Atomic Absorption

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Atomic Absorption
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Where g degeneracy
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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.

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Atomic Absorption
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Ideally, we want the selected emission line from
the lamp to be narrower than the absorption
spectrum
Monochromator is used to select one of the
emitted lines
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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

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Hollow Cathode Lamp
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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.

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Electrodeless Discharge Lamp
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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

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High-resolution continuum source AASthe better
way to perform atomic absorption spectrometry
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• 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.

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• 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.

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Sample

• There are a variety of different sampling
  methods
• Flame
• Furnace (electrothermal atomizer)
• Arc, spark
• ICP
• Cold vapour atomization
• Hydride generation

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Flame

• Stable
• Safe
• Cheap to maintain
• High temperature
• Reducing Atmosphere - many metals form stable
  oxides, not easily atomized just by flame
  temperatures

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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

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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)

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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

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Ultrasonic breakup of drops

• High frequency vibrations
• Uniform and controllable drop size
• BUT
• Drops are larger and equipment more expensive

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Desolvation

• Critical to number of free atoms
• Usually occurs at base of flame
• Solvent then water of crystallization
• Depends on droplet size and solvent

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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

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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

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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

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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

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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

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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.

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Air-C2H2 Yellow Colorless Blue
C2H2-N2O Red (CN., NH.) Whitish-blue
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Flame AtomizerAdvantages

• Convenient
• Rapid
• Suitable for all AA-determinable elements
• Limitations
• Limited Sensitivity
• Large Sample Volume
• Cannot handle some sample types

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Sensitivity Limitation
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Interferences

• Spectral
• Vaporization
• Chemical

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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

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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

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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

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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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