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Spectroscopy

Spectroscopy

JULI BERWALD

Spectroscopy is the measurement of the absorption, scattering, or emission of electromagnetic radiation by atoms or molecules. Absorption is the transfer of electromagnetic energy from a source to an atom or molecule. Scattering is the redirection of light as a result of its interaction with matter. Emission is the transition of electromagnetic energy from a one energy level to another energy level that results in the emission of a photon.

When atoms or molecules absorb electromagnetic energy, the incoming energy transfers the quantized atomic or molecular system to a higher energy level. Electrons are promoted to higher orbitals by ultraviolet or visible light; vibrations are excited by infrared light, and rotations are excited by microwaves. Atomic-absorption spectroscopy measures the concentration of an element in a sample, whereas atomic-emission spectroscopy aims at measuring the concentration of elements in samples. UV-VIS absorption spectroscopy is used to obtain qualitative information from the electronic absorption spectrum, or to measure the concentration of an analyte molecule in solution. Molecular fluorescence spectroscopy is a technique for obtaining qualitative information from the electronic fluorescence spectrum, or for measuring the concentration of an analyte in solution.

Infrared spectroscopy has been widely used in the study of surfaces. The most frequently used portion of the infrared spectrum is the region where molecular vibrational frequencies occur. This technique was first applied around the turn of the twentieth century in an attempt to distinguish water of crystallization from water of constitution in solids.

Ultraviolet spectroscopy takes advantage of the selective absorbance of ultraviolet radiation by various substances. The technique is especially useful in investigating biologically active substances such as compounds in body fluids, and drugs and narcotics either in the living body (in vivo ) or outside it (in vitro ). Ultraviolet instruments have also been used to monitor air and water pollution, to analyze dyestuffs, to study carcinogens, to identify food additives, to analyze petroleum fractions, and to analyze pesticide residues. Ultraviolet photoelectron spectroscopy, a technique that is analogous to x-ray photoelectron spectroscopy, has been used to study valence electrons in gases.

Microwave spectroscopy, or molecular rotational resonance spectroscopy, addresses the microwave region of the electromagnetic spectrum and the absorption of energy by molecules as they undergo transitions between rotational energy levels. From these spectra, it is possible to obtain information about molecular structure, including bond distances and bond angles. One example of the application of this technique is in the distinction of trans and gauche rotational isomers. It is also possible to determine dipole moments and molecular collision rates from these spectra.

In nuclear magnetic resonance (NMR), resonant energy is transferred between a radio-frequency alternating magnetic field and a nucleus placed in a field sufficiently strong to decouple the nuclear spin from the influence of atomic electrons. Transitions induced between substrates correspond to different quantized orientations of the nuclear spin relative to the direction of the magnetic field. Nuclear magnetic resonance spectroscopy has two sub-fields: broadline NMR and high resolution NMR. High resolution NMR has been used in inorganic and organic chemistry to measure subtle electronic effects, to determine structure, to study chemical reactions, and to follow the motion of molecules or groups of atoms within molecules.

Electron paramagnetic resonance is a spectroscopic technique similar to nuclear magnetic resonance except that microwave radiation is employed instead of radio frequencies. Electron paramagnetic resonance has been used extensively to study paramagnetic species present on various solid surfaces. These species may be metal ions, surface defects, or adsorbed molecules or ions with one or more unpaired electrons. This technique also provides a basis for determining the bonding characteristics and orientation of a surface complex. Because the technique can be used with low concentrations of active sites, it has proven valuable in studies of oxidation states.

Atoms or molecules that have been excited to high energy levels can decay to lower levels by emitting radiation. For atoms excited by light energy, the emission is referred to as atomic fluorescence; for atoms excited by higher energies, the emission is called atomic or optical emission. In the case of molecules, the emission is called fluorescence if the transition occurs between states of the same spin, and phosphorescence if the transition takes place between states of different spin.

In x-ray fluorescence, the term refers to the characteristic x-rays emitted as a result of absorption of x-rays of higher frequency. In electron fluorescence, the emission of electromagnetic radiation occurs as a consequence of the absorption of energy from radiation (either electro-magnetic or particulate), provided the emission continues only as long as the stimulus producing it is maintained.

