle of Flame AAS
• Sample solution is sucked into a spray chamber to produce
an aerosol

• The aerosol is introduced into a flame with the fuel gas

• The solution is evaporated and the elements are atomised but
not excited or ionised

• A lamp containing the element being analysed is used to
produce light of the correct wavelength

• Absorbance of this light by the sample atoms is measured

                                                             20
                  Analysis by AAS
1. Measure the absorbance of a blank solution Abl and a
   series of standard solutions Ax
2. Plot the calibration line (Ax – Abl) vs Cx

3. Measure the absorbance of the unknown solutions

Calculate the concentrations
of the unknown solutions

      A A
    c m
     x
            x     bl




Where m is the gradient of
the calibration line                                      21
                Analysis by AAS
Standard Addition Method
   – Compensates for chemical interferences
   – Cannot eliminate spectral interferences




Concentration                          Sample plus
of Unknown                             Standard Addition

                       Sample

                                                           22
                     Volume of Standard Added (mL)
Use of AAS in Forensic Science




                                 23
Who fired the
murder weapon ?
                  24
      Gunshot residue (GSR)

• Bullets contain “primer” and “propellant”
  which result in GSR on the firing hand

• Swabbing hands with mild acid will
  release barium (Ba) and antimony (Sb)

• Measure by Atomic Absorption
                                         25

								
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