Atomic Fluorescence Spectroscopy

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Atomic Fluorescence Spectroscopy
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Background

• First significant research by Wineforder and
  Vickers in 1964 as an analytical technique
• Used for element analysis
• Example Trace elements in ground water
• Has not found wide spread success because there
  does not seem to be a distinct advantage over
  established methods, i.e. atomic absorbance

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What is Atomic Fluorescence?

• Atomic fluorescence spectroscopy (AFS) is the
  optical emission from gas-phase atoms that have
  been excited to higher energy levels by
  absorption of radiation.
• AFS is useful to study the electronic structure
  of atoms and to make quantitative measurements of
  sample concentrations.

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Why use AFS?

• it is a quantitative technique for determination
  of a large number of elements
• used mostly in analysis of metals in biological
  samples, agricultural samples, water, and
  industrial oils

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Jablonski Diagram
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Instrumentation

• Virginia Tech website maintained by Professor
  Brian Tissue Department of Chemistry

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General process for AFS

• nebulization - converts the sample solution into
  a mist made up of tiny liquid droplets
• atomization - flow of gas carries sample into
  heated region where sample molecules are broken
  into free atoms
• desolvation - the solvent is evaporated to
  produce a solid molecular aerosal
• dissociation - molecules dissociate to produce an
  atomic gas
• The atoms dissociate to produce ions and electrons

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General Process continued

• Excitation due to light source
• Fluorescence of sample
• This fluorescence can be selected for certain
  wavelengths by a monochromator
• Then the detector reads the emission and
  amplifies the signal
• Then the readout device relays the data

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Problems with Technique

• Precision and accuracy are highly dependent on
  the atomization step
• Light source
• molecules, atoms, and ions are all in heated
  medium thus producing three different atomic
  emission spectra

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Problems continued

• Line broadening occurs due to the uncertainty
  principle
• limit to measurement of exact lifetime and
  frequency, or exact position and momentum
• Temperature
• increases the efficiency and the total number of
  atoms in the vapor
• but also increases line broadening since the
  atomic particles move faster.
• increases the total amount of ions in the gas and
  thus changes the concentration of the unionized
  atom

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Interferences

• If the matrix emission overlaps or lies too close
  to the emission of the sample, problems occur
  (decrease in resolution)
• This type of matrix effect is rare in hollow
  cathode sources since the intensity is so low
• Oxides exhibit broad band absorptions and can
  scatter radiation thus interfering with signal
  detection
• If the sample contains organic solvents,
  scattering occurs due to the carbonaceous
  particles left from the organic matrix

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Interferences continued
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Detection Limits

• Are similar to those for Atomic Absorption and
  Atomic Emission
• Varies for different elements

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Final conclusion

• This technique offers some advantages for some
  elements while other atomic spectroscopy
  techniques may be better for other elements
• Future work on light sources and atomizers will
  increase the analytical uses of this technique