The effects governing x-ray photoelectron spectroscopy were first explained by Albert Einstein in 1905, who showed that the energy of an electron ejected in photoemission was equal to the difference between the photon and the binding energy of the electron in the target. In the 1950s, researchers began measuring binding energies of core electrons by x-ray photoemission. The discovery that these binding energies could vary as much as 6 eV, depending on the chemical state of the atom, led to rapid development of x-ray photoelectron spectroscopy, also known as Electron Spectroscopy for Chemical Analysis (ESCA). This technique has provided valuable information about chemical effects at surfaces. Unlike other spectroscopies in which the absorption, emission, or scattering of radiation is interpreted as a function of energy, photoelectron spectroscopy measures the kinetic energy of the electrons(s) ejected by x-ray radiation.

Mössbauer spectroscopy was invented in the late 1950s by Rudolf Mössbauer, who discovered that when solids emit and absorb gamma rays, the nuclear energy levels can be separated to one part in 1014, which is sufficient to reflect the weak interaction of the nucleus with surrounding electrons. The Mössbauer effect probes the binding, charge distribution and symmetry, and magnetic ordering around an atom in a solid matrix. An example of the Mössbsauer effect involves the Fe57 nuclei (the absorber) in a sample to be studied. From the ground state, the Fe57 nuclei can be promoted to their first excited state by absorbing a 14.4-keV gamma-ray photon produced by a radioactive parent, in this case Co57. The excited Fe57 nucleus then decays to the ground state via electron or gamma ray emission. Classically, one would expect the Fe57 nuclei to undergo recoil when emitting or absorbing a gamma-ray photon (somewhat like what a person leaping from a boat to a dock observes when his boat recoils into the lake); but according to quantum mechanics, there is also a reasonable possibility that there will be no recoil (as if the boat were embedded in ice when the leap occurred).

When electromagnetic radiation passes through matter, most of the radiation continues along its original path, but a tiny amount is scattered in other directions. Light that is scattered without a change in energy is called Rayleigh scattering; light that is scattered in transparent solids with a transfer of energy to the solid is called Brillouin scattering. Light scattering accompanied by vibrations in molecules or in the optical region in solids is called Raman scattering.

In vibrational spectroscopy, also known as Raman spectroscopy, the light scattered from a gas, liquid, or solid is accompanied by a shift in wavelength from that of the incident radiation. The effect was discovered by the Indian physicist C. V. Raman in 1928. The Raman effect arises from the inelastic scattering of radiation in the visible region by molecules. Raman spectroscopy is similar to infrared spectroscopy in its ability to provide detailed information about molecular structures. Before the 1940s, Raman spectroscopy was the method of choice in molecular structure determinations, but since that time infrared measurements have largely supplemented it. Infrared absorption requires that a vibration change the dipole moment of a molecule, but Raman spectroscopy is associated with the change in polarizability that accompanies a vibration. As a consequence, Raman spectroscopy provides information about molecular vibrations that is particularly well suited to the structural analysis of covalently bonded molecules, and to a lesser extent, of ionic crystals. Raman spectroscopy is also particularly useful in studying the structure of polyatomic molecules. By comparing spectra of a large number of compounds, chemists have been able to identify characteristic frequencies of molecular groups, e.g., methyl, carbonyl, and hydroxyl groups.

Spectroscopy has great potential to enhance military and defense capabilities. Both chemical and biological warfare agents are detectable, and potentially identifiable, by spectroscopic imaging. New technology involving fiber optic systems and lasers that can quickly change frequencies provides the opportunity to miniaturize spectroscopic equipment. Systems are currently being developed, which will take this technology into the battle-field in order to target surface and ground contamination by chemical and biological weapons. Spectroscopic examination can also aid in the identification and measurement of subcellular processes, such as carbon dioxide production or oxygen use. These measurements facilitate the understanding of cell growth, cellular response to environmental stimuli, and cellular reactions to drugs and biological and chemical warfare agents.

