Atomic-absorption _AA_ spectroscopy

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					Atomic-Absorption (AA)
    Spectroscopy
                           Overview

   Excitation and Decay: Interplay of electron state
    and photons
     Emission
     Absorption

   Instrument Components
     Double beam configuration
     Light source
     Production of free atoms
            Graphite furnace
            Flame
       Detector
 Excitation and Decay: Interplay
  of electron state and photons
 Emission  – decay from high energy state
 Absorption – excitation to high energy state
 Two fundamentally different ways of
  quantifying the concentration of an element!
   Emission   – flame provides energy to excite
    atoms
   Absorption – Gas phase atoms absorb photons
    from light source
 Narrow   band! (unlike molecular absorption)
          Instrument Schematic
          Reference Beam




Lamp             Flame



                         V ref
       Chopper               V
                     V sample
                                 t
  Light Source: Hollow Cathode
              Lamp
                     Cup made of
Power Supply         metal of interest    window
             anode
        +                                    light
        -
             cathode
 The electric potential ionizes rare gas atoms and
  accelerates them into the cathode where they
  sputter metal atoms into the gas phase
 Collisions with gas atoms or electrons excite the
  metal atoms
 On decay the metal atoms emit light
                   Atomization

 Atomic absorption and emission requires gas-
  phase atoms
 Liquid and solid samples must be vaporized
     Flame
     graphite furnace
 The gas-phase atoms absorb ultraviolet or visible
  light and make transitions to higher electronic
  energy levels.
 What about the inevitable subsequent emission of
  photons on decay?
                 Detector

 Photo  multiplier tube
 Extremely sensitive
 Can detect single photons
 Produce a small current that is proportional
  to the number of photons
 Op-amps produce a voltage that is then
  digitized for analysis.
                     Flame

 Different   temperatures required for different
  elements
 Air-Acetylene flame
            flame for 35 elements
   Preferred
   Temperature of 2300 C
 Nitrous   Oxide-Acetylene flame
   Temperature   of 2900 C
 Optimal  temperature produces free gas
  phase atoms but doesn’t ________ them
                           ionize
               Calibration

   Beer’s
 ________  Law
 Non linearity (according to Perkin Elmer)
   Stray light
   Non homogeneities of temperature and space in
    the absorbing cell
   Line broadening
   Absorption at nearby lines
      Getting a Concentration

 Flame
   Measures  something proportional to the
    ______________ of the sample
    concentration
   Analysis in the mg/L range (ppm)

 Graphite   Furnace
   Measures  a _______
                 mass
   Analysis in the mg/L range (ppb)

				
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posted:2/13/2012
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