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Spectroscopy is often used in physical and analytical chemistry for the identification of substances, through the spectrum emitted or absorbed. A device for recording a spectrum is a spectrometer. Spectroscopy can be classified according to the physical quantity which is measured or calculated or the measurement process.
Spectroscopy is also heavily used in astronomy.
Physical quantity measured
The type of spectroscopy depends on the physical quantity measured. Normally, the quantity that is measured is an amount or intensity of something.
- The intensity of emitted electromagnetic radiation and the amount of absorbed electromagnetic radiation is studied by electromagnetic spectroscopy.
- The amplitude of a macroscopic vibrations are studied by acoustic spectroscopy , and dynamic mechanical spectroscopy.
- Kinetic energy of particles is studied by electron energy loss spectroscopy, Auger electron spectroscopy
- The mass-to-charge ratio of molecules and atoms are studied in mass spectrometry. Note that a mass spectrometer does not measure the kinetic energy of particles: all particles have the same known kinetic energy (or an integer multiple thereof, depending on the charge). It is disputable whether this field strictly is a type of spectroscopy.
- The number of molecules or atoms or quantum-mechanical states to which the frequency or energy parameter applies.
Different types of spectroscopy use different measurement processes:
Two main types of spectroscopy
Absorption Spectroscopy uses the range of electromagnetic spectra in which a substance absorbs. It is more commonly used. In atomic absorption spectroscopy, the sample is atomized and then light of a particular frequency is passed through the vapour. After calibration, the amount of absorption can be related to the concentrations of various metal ions through the Beer-Lambert law. The method can be automated and is widely used to measure concentrations of ions such as sodium and calcium in blood. Other types of spectroscopy may not require sample atomization. For example, UV/Vis absorption spectroscopy is most often performed on liquid samples to detect molecular content and IR spectroscopy is most often performed on dried samples to determine molecular information, including structureal information.
Emission Spectroscopy uses the range of electromagnetic spectra in which a substance radiates. The substance first absorbs energy and then radiates this energy as light. This energy can be from a variety of sources, including collision (either due to high temperatures or otherwise), chemical reactions, and light.
Common types of spectroscopy
Fluorescence spectroscopy Fluorescence spectroscopy uses higher energy photons to excite a sample, which will then emit lower energy photons. This technique has become popular for its biochemical and medical applications, and can be used for confocal microscopy, fluorescence resonance energy transfer, and fluorescence lifetime imaging.
X-ray spectroscopy and X-ray crystallography When X-rays of sufficient frequency (energy) interact with a substance, inner shell electrons in the atom are excited to outer empty orbitals, or they may be removed completely, ionizing the atom. The inner shell "hole" will then be filled by electrons from outer orbitals. The energy available in this de-excitation process is emitted as radiation (fluorescence) or will remove other less-bound electrons from the atom (Auger effect). The absorption or emission frequencies (energies) are characteristic of the specific atom. In addition, for a specific atom small frequency (energy) variations occur which are characteristic of the chemical bonding. With a suitable apparatus, these characteristic X-ray frequencies or Auger electron energies can be measured. X-ray absorption and emission spectroscopy is e.g. used in chemistry and material sciences to determine elemental composition and chemical bonding.
X-ray crystallography is a process in which X-rays are shone onto crystals at a certain angle. The wavelength of the X-rays is known and so the distance apart of the crystal planes can be calculated. Combining all information enables crystal structure to be detected.
Many atoms emit or absorb visible light. In order to obtain a fine line spectrum, the atoms must be in a gas phase. This means that the substance has to be vaporised. Spectrum is studied in absorption or emission.
All atoms absorb in the UV region because photons are energetic enough to excite outer electrons. If the frequency is high enough, Photoionisation takes place.
Infra-red spectroscopy In Organic chemistry different types of interatomic bond vibrate at different frequencies in the infra-red part of the spectrum. The analysis of IR absorption spectra shows what type of bonds are present in the sample.
Nuclear Magnetic Resonance spectroscopy NMR spectroscopy determines the different local environments of hydrogen or carbon atoms in an organic compound. This is used to determine the structure of the compound.
Less frequently used / combined spectroscopy
- Raman spectroscopy uses the inelastic scattering of light to analyse vibrational and rotational modes of molecules. The resulting 'fingerprints' are an aid to analysis.
- Fourier transform is an efficient method for collecting various spectra. The use of Fourier transform in spectroscopy is called Fourier transform spectroscopy. Nearly all infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR) spectroscopy are performed with Fourier transforms.
- Spectroscopy of matter in situations where the properties are changing with time is called Time-resolved spectroscopy.
- Spectroscopy using an AFM-based analytical technique is called Force spectroscopy.
- Dielectric spectroscopy
- Astronomical spectroscopy
- Rotational spectroscopy
- Vibrational spectroscopy
- Infrared spectroscopy
- Rigid rotor
- EPR spectroscopy
- Spectral power distributions
- Metamerism (color)
- Spectral reflectance
- Laser Induced Breakdown Spectroscopy (LIBS)
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