FURTHER READING:

PERIODICALS:

Behnisch, P.A. "Biodetectors in Environmental Chemistry: Are We at a Turning Point?" Environ Int 27(2001): 44142.

"Early Warning Technology." Med Device Technol 13 (2002): 7072.

Casagrande R. "Technology against Terror." Scientific American. 287 (2002): 5965.

ELECTRONIC:

Scripps Center for Mass Spectrometry (BC-007), 10550 North Torrey Pines Rd., La Jolla, CA 92037. (858) 7849596. Gary Suizdak, director. <http://masspec.scripps.edu/information/intro/index.html.> (January 5, 2003).

SEE ALSO

Biological Warfare, Advanced Diagnostics
Biomedical Technologies
Chemical and Biological Detection Technologies
Electromagnetic Spectrum
Electromagnetic Weapons, Biochemical Effects
Microscopes

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Spectroscopy

Spectroscopy

Because organisms present unique spectroscopic patterns, spectroscopic examination (e.g., Raman spectroscopy) of microorganisms (e.g., microbial cells) can help to differentiate between species and strains of microbes. Spectroscopic examination can also aid in the identification and measurement of subcellular processes (e.g., CO2 production) that facilitate the understanding of cell growth, response to environmental stimuli, and drug actions.

The measurement of the absorption, emission, or scattering of electromagnetic radiation by atoms or molecules is referred to as spectroscopy. A transition from a lower energy level to a higher level with transfer of electromagnetic energy to the atom or molecule is called absorption; a transition from a higher energy level to a lower level results in the emission of a photon if energy is transferred to the electromagnetic field; and the redirection of light as a result of its interaction with matter is called scattering.

When atoms or molecules absorb electromagnetic energy, the incoming energy transfers the quantized atomic or molecular system to a higher energy level. Electrons are promoted to higher orbitals by ultraviolet or visible light; vibrations are excited by infrared light, and rotations are excited by microwaves. Atomic-absorption spectroscopy measures the concentration of an element in a sample, whereas atomic-emission spectroscopy aims at measuring the concentration of elements in samples. UV-VIS absorption spectroscopy is used to obtain qualitative information from the electronic absorption spectrum, or to measure the concentration of an analyte molecule in solution. Molecular fluorescence spectroscopy is a technique for obtaining qualitative information from the electronic fluorescence spectrum, or, again, for measuring the concentration of an analyte in solution.

Infrared spectroscopy has been widely used in the study of surfaces. The most frequently used portion of the infrared spectrum is the region where molecular vibrational frequencies occur. This technique was first applied around the turn of the twentieth century in an attempt to distinguish water of crystallization from water of constitution in solids.

Ultraviolet spectroscopy takes advantage of the selective absorbance of ultraviolet radiation by various substances. The technique is especially useful in investigating biologically active substances such as compounds in body fluids, and drugs and narcotics either in the living body (in vivo) or outside it (in vitro). Ultraviolet instruments have also been used to monitor air and water pollution , to analyze dyestuffs, to study carcinogens, to identify food additives, to analyze petroleum fractions, and to analyze pesticide residues. Ultraviolet photoelectron spectroscopy, a technique that is analogous to x-ray photoelectron spectroscopy, has been used to study valence electrons in gases.

Microwave spectroscopy, or molecular rotational resonance spectroscopy, addresses the microwave region and the absorption of energy by molecules as they undergo transitions between rotational energy levels. From these spectra, it is possible to obtain information about molecular structure, including bond distances and bond angles. One example of the application of this technique is in the distinction of trans and gauche rotational isomers. It is also possible to determine dipole moments and molecular collision rates from these spectra.

In nuclear magnetic resonance (NMR), resonant energy is transferred between a radio-frequency alternating magnetic field and a nucleus placed in a field sufficiently strong to decouple the nuclear spin from the influence of atomic electrons. Transitions induced between substrates correspond to different quantized orientations of the nuclear spin relative to the direction of the magnetic field. Nuclear magnetic resonance spectroscopy has two subfields: broadline NMR and high resolution NMR. High resolution NMR has been used in inorganic and organic chemistry to measure subtle electronic effects, to determine structure, to study chemical reactions, and to follow the motion of molecules or groups of atoms within molecules.

