Spectroscopic mehtod

SPECTROSCOPIC METHOD

Atomic Absorption Spectrometry

atomic-absorption-spectroscopy

  • The absorption of electromagnetic radiation by atoms allows both qualitative and quantitative determination of a wide range of elements. Alternatively, fluorescence radiation may be emitted and measured. Narrow band sources of radiation specific to particular elements irradiate the atomic vapor produced by flame, furnace or other methods and the absorption is measured. For fluorescence, both continuum and element specific sources are used.
  • Atomic absorption spectrometry is used widely for the quantitative determination of metals at trace levels. Atomic fluorescence spectrometry is usually limited to mercury and other volatile species.

Ultra Violet and visible spectrum Spectrophotometry

UV visible spectrum spectrophotometry

  • Absorption in the ultraviolet and visible regions of the electromagnetic spectrum corresponds to transitions between electronic energy levels and provides useful analytical information for both inorganic and organic samples.
  • Molecules may possess several excited states. After excitation by absorption of radiation, rapid transitions can occur to lower energy excited states, which then revert to the ground state, emitting electromagnetic radiation at a lower energy and by slower processes referred to as photoluminescence.
  • The components of ultraviolet and visible spectrometers include a source of radiation, a means of dispersion and a detector specific to this spectral region.

Infrared and Ramans spectrophotometry

IR_and_Raman_spectrophotometer

  • Vibrational transitions in molecules cause absorption in the infrared region of the electromagnetic spectrum. They may also be studied using the technique of Raman spectrometry, where they scatter exciting radiation with an accompanying shift in its wavelength.
  • Vibrational spectra give information about the functional groups in molecules, and the observed group frequencies are affected by molecular interactions such as hydrogen bonding.
  • Infrared and Raman instruments include a radiation source, a means of analyzing the radiation and a detection and data processing system. Additionally, sampling methods to deal with gases, liquids, solids, microsamples and mixtures are available.

Nuclear magnetic resonance (NMR) spectrometry

  • Nuclear magnetic resonance (NMR) spectrometry is based on the net absorption of energy in the radiofrequency region of the electromagnetic spectrum by the nuclei of those elements that have spin angular momentum and a magnetic moment. For the nuclei of a particular element, characteristic absorption, or resonance frequencies, and other spectral features provide useful information on identity and molecular structure.
  • Nuclei of elements that possess spin angular momentum and generate a magnetic moment are assigned a half-integral or integral spin quantum number. This determines the number of orientations in space that can be adopted by the spinning nuclei when subjected to an external magnetic field. Electrons also possess spin angular momentum, which generates a magnetic moment that affects the magnitude of the external field experienced by nuclei.
  • Nuclei of a particular element that are in different chemical environments within the same molecule generally experience slightly different applied magnetic field strengths due to the shielding and deshielding effects of nearby electrons. As a result, their resonance frequencies differ, and each is defined by a characteristic chemical shift value.
  • The spin states of one group of nuclei can affect the magnetic field experienced by neighboring groups through intervening bonds in the molecule in such a way that the absorption peaks of each group are split into a number of components. This effect can provide useful information for spectral interpretation.
  • Spectrometers comprise a superconducting solenoid or electromagnet to provide a powerful, stable and homogeneous magnetic field, a transmitter to generate the appropriate radiofrequencies, and a receiver coil and circuitry to monitor the detector signal. A dedicated microcomputer controls the recording of spectra and processing of the data.
Last modified: Tuesday, 23 August 2011, 12:19 PM