Spectroscopy

• The wavelength range is the distance between two consecutive peaks.
• Atomic absorption spectroscopy (AAS) and atomic emission spectroscopy (AES) principle are based on the absorption and emission of light by atoms in the gaseous state
• Atomic absorption spectrometry (AAS)


  A technique used for determining the concentration of a particular metal element within a sample
• AAS can be used to analyze the concentration of over 62 different metals in a solution
• AAS analysis


  1. Aspiration of a solution of metallic salt into a flame

  2. Absorption of radiation by atoms
• AAS method


  • Similar to spectrophotometer, with the sample cell replaced by a flame
  • Uses a monochromatic light produced by a hollow cathode lamp
• AAS process


  1. Dissociation of molecules by heat energy

  2. Conversion of components to atoms

  3. Absorption of resonance lines by ground state atoms

  4. Measurement of transmitted light that is not absorbed
• Components of AAS


  • Radiation source
  • Atomizer
  • Flame/Burner
  • Nebulizer
  • Monochromator
  • Detector
  • Amplifier
  • Recorder
• Radiation source


  Source of light with varying wavelength range, e.g. hollow cathode lamp, deuterium lamp, xenon lamp
• Atomizer


  Converts the analyte molecules to gaseous atoms, e.g. flame atomizer, electrothermal atomizer
• Flame/Burner system


  Air and fuel combine to produce a flame that destroys analyte ions and breaks down complexes, creating atoms of the element of interest
• Nebulizer and mixing chamber

  Sucks up liquid sample at a controlled rate, creates a fine aerosol for introduction into the flame, and mixes the aerosol and fuel and oxidant thoroughly
• Monochromator


  Selectively provides radiation of desired wavelength, isolating the analytical line from other lines
• Detectors (Photomultiplier Tubes)


  Determines the intensity of photons of the analytical line exiting the monochromator
• Amplifier


  Amplifies the signal emitted by the detector
• Recorder


  Receives electrical signals from the detector and converts them into a readable response
• Analytical procedure for AAS

  1. Sample preparation

  2. Hollow cathode lamp warming up

  3. Monochromator wavelength selection and slit width adjustment

  4. Blank solution aspiration and 100% transmission adjustment

  5. Standards and samples aspiration and readings

  6. Calibration curve plotting and concentration calculation
• AAS can be used for qualitative and quantitative analysis
• Applications of AAS


  • Food analysis
  • Water and wastewater analysis
  • Analysis of animal feedstuffs
  • Clinical analysis
  • Analysis of metals in soils, sediments and rocks
  • Analysis of metals in industrial products and biological fluids
• Advantages of AAS


  • High sample throughput
  • Easy to use
  • High sensitivity
  • High accuracy
  • Inexpensive
  • Wide applications
• Disadvantages of AAS


  • Lack of versatility (only solutions can be analyzed)
  • Cannot detect non-metals
  • Relatively large sample quantities required
  • Sample is destroyed
• Flame emission spectrometry (FES)


  Also known as Flame photometry, used to determine the concentration of certain metal ions (Na, K, Ca and Li) by measuring the intensity of the light emitted when a metal is introduced into a flame
• Basic principles of FES

  1. Liquid sample containing metal salt solution is introduced into flame

  2. Solvent is vaporized, leaving solid salt particles which are vaporized into gaseous state

  3. Gaseous molecules dissociate to give neutral atoms which are excited by thermal energy of flame

  4. Excited atoms emit photons of characteristic wavelength while returning to lower energy state

  5. Measurement of emitted photons forms the basis of flame photometry
• Major components of FES instrumentation

  • Burner
  • Nebulizer and mixing chamber
  • Monochromator/filter
  • Detector
  • Meter/read-out device
• Burner


  Air and fuel combine to produce the flame
• Nebulizer and mixing chamber

  Nebulizer turns the liquid sample into a fine mist for introduction into the flame
• Monochromator/Filter


  Allows only the chosen wavelength and absorbs all other wavelengths
• Detector


  Detects the intensity of the emitted light and generates a current proportional to it
• Meter or read-out device


  Displays the intensity of the emitted light, which is proportional to the concentration of the element
• Interferences in FES


  • Spectral interferences (overlapping emission lines)
  • Ionic interferences (ionization of atoms)
  • Chemical interferences (reactions between interferents and analyte)
• Advantages of FES


  • Simple quantitative analytical test
  • Inexpensive
  • Easy, reliable and convenient for alkali and alkaline earth metals
  • Selective and sensitive
• Disadvantages of FES


  • Inaccurate concentration measurement
  • Requires standard solutions
  • Less accuracy at high concentrations
  • Cannot determine molecular structure
  • Cannot detect C, H and halides
• Applications of FES


  • Analysis of alkali and alkaline earth metals in water, glass, cement, body fluids, metallurgical products, agricultural samples, and food and beverages