Electron paramagnetic resonance is a spectroscopic technique similar to nuclear magnetic resonance except that microwave radiation is employed instead of radio frequencies. Electron paramagnetic resonance has been used extensively to study paramagnetic species present on various solid surfaces. These species may be metal ions, surface defects, or adsorbed molecules or ions with one or more unpaired electrons. This technique also provides a basis for determining the bonding characteristics and orientation of a surface complex. Because the technique can be used with low concentrations of active sites, it has proven valuable in studies of oxidation states.

Atoms or molecules that have been excited to high energy levels can decay to lower levels by emitting radiation. For atoms excited by light energy, the emission is referred to as atomic fluorescence; for atoms excited by higher energies, the emission is called atomic or optical emission. In the case of molecules, the emission is called fluorescence if the transition occurs between states of the same spin, and phosphorescence if the transition takes place between states of different spin.

In x-ray fluorescence, the term refers to the characteristic x rays emitted as a result of absorption of x rays of higher frequency. In electron fluorescence, the emission of electromagnetic radiation occurs as a consequence of the absorption of energy from radiation (either electromagnetic or particulate), provided the emission continues only as long as the stimulus producing it is maintained.

The effects governing x-ray photoelectron spectroscopy were first explained by Albert Einstein in 1905, who showed that the energy of an electron ejected in photoemission was equal to the difference between the photon and the binding energy of the electron in the target. In the 1950s, researchers began measuring binding energies of core electrons by x-ray photoemission. The discovery that these binding energies could vary as much as 6 eV, depending on the chemical state of the atom, led to rapid development of x-ray photoelectron spectroscopy, also known as Electron Spectroscopy for Chemical Analysis (ESCA). This technique has provided valuable information about chemical effects at surfaces. Unlike other spectroscopies in which the absorption, emission, or scattering of radiation is interpreted as a function of energy, photoelectron spectroscopy measures the kinetic energy of the electrons(s) ejected by x-ray radiation.

Mössbauer spectroscopy was invented in the late 1950s by Rudolf Mössbauer, who discovered that when solids emit and absorb gamma rays, the nuclear energy levels can be separated to one part in 1014, which is sufficient to reflect the weak interaction of the nucleus with surrounding electrons. The Mössbauer effect probes the binding, charge distribution and symmetry, and magnetic ordering around an atom in a solid matrix. An example of the Mössbauer effect involves the Fe-57 nuclei (the absorber) in a sample to be studied. From the ground state, the Fe-57 nuclei can be promoted to their first excited state by absorbing a 14.4-keV gamma-ray photon produced by a radioactive parent, in this case Co-57. The excited Fe-57 nucleus then decays to the ground state via electron or gamma ray emission. Classically, one would expect the Fe-57 nuclei to undergo recoil when emitting or absorbing a gamma-ray photon (somewhat like what a person leaping from a boat to a dock observes when his boat recoils into the lake); but according to quantum mechanics, there is also a reasonable possibility that there will be no recoil (as if the boat were embedded in ice when the leap occurred).

When electromagnetic radiation passes through matter, most of the radiation continues along its original path, but a tiny amount is scattered in other directions. Light that is scattered without a change in energy is called Rayleigh scattering; light that is scattered in transparent solids with a transfer of energy to the solid is called Brillouin scattering. Light scattering accompanied by vibrations in molecules or in the optical region in solids is called Raman scattering.

In vibrational spectroscopy, also known as Raman spectroscopy, the light scattered from a gas, liquid, or solid is accompanied by a shift in wavelength from that of the incident radiation. The effect was discovered by the Indian physicist C. V. Raman in 1928. The Raman effect arises from the inelastic scattering of radiation in the visible region by molecules. Raman spectroscopy is similar to infrared spectroscopy in its ability to provide detailed information about molecular structures. Before the 1940s, Raman spectroscopy was the method of choice in molecular structure determinations, but since that time infrared measurements have largely supplemented it. Infrared absorption requires that a vibration change the dipole moment of a molecule, but Raman spectroscopy is associated with the change in polarizability that accompanies a vibration. As a consequence, Raman spectroscopy provides information about molecular vibrations that is particularly well suited to the structural analysis of covalently bonded molecules, and to a lesser extent, of ionic crystals. Raman spectroscopy is also particularly useful in studying the structure of polyatomic molecules. By comparing spectra of a large number of compounds, chemists have been able to identify characteristic frequencies of molecular groups, e.g., methyl, carbonyl, and hydroxyl groups.

