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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. Measurement process Different types of spectroscopy use different measurement processes: Three main types of spectroscopy Absorption spectroscopy uses the range of electromagnetic spectra in which a substance absorbs. 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, ultraviolet/visible (UV/ Vis) absorption spectroscopy is most often performed on liquid samples to detect molecular content and infrared (IR) spectroscopy is most often performed on liquid, semi-liquid (paste or grease), dried, or solid samples to determine molecular information, including structural 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), and chemical reactions. Scattering spectroscopy measures certain physical properties by measuring the amount of light that a substance scatters at certain wavelengths, incident angles, and polarization angles. Scattering spectroscopy differs from emission spectroscopy due to the fact that the scattering process is much faster than the absorption/emission process. One of the most useful applications of light scattering spectroscopy is Raman spectroscopy. 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 used in chemistry and material sciences to determine elemental composition and chemical bonding. X-ray crystallography is a scattering process; crystalline materials scatter X-rays at well-defined angles. If the wavelength of the incident X-rays is known, this allows calculation of the distances between planes of atoms within the crystal. The intensities of the scattered X-rays give information about the atomic positions and allow the arrangement of the atoms within the crystal structure to be calculated. Flame Spectroscopy Liquid solution samples are aspirated into a burner or nebulizer/burner combination, desolvated, atomized, and sometimes excited to a higher energy electronic state. The use of a flame during analysis requires fuel and oxidant, typically in the form of gases. Common fuel gases used are acetylene (Ethyne) or hydrogen. Common oxidant gases used are oxygen, air, or nitrous oxide. These methods are often capable of analyzing metallic element analytes in the part per million, billion, or possibly lower concentration ranges. Light detectors are needed to detect light with the analysis information coming from the flame. Plasma Emission Spectroscopy In some ways similar to flame atomic emission spectroscopy, it has largely replaced it. Spark or arc (emission) spectroscopy - is used for the analysis of metallic elements in solid samples. For non-conductive materials, a sample is ground with graphite powder to make it conductive. In traditional arc spectroscopy methods, a sample of the solid was commonly ground up and destroyed during analysis. An electric arc or spark is passed through the sample, heating the sample to a high temperature to excite the atoms in it. The excited analyte atoms glow emitting light at various wavelengths which could be detected by common spectroscopic methods. Since the conditions producing the arc emission typically are not controlled quantitatively, the analysis for the elements is qualitative. Nowadays, the spark sources with controlled discharges under an argon atmosphere allow that this method can be considered eminently quantitative, and its use is widely expanded worldwide through production control laboratories of foundries and steel mills. Visible spectroscopy 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. The spectrum is studied in absorption or emission. Visible absorption spectoscopy is often combined with UV absorption spectroscopy in UV/Vis spectroscopy. Ultraviolet spectroscopy 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. Infrared spectroscopy Infrared spectroscopy offers the possibility to measure different types of interatomic bond vibrations at different frequencies. Especially in organic chemistry the analysis of IR absorption spectra shows what type of bonds are present in the sample. Thermal infrared spectroscopy Thermal infrared spectroscopy measures thermal radiation emitted from materials and surfaces and is used to determine the type of bonds present in a sample as well as their lattice environment. The techniques are used by organic chemists, mineralogists, and planetary scientists. Nuclear magnetic resonance spectroscopy Nuclear magnetic resonance spectroscopy analyzes certain atomic nuclei to determine different local environments of hydrogen, carbon, or other atoms in the molecule of an organic compound or other compound. This is used to help determine the structure of the compound. Less frequently used / combined spectroscopy See also | |||||||||
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