Atomic Absorption - PowerPoint by O8Ne5w

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									 *    *       E
N    g      
       e     kT
                   Where g = degeneracy
N0   g0
    Lines are broadened by two
              effects:
• Doppler
• Collisional
• Operating conditions for lamp are chosen
  so that the Doppler broadening in the lamp
  (low P, few collisions) is less than the
  Doppler and collisional broadening in the
  flame or furnace.
Atomic Absorption
Monochromator is used to
select one of the emitted
lines



    Ideally, we want
      the selected
   emission line from
     the lamp to be
   narrower than the
       absorption
        spectrum
   Absorption lines are narrow
• The selected bandwidth of light from
  source must be narrower than chosen
  absorption line
• If a monochromator was able to select a
  narrow enough bandwidth from the output
  of a deuterium lamp, the power of the light
  would be negligible
• Therefore lamps that emit spectral lines
  are used
Hollow Cathode Lamp
                   HCL
• Inert gas is ionized by discharge
• Is accelerated to cathode
• Causes some element to dislodge and
  form atomic cloud (sputtering)
• Some are excited (in collisions with ions)
  and emit line spectra.
• Usually lamps are for one element – but
  can be for as many as six.
Electrodeless Discharge Lamp
 Electrodeless Discharge Lamp
• Microwave excited discharge tubes
• Intensities 10-100 x greater than from HCL
• Small amount of element or halide of an element
  in a small sealed tube containing a few torr of
  inert gas
• Placed in microwave cavity (2450 MHz)
• Argon is ionized, the ions are accelerated and
  excite the metal atoms
• Less stable than HCL, but more intense.
• Not available for all elements
  High-resolution continuum source AAS:
the better way to perform atomic absorption
                spectrometry
• Single xenon arc lamp

• Today, multiple hollow cathode lamps are
  no longer used.
• With the use of a single xenon arc lamp,
  all the elements can be measured from
  185-900 nm.
• This takes AAS into a true multi-element
  technique with the analysis of 10 elements
  per minute.
• CCD technology - For the first time in an AAS
  CCD chips are now available with 200 pixels
  which act as independent detectors.

• Simultaneous background correction -
  Background is now measured simultaneously
  compared to sequential background on
  conventional AAS.

• Better detection limits - Due to the high
  intensity of the Xenon Lamp there is better
  signal/noise ratio thus giving better detection
  limits. In some cases it is up to 10 times better
  than conventional AAS.
                 Sample
• There are a variety of different sampling
  methods:
• Flame
• Furnace (electrothermal atomizer)
• Arc, spark
• ICP
• Cold vapour atomization
• Hydride generation
                  Flame
•   Stable
•   Safe
•   Cheap to maintain
•   High temperature
•   Reducing Atmosphere - many metals
    form stable oxides, not easily atomized
    just by flame temperatures
                  Flame
• Typical system – spray chamber and burner
• Sample is aspirated into spray chamber
  using nebulizer (sucked in by Venturi effect)
• Produces aerosol.
• Aerosol strikes obstruction – spoiler – to
  break it into smaller drops
• Only smallest drops proceed to flame
• Larger drops go down drain
  Sequence of Events in Flame
• Evaporation of Solvent (leaving fine salt
  particles suspended in flame)
• Loss of water of hydration
• Vaporization of solid particles to free
  atoms (due to heat and chemical reaction)
• Excitation
• Ionization (not always desirable)
               Nebulization
• Controls fraction of sample to reach flame
• Drop size is governed by viscosity, surface
  tension, gas flow, density, design of nebulizer
• Organic solvents have lower viscosity and lower
  surface tension than water (0.25 - 0.3 x) They
  also allow preconcentration
• But change flame conditions – not always so
  beneficial
• Salt increases viscosity, decreasing efficiency
• The smaller the drop, the more easily it is
  desolvated and vaporized
     Ultrasonic breakup of drops
•   High frequency vibrations
•   Uniform and controllable drop size
•   BUT
•   Drops are larger and equipment more
    expensive
                 Desolvation
•   Critical to number of free atoms
•   Usually occurs at base of flame
•   Solvent then water of crystallization
•   Depends on droplet size and solvent
               Vaporization
• Atomization to free atoms
• Ideally – want high temperature and long
  residence time (slow burn rate of the gases)
  - lots of time for atomization
• Depends on nature of molecules and atoms
• Al2O3 atomizes more slowly than NaCl
  particle of the same size
• Important if analyzing mixtures – different
  conditions are needed for different atoms
                Ionization
• Ions undergo different transitions than
  atoms
• Want one or the other
• Ions not desirable in flame method
• In ICP, ions are the desired species
• For alkali and alkaline earths, ions form
  above 2000K
                  Ionization
•   Increases
•    at low sample concentration
•   With increasing flame temperature
•   With decreasing ionization potential
•   Prevent by:
•   Low flame temperature
•   Excess of easily ionizable metal eg Li
•   Called a SUPPRESSOR
•   Eg: add lots of Li to solution to be
    analyzed for K
    Premix (Laminar flow) Burner
• Most common
• Gases premixed before entering burner
• Stable flame
• Use long narrow flames – long path length
  for light absorption
• Use at right angles for emission
• Small (narrow) flame keeps atom
  concentration high
Width of slot depends on gases
• Narrow slots prevent flame backing into
  mixing chamber and causing explosion
• But must allow enough gas through to
  support rate of burning
• Too narrow:
• Cooling by adjacent air
• Salt deposition clogs burner
• Use different burners for different gases
                 Gases
• Gas mixtures with high-burning velocities
  are less safe
• Also want long residence times
• C2H2-N2O (220cm/s) is better than C2H2-
  O2 (1130 cm/s). They have similar flame
  temperatures.
Air-C2H2    C2H2-N2O

Yellow

Colorless   Red (CN., NH.)

Blue        Whitish-blue
            Flame Atomizer
              Advantages
• Convenient
• Rapid
• Suitable for all AA-determinable elements
                   Limitations
• Limited Sensitivity
• Large Sample Volume
• Cannot handle some sample types
Sensitivity Limitation
             Interferences
• Spectral

• Vaporization

• Chemical
                Spectral
• Mg 285.21 nm
• Na     285.28 nm
• Not usually much of a problem – can
  change to another wavelength
• Problem worse in emission because more
  lines – High T – lots of excitation
• Choice of line dictates concentration range
   able to be analyzed
    Vaporization Interferences
• When one component of a sample
  influences the rate of vaporization of the
  species of interest
• Physical – changes matrix it vaporizes
  from
• Chemical – changes the species to be
  vaporized
        Chemical Vaporization
           Interferences
• Metal oxides form
• Metal ions form thermally stable
  complexes with anions
• The effects usually occur during formation
  of the solid particle
• CaPO4 formation – a well known example.
• CaPO4 is harder to vaporize than Ca2+
  CaPO4 - Interference Prevention
• Put light path higher in flame to allow a longer
  residence time
• Add releasing agent – La2+ or Sr2+ (added in
  excess) will preferentially combine with PO43-
  and leave Ca2+ free to be analyzed
• Protective agent – add EDTA. Ca-EDTA
  complex is easily destroyed in flame
• Glucose – burns easily and helps droplets
  shatter apart
• Hotter flame – then need ionization suppressor

								
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