See also Biotechnology; Electron microscope, transmission and scanning; Electron microscopic examination of microorganisms; Electrophoresis; Enzyme-linked immunosorbant assay (ELISA); Epidemiology, tracking diseases with technology; Fluorescence in situ hybridization (FISH); Laboratory techniques in immunology; Laboratory techniques in microbiology; Microscope and microscopy

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Spectrograph

Spectrograph

Forensic analysis of a wide and diversified range of samples seized at crime scenes, accidents, fire debris , explosions, and autopsies requires the use of several analytical methods and tools, such as gas chromatography/mass spectrometry (GC/MS), atomic absorption spectroscopy (AAS), inductively coupled plasma spectroscopy (ICP), infrared spectroscopy (IRS), nuclear magnetic resonance (NMR) spectroscopy, and high performance liquid chromatography (HPLC). Spectrography and spectroscopy are basically synonyms, referring to "a picture of a spectrum." The terms spectroscopy and spectroscope are more commonly used because they are older and easier to pronounce than the denominations spectrography and spectrograph. Spectrography is also known as spectral imaging techniques.

Spectrographs or spectroscopes are optical instruments that measure wavelengths and energy levels, radiated from atomic bonds between elements in molecules, or from other light sources such as stars. Spectrographs and spectroscopes disperse light into wave patterns known as a spectral image. The first spectroscope was developed at the beginning of the nineteenth century and consisted of three metallic tubes containing lenses disposed with converging axes and a flint glass prism, which dispersed light originated from a light source or the radiant energy emitted by chemical compounds into a wave spectrum. The spectral image allowed the quantitative analysis of chemical elements, index of refraction, wavelengths, and mass as well as the composition of chemical molecules. Its first application was in astronomy (telescopes) and chemistry (analytical spectroscopy) to determine the composition of chemical elements present in nebulas, stars, and in unknown chemical compounds.

With the advent of photography, spectroscopes were renamed spectrographs because a camera was coupled to the device instead of a telescope, allowing the development of the resulting spectral image into a photographic picture. During the twentieth century, with the advances in physics and electronic technology, the photographic camera was substituted by a photomultiplier that permitted instantaneous spectrographic analyses. A variety of spectral imaging technologies are presently available that are supported by computer software. Examples of forensic applications for these technologies include: isolating trace residues on surfaces; identifying fibers and micro particles; detection and quantification of organic and inorganic contaminants in food, water, and air; identifying semivolatile and volatile (explosive) fuel residues; and analyzing paint fragments.

Infrared (IR) spectroscopy uses infrared light to identify substances, due to chemical bonds vibrating in different frequencies, absorbing different amounts of infrared wavelengths, and emitting specific quanta of radiation (e.g., energy at known wavelengths). The device registers the absorbed wavelengths and produces a graph that is compared to those of known substances, which are recorded in a computer database. Each peak in the spectrum represents a different chemical element with unique properties. Each chemical molecule gives a unique spectrum, known as a fingerprint region. Forensic experts may use this method to identify types of drugs in a sample or paint chips from a car. Forensic analysts can gather physical evidence to support claims of sexual assault by testing samples of blood or urine from the victim with infrared spectroscopy. If Rohypnol or other "date-rape" drugs are found, investigators have not only physical evidence of the crime, but also information about what investigators should search for in the suspect's house.

Gas chromatography/mass spectroscopy is a combined method used in forensics to identify residual fuels, such as accelerants and chemical residues, in the debris of a fire scene in order to determine whether the fire was accidental or was caused intentionally. These methods are also used to verify the purity of chemical products and the presence of contaminants in cosmetics, hygiene products, and food products. High performance liquid chromatography is another forensic method for identification of food and cosmetic contaminants.

