Spectroscopy

• Spectroscopy


  Uses absorption, emission, or scattering of electromagnetic radiation by matter to qualitatively or quantitatively study matter or to study physical processes
• The matter can be atoms, molecules, atomic or molecular ions, or solids
• The interaction of radiation with matter can cause redirection of the radiation and/or transition between the energy levels of the atoms or molecules
• Electromagnetic Radiation


  • Form of energy that is transmitted through space at enormous velocities
  • Has an electric and magnetic field components which oscillate perpendicular to each other and in the direction of energy propagation
• Types of electromagnetic radiation


  • Gamma rays
  • X-rays
  • UV light
  • Visible
  • Infrared
  • Radio
• Wavelength


  Distance between wave crests
• Frequency


  Number of crests that pass a given point in a second
• Higher the frequency


  • Shorter the wavelength
  • Higher the energy
• EMR Regions Used in Analyses


  • NMR
  • ESR
  • Microwave
  • Infrared
  • Visible and ultraviolet
  • X-ray
  • Gamma-ray
• Absorption spectroscopy


  Transition from lower level to a higher level with transfer of energy from the radiation field to an absorber, atom, molecule, or solid
• Emission spectroscopy


  Transition from a higher level to a lower level with transfer of energy from the emitter to the radiation field
• Scattering spectroscopy


  Redirection of light due to its interaction with matter
• Spectroscopists use the interactions of radiation with matter to obtain information about a sample
• Emission or chemiluminescence processes involve the sample being excited by the application of thermal, electrical, or chemical energy
• Absorption methods involve radiation of incident radiant power being absorbed by the analyte, resulting in a transmitted beam of lower radiant power
• Photoluminescence methods (fluorescence and phosphorescence) result from absorption of electromagnetic radiation and then dissipation of the energy by emission of radiation
• Spectrometer


  Spectroscopic instrument that employs a monochromator or polychromator in conjunction with a transducer to convert the radiant intensities into electrical signals
• Spectrophotometer


  Spectrometer that allows measurement of the ratio of the radiant powers of two beams, a requirement to measure absorbance
• Photometer


  Employs a filter for wavelength selection in conjunction with a suitable radiation transducer
• Spectrophotometry is the quantitative measurement of the reflection or transmission properties of a material as a function of wavelength
• Spectrophotometry deals with visible, near-ultraviolet, and near-infrared light
• The amount of light absorbed is proportional to the concentration of the substance, as described by Beer's Law
• Transmittance


  Ratio of the transmitted radiant power to the incident radiant power
• Absorbance


  Logarithm of the reciprocal of the transmittance
• Beer-Lambert law tells us quantitatively how the amount of attenuation depends on the concentration of the absorbing molecules and the pathlength over which absorption occurs
• Continuum Sources for Optical Spectroscopy


  • Xenon arc lamp
  • H2 and D2 lamps
  • Tungsten/halogen lamp
  • Tungsten lamp
  • Nernst glower
  • Nichrome wire
  • Globar
• Entrance Slit


  • Minimizes unwanted light or stray light and prevents the entrance of scattered light into the monochromator system
• Monochromator


  Isolates specific or individual wavelength of light
• Monochromators generally employ a diffraction grating to disperse the radiation into its component wavelengths
• By rotating the grating, different wavelengths can be made to pass through an exit slit
• Spectral bandpass or effective bandwidth


  Wavelength range passed by a monochromator
• Polychromator contains multiple exit slits and multiple detectors that allow many discrete wavelengths to be measured simultaneously
• Exit Slit


  • Controls the width of the wavelength band that reaches the detector
• Monochromator


  • Employs a diffraction grating to disperse the radiation into its component wavelengths
• Selecting the desired wavelength


  1. Rotating the grating

  2. Different wavelengths can be made to pass through an exit slit
• Spectral bandpass


  The wavelength range passed by a monochromator
• Polychromator


  Contains multiple exit slits and multiple detectors that allow many discrete wavelengths to be measured simultaneously
• Exit slit


  Controls the width of light beam (band pass)
• Sample container


  Holds the solution whose concentration is to be measured
• Transmittance


  The power of the beam transmitted through a cell containing the analyte solution compared to one that traverses an identical cell containing only the solvent or a reagent blank