Atomic-absorption (AA) spectroscopy by malj

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									          Atomic-Absorption (AA)

                             School of Civil and
Monroe L. Weber-Shirk   Environmental Engineering

   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
   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
  Light Source: Hollow Cathode
                     Cup made of
Power Supply         metal of interest    window
        +                                    light
 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

 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?

 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.

 Different   temperatures required for different
 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

 ________  Law
 Non linearity (according to Perkin Elmer)
   Stray light Correct for this by subtracting Vdark
   Non homogeneities of temperature and space in
    the absorbing cell
   Line broadening
                                  What is the physics?
   Absorption at nearby lines

 What   else could be causing non linearity?
      Getting a Concentration

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

 Graphite   Furnace
   Measures  an absorbance peak with an area
    proportional to _______ of the sample
   Analysis in the mg/L range (ppb)

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