Mass spectrometry is used to measure the masses of chemical isotopes (e.g., molecular mass) and to detect impurities in materials. Beams of ionized gas molecules are accelerated in the mass spectrograph, passing through a combined electric and magnetic selector that deflects them, before entering into a vacuum chamber. The amount of deflection is given by the mass/charge ratio, with each molecule being fragmented into smaller particles. In the vacuum chamber, a magnetic field interferes with the beam trajectory creating a spectrum on a photographic plate. Each peak in the spectrum represents a specific mass/charge ratio of a charged fragment and the largest mass/charge ratio indicates the molecular ion used to determine the molecular mass.

Another application of spectrometry is in the forensic analysis of questioned documents . Imaging spectrometers equipped with spectral scanners permit the detection of slight differences in inks and paper surfaces, as well as the presence of erased or added lines in numbers or letters. These optical instruments scan the document point by point through absorption, reflection, and fluorescence of materials, forming a spectral image where existing adulterations become evident to the naked eye. Spectral imaging is a convenient forensic method because it does not destroy evidence during analysis.

Atomic absorption spectroscopy (AAS) allows the precise quantification of inorganic elements in paints, water, air, or soils . AAS can, for instance, determine environmental contamination of water by mercury or other heavy metals. However, when multiple inorganic elements need to be simultaneously analyzed in a sample, inductively coupled plasma (ICP) spectroscopy is the method utilized. The ICP method can detect multiple metals in a solid matrix, in welding fumes, or in water or paints. Another analytical method used in forensics is Raman spectroscopy, especially when the preservation of samples is important as with court exhibits. This method can identify drugs, chemicals, fibers, and paints through spectral microscopy.

Determining the postmortem interval (PMI) or the time elapsed since death is crucial for investigators of a murder , especially when the body was subjected to environmental influences such as water, soil, or insects. In these cases, postmortem metabolic changes can be assessed through high-resolution magnetic resonance spectroscopy (H-MRS). It also provides additional valuable information to other traditional forensic methods used to determine PMI. In one study, decomposing brain tissue was used in H-MRS to identify metabolites and gases that helped to determine the time elapsed since death. The brain metabolites showed expected decreased concentrations that correlated with the estimated PMI of known samples.

In spite of the great utility of analytical instruments in forensic investigations, it is important to keep in mind that nothing substitutes human scientific and technical competence along with the exchange of information when interpreting data, especially when lives are at stake in a criminal court. One example of this was given by a scientist in a 2004 report alerting that bullet matching based on chemical analysis has sometimes been biased by errors in analysts' interpretation of data. In the report, "Forensic Analysis: Weighting Bullet Lead Evidence," the limitations of lead content analysis as a tool for matching evidence and evidence validation were described. Chemical analysis through ICP spectroscopy detects minute amounts of trace elements in bullet fragments such as arsenic, antimony, copper, silver, cadmium, tin, and bismuth, which are present in less than 1% of bullet lead alloys. Although the resulting bullet characteristics are accurate, the way data is interpreted may be misleading. It was long assumed that if two bullets are chemically identical, they originated from the same smelting source or were manufactured at the same day at the same factory. FBI examiners even assumed in courts that they came from the same bullet box. The report featured evidence from forensic chemists that even in a single lead smelting pot, sometimes the composition varied from one batch of bullets to the next batch, whereas the composition of different pots matched, implying that bullets made from different pots, by different manufacturers, sometimes matched.

Another forensic analytical chemist at the Committee of the National Academies discovered that bullets made of lead from different sources can get mixed into the supply and manufacturing processes, which can lead to the same ammunition box containing bullets with different elemental compositions. The National Academies Committee concluded in their report that it is impossible to determine with absolute certainty that a bullet from a crime scene came from a specific box of bullets, or that two bullets were manufactured on the same day by the same manufacturer. In the face of these and other gathered data generated from spectrography and shown in the report, the committee has asked the FBI to revise its rules on the interpretation of results from bullet chemical analysis.

see also Accelerant; Analytical instrumentation; Ballistic fingerprints; Ballistics; Chemical and biological detection technologies; Chromatography; Circumstantial evidence; Document forgery; Electromagnetic spectrum; Energy dispersive spectroscopy; Fire debris; Gas chromatograph-mass spectrometer; Ink analysis; Isotopic analysis; Microspectrophotometry; Paint analysis; Spectroscopy; Trace evidence.

