The study of energy levels in atoms or molecules, usually using absorbed or emitted electromagnetic radiation. Inner atomic electrons give spectra in the X-ray region; outer atomic electrons give visible light spectra; the rotation and vibration of molecules give infrared spectra; the precession of nuclear magnetic moments gives radio-wave spectra. Many types of spectroscopy exist, often used to identify the structure of an unknown substance or to detect the presence of known substances.
Spectroscopy is the study of matter and its properties by investigating light, sound, or particles that are emitted, absorbed or scattered by the matter under investigation. Historically, spectroscopy referred to a branch of science in which visible light was used for theoretical studies on the structure of matter and for qualitative and quantitative analyses.
Spectroscopy is often used in physical and analytical chemistry for the identification of substances through the spectrum emitted from them or absorbed in them. 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 and remote sensing.
Physical quantity measured
The type of spectroscopy depends on the physical quantity measured.
The intensity of emitted electromagnetic radiation and the amount of absorbed electromagnetic radiation are studied by electromagnetic spectroscopy (see also cross section). The amplitude of macroscopic vibrations is studied by acoustic spectroscopy and dynamic mechanical spectroscopy. Kinetic energy of particles is studied by electron energy loss spectroscopy and Auger electron spectroscopy (see also cross section). Mass spectrometry is more of a measuring technique (metric) than an observation (scopic) technique but can produce a spectrum of masses, a mass spectrum, similar in appearance to other spectroscopy techniques.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. 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.
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.
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. X-ray absorption and emission spectroscopy is used in chemistry and material sciences to determine elemental composition and chemical bonding.
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. A higher temperature flame than atomic absorption spectroscopy (AA) is typically used to produce excitation of analyte atoms. Plasma emission spectroscopy is a more modern version of this method. See Flame emission spectroscopy for more details. Atomic absorption spectroscopy (often called AA) - This method commonly uses a pre-burner nebulizer (or nebulizing chamber) to create a sample mist and a slot-shaped burner which gives a longer pathlength flame. The temperature of the flame is low enough that the flame itself does not excite sample atoms from their ground state. The nebulizer and flame are used to desolvate and atomize the sample, but the excitation of the analyte atoms is done by the use of lamps shining through the flame at various wavelengths for each type of analyte. In AA, the amount of light absorbed after going through the flame determines the amount of analyte in the sample. A graphite furnace for heating the sample to desolvate and atomize is commonly used for greater sensitivity. Because of its good sensitivity and selectivity, it is still a commonly used method of analysis for certain trace elements in aqueous (and other liquid) samples. The flame is used to solvate and atomize the sample, but a lamp shines light at a specific wavelength into the flame to excite the analyte atoms in the flame. The intensity of this fluorescing light is used for quantifying the amount of analyte element in the sample. A graphite furnace can also be used for atomic fluorescence spectroscopy. This method is not as commonly used as atomic absorption or plasma emission spectroscopy.
Plasma Emission Spectroscopy In some ways similar to flame atomic emission spectroscopy, it has largely replaced it.
Direct-current plasma (DCP) Glow discharge-optical emission spectrometry (GD-OES) Inductively coupled plasma-atomic emission spectrometry (ICP-AES) Laser induced breakdown spectrometry (LIBS), also called Laser-induced plasma spectrometry (LIPS) Microwave-induced plasma (MIP)Spark or arc (emission) spectroscopy - is used for the analysis of metallic elements in solid samples. 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.
Visible spectroscopy
Many atoms emit or absorb visible light. 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.
Infrared spectroscopy
Infrared spectroscopy offers the possibility to measure different types of interatomic bond vibrations at different frequencies.
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.
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.
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