Docstoc

AA and Atomic Fluorescence Spectroscopy Chapter 9

Document Sample
AA and Atomic Fluorescence Spectroscopy Chapter 9 Powered By Docstoc
					      AA and Atomic Fluorescence Spectroscopy
                    Chapter 9
•   Sample Atomization
•   Atomic Absorption Instrumentation
•   Interference
•   Atomic Absorption Techniques
•   Atomic Fluorescence

• Sample Atomization
    For techniques samples need to be atomized
    Techniques are useful for element identification
         Molecular information destroyed by atomization
• Flame Atomization
    Sample nebulized
    Mixed with fuel
    Carried to flame for atomization                     5-1
Technique




            5-2
                       Flame Atomization
           • Evaporation of solvent
               Produces molecular aerosol
           • Molecules dissolution leads to atomic gas
           • Atoms ionize to product cations and electrons
           • Property of flame can affect process
Fuel Gas                        Oxygen ºC          Air ºC

Methane                          2810              1957
Ethane                                             1960
Propane                          2820              1980
Butane                                             1970
Hydrogen                         2660              2045
Acetylene                        3100              2400

                                                            5-3
              Flame ionization
• Flame temperature in range of 1700 °C to 2400
  °C in air
    From 2500 °C to 3100 °C with oxidant
    Need to keep flame stable
• Flame structure
    Different zones are properties of fuel and
      oxidant
         Primary combustion zone
          * Blue luminescence due to C2 and CH
          * Thermal equilibrium not reached in
            primary zone
                                             5-4
             Flame ionization
• Interzonal region
    Central part of flame
    High concentration of free atoms
         Used for spectroscopy
• Secondary combustion region
    Convert elements to oxides
    Disperse sample to air




                                        5-5
              Flame Structure
 secondary
                                interzonal




                                Primary
                                zone


Maximum
temperature


                                      5-6
         Best location for absorbance?
• Variation due to the degree of oxidation for a given element
      Mg
            Atomizes then oxidizes as Mg approaches secondary
            combustion area
             * Formation of MgO reduces absorbance
      Ag
            Does not readily oxidize
            Atomization over flame area
      Cr
            Forms oxidizes readily so that oxide is main species in
            flame
• Need to consider based on flame sample area
      Does instrument sample entire flame or just small area?



                                                                  5-7
Absorbance Profile




                     5-8
        Electrothermal Atomization
• Atomization of entire sample in short period
• Average sample time in optical path is seconds
    Evaporation of sample
          Microliter volume
          Low temperature
    Sample ashed at higher temperature
    Increase current
          Sample temperature goes to 2000-3000 °C
    Sample measured above heated surface
• High sensitivity for small samples


                                                    5-9
Electrothermal atomizer




                 Sample concentration




                                5-10
Graphite Furnace




                   5-11
                Atomization Techniques
• Glow Discharge
    Sputtering of
      samples due to Ar
      ion acceleration
    Mixture of atoms
      and ions
• Hydride generator
    Forms volatile
      species
          As, Sb, Sn, Se,
          Bi, Pb
• Cold Vapor (Hg)


                                         5-12
    Atomic Absorption Instrumentation
•   Radiation Source
•   Sample Holder
•   Wavelength selector
•   Detector

• Radiation sources
    AA has narrow lines (0.005 nm)
    Most light sources provide light with greater
      bandwidths
          Absorption of source light
    Need narrow source

                                                     5-13
  Atomic Absorption Instrumentation
• Light source
    Use source for element detection
          For Na, use Na vapor lamp
           * 3p to 2s transition at 589.6 nm
    Minimize line broadening
          Doppler
          Pressure
          Temperature
    Need a separate light source for each element



                                                     5-14
  Atomic Absorption Instrumentation
• Hollow Cathode Lamp
   Ionization of inert
      gas by potential
   Gas acceleration to
      cathode
   Atoms on cathode
      into gas state
          Some excited
          Deexcite with
          photon emission
   Need to excite
      specific elements for
      measurement
                                 5-15
    Atomic Absorption Instrumentation
• Electrodeless Discharge
  Lamps
    Inert gas in quartz
       tube
    Excite gas with RF

      Similar to cathode
       expect excitation




                                   5-16
             Spectrophotometers
• Single Beam
    Shutter controls
       beam
    Collect blank


         Blank provides
         100%
         transmission
         Insert sample
         and measure
         absorbance


                                  5-17
                 Spectrophotometers
• Double Beam
    Light source split
    Measure light through
     flame and light reference
     light




      Determine %T


          Does not consider
          light absorption in
          flame
                                      5-18
                    Interference
• Spectral interference
    Overlap of sample spectra
          Not very common due to narrow line widths
           * If occurs select different transition
    Scattering
          Formation of oxides
    Correct with different methods
          Two line method
          Continuum source
          Zeeman effect
           * Polarize and split light with magnetic field
                                                        5-19
                 Interference
• Chemical Interference
   More common than spectral interference
   Formation of compounds with low
     volatility
        Additives to remove such compounds
         * EDTA
   Dissociation equilibria
        Reaction of oxide species
   Ionization equilibria
        Formation of ion species, liberation of
        electron
                                              5-20
Interference




               5-21
Detection Limits




                   5-22
   Atomic Fluorescence Spectroscopy
• optical emission from gas-phase atoms that have been
  excited to higher energy levels
    Enhancement of sensitivity over AA
    Examine electronic structure of atoms
• Light source
    Hollow Cathode Lamp
    Laser
• Detection
    Similar to AA



                                                    5-23

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:8
posted:2/12/2012
language:Latin
pages:23