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Spectroscopy

Spectroscopy

Forensic analysis utilizes a variety of physical, chemical, and molecular techniques to detect and, in many cases, determine the quantity and composition of a specific compound. Some of these techniques are extremely sensitive and accurate. One such example is spectroscopy.

Spectroscopy is the measurement of the absorption, scattering, or emission of electromagnetic radiation by atoms or molecules. Absorption is the transfer of electromagnetic energy from a source to an atom or molecule. Scattering is the redirection of light as a result of its interaction with matter. Emission is the transition of electromagnetic energy from a one energy level to another energy level that results in the emission of a photon.

When atoms or molecules absorb electromagnetic energy, the incoming energy transfers the quantized atomic or molecular system to a higher energy level. Electrons are promoted to higher orbitals by ultraviolet or visible light; vibrations are excited by infrared light (thermal energy). Atomic emission spectroscopy (AES) is designed to measure the amount of light emitted by excited atoms. Atomic absorption spectroscopy (AAS) is more sensitive, because it measures the amount of light absorbed by ground state atoms. Atomic absorption tests are more sensitive because there are more ground state electrons than excited electrons in a sample. UV-VIS absorption spectroscopy is used to obtain qualitative information from the electronic absorption spectrum, or to measure the concentration of an analyte molecule in solution. Molecular fluorescence spectroscopy is a technique for obtaining qualitative information from the electronic fluorescence spectrum, or for measuring the concentration of a chemical compound undergoing analysis, also known as an analyte, in solution.

Infrared spectroscopy has been widely used in the study of surfaces. The most frequently used portion of the infrared spectrum is the region where molecular vibrational frequencies occur. This technique was first applied around the turn of the twentieth century in an attempt to distinguish water of crystallization from water of constitution in solids.

Ultraviolet spectroscopy takes advantage of the selective absorbance of ultraviolet radiation by various substances. The technique is especially useful in investigating biologically active substances such as compounds in body fluids , and drugs and narcotics either in the living body (in vivo ) or outside it (in vitro ). Ultraviolet instruments have also been used to monitor air and water pollution, to analyze dyes, to study carcinogens, to identify food additives, to analyze petroleum fractions, and to analyze pesticide residues. All of these can be forensically relevant. Ultraviolet photoelectron spectroscopy, a technique that is analogous to x-ray photoelectron spectroscopy, has been used to study valence electrons in gases.

Microwave spectroscopy, or molecular rotational resonance spectroscopy, addresses the microwave region of the electromagnetic spectrum and the absorption of energy by molecules as they undergo transitions between energy levels. From these spectra, it is possible to obtain information about molecular structure.

In nuclear magnetic resonance (NMR), resonant energy is transferred between a radio-frequency alternating magnetic field and a nucleus placed in a field sufficiently strong to separate the nuclear spin from the influence of atomic electrons. Transitions induced between substrates correspond to different quantized orientations of the nuclear spin relative to the direction of the magnetic field. Nuclear magnetic resonance spectroscopy has two subfields: broadline NMR and high resolution NMR. High resolution NMR has been used in inorganic and organic chemistry to measure subtle electronic effects, to determine structure, to study chemical reactions, and to follow the motion of molecules or groups of atoms within molecules.

Electron paramagnetic resonance is a spectroscopic technique similar to nuclear magnetic resonance except that microwave radiation is employed instead of radio frequencies. Electron paramagnetic resonance has been used extensively to study paramagnetic species present on various solid surfaces. These species may be metal ions, surface defects, or adsorbed molecules or ions with one or more unpaired electrons. This technique also provides a basis for determining the bonding characteristics and orientation of a surface complex. Because the technique can be used with low concentrations of active sites, it has proven valuable in studies of oxidation states.

