Spectroscopy

Extremely high resolution spectrum of the Sun showing thousands of elemental absorption lines (fraunhofer lines)

Spectroscopy is the study of spectra, that is, the dependence of physical quantities on frequency.

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. Most large telescopes have spectrographs, which are used either to measure the chemical composition and physical properties of astronomical objects or to measure their velocities from the Doppler shift of spectral lines.

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 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).
• The mass-to-charge ratios of molecules and atoms are studied in mass spectrometry. 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) - so 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. In this case the spectra is usually called cross section.

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), chemical reactions, and light.

Scattering spectroscopy measures certain physical properties by measuring the amount of light that a substance scatters at cetain 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

Spectrum of fluorescent lights showing prominent mercury peaks.

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 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.

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 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.

• Atomic Emission Spectroscopy - This method uses flame excitation; atoms are excited from the heat of the flame to emit light. This method commonly uses a total consumption burner with a round burning outlet. A higher temperature flame than atomic absorption spectroscopy (AA) is typically used to produce excitation of analyte atoms. Since analyte atoms are excited by the heat of the flame, no special elemental lamps to shine into the flame are needed. A high resolution polychromator can be used produce an emission intensity vs. wavelength spectrum over a range of wavelengths showing multiple element excitation lines, meaning multiple elements can be detected in one run. Alternatively, a monochromator can be set at one wavelength to concentrate on analysis of a single element at a certain emission line. 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. The graphite furnace method can also analyze some solid or slurry samples. 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.
• Atomic Fluorescence Spectroscopy - This method commonly uses a burner with a round burning outlet. 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 atoms of certain elements can then fluoresce emitting light in a different direction. 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)

• Inductively coupled plasma-atomic emission spectroscopy (ICP-AES)

• Laser induced breakdown spectroscopy (LIBS)

• Laser-induced plasma

• 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. 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. Spectrum is studied in absorption or emission.

UV 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.

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 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.

Photoemission spectroscopy

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.
• Raman Optical Activity spectroscopy exploits Raman scattering and optical activity effects to reveal detailed information on chiral centres in molecules.
• 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
• Circular Dichroism spectroscopy
• Cavity ring down spectroscopy

See also

• Atomic spectroscopy
• Astronomical spectroscopy
• Diffuse Reflectance Spectroscopy
• Rotational spectroscopy
• Vibrational spectroscopy
• Infrared spectroscopy
• Rigid rotor
• EPR spectroscopy
• Spectral power distributions
• Metamerism (color)
• Spectral reflectance
• Spectrophotometry
• Laser Induced Breakdown Spectroscopy (LIBS)
• Cross section
• Scattering theory
• UV/VIS spectroscopy

External links

• The Science of Spectroscopy - supported by NASA, includes OpenSpectrum, a Wiki-based learning tool for spectroscopy that anyone can edit

• Atomic Emission Spectroscopy

• A Short Study of the Characteristics of two Lab Spectroscopes