Atoms or molecules that have been excited to high energy levels can decay to lower levels by emitting radiation. For atoms excited by light energy, the emission is referred to as atomic fluorescence; for atoms excited by higher energies, the emission is called atomic or optical emission. In the case of molecules, the emission is called fluorescence if the transition occurs between states of the same spin, and phosphorescence if the transition takes place between states of different spin.

In x-ray fluorescence, the term refers to the characteristic x rays emitted as a result of absorption of x rays of higher frequency. In electron fluorescence, the emission of electromagnetic radiation occurs as a consequence of the absorption of energy from radiation (either electromagnetic or particulate), provided the emission continues only as long as the stimulus producing it is maintained.

The effects governing x-ray photoelectron spectroscopy were first explained by Albert Einstein in 1905, who showed that the energy of an electron ejected in photoemission was equal to the difference between the photon and the binding energy of the electron in the target. In the 1950s, researchers began measuring binding energies of core electrons by x-ray photoemission. The discovery that these binding energies could vary as much as 6 eV, depending on the chemical state of the atom, led to rapid development of x-ray photoelectron spectroscopy, also known as electron spectroscopy for chemical analysis (ESCA). This technique has provided valuable information about chemical effects at surfaces. Unlike other spectroscopy techniques in which the absorption, emission, or scattering of radiation is interpreted as a function of energy, photoelectron spectroscopy measures the kinetic energy of the electrons(s) ejected by x-ray radiation.

Mössbauer spectroscopy was invented in the late 1950s by Rudolf Mössbauer, who discovered that when solids emit and absorb gamma rays, the nuclear energy levels can be separated to one part in 1014, which is sufficient to reflect the weak interaction of the nucleus with surrounding electrons. The Mössbauer effect probes the binding, charge distribution and symmetry, and magnetic ordering around an atom in a solid matrix. An example of the Mössbauer Mo 57Fe nuclei (the absorber) in a sample to be studied. From the ground state, the 57Fe nuclei can be promoted to their first excited state by absorbing a 14.4-keV gamma-ray photon produced by a radioactive parent, in this case 57Co. The effect involves the 57Fe nucleus then decays to the ground state via electron or gamma ray emission. Classically, one would expect the 57Fe nuclei to undergo recoil when emitting or absorbing a gamma-ray photon (somewhat like what a person leaping from a boat to a dock observes when his boat recoils into the lake); but according to quantum mechanics, there is also a reasonable possibility that there will be no recoil (as if the boat were embedded in ice when the leap occurred).

When electromagnetic radiation passes through matter, most of the radiation continues along its original path, but a tiny amount is scattered in other directions. Light that is scattered without a change in energy is called Rayleigh scattering; light that is scattered in transparent solids with a transfer of energy to the solid is called Brillouin scattering. Light scattering accompanied by vibrations in molecules or in the optical region in solids is called Raman scattering.

In vibrational spectroscopy, also known as Raman spectroscopy, the light scattered from a gas, liquid, or solid is accompanied by a shift in wavelength from that of the incident radiation. The effect was discovered by the Indian physicist C. V. Raman in 1928. The Raman effect arises from the inelastic scattering of radiation in the visible region by molecules. Raman spectroscopy is similar to infrared spectroscopy in its ability to provide detailed information about molecular structures. Before the 1940s, Raman spectroscopy was the method of choice in molecular structure determinations, but since that time infrared measurements have largely supplemented it. Infrared absorption requires that a vibration change the dipole moment of a molecule, but Raman spectroscopy is associated with the change in polarizability that accompanies a vibration. As a consequence, Raman spectroscopy provides information about molecular vibrations that is particularly well suited to the structural analysis of covalently bonded molecules, and to a lesser extent, of ionic crystals. Raman spectroscopy is also particularly useful in studying the structure of polyatomic molecules. By comparing spectra of a large number of compounds, chemists have been able to identify characteristic frequencies of molecular groups, e.g., methyl, carbonyl, and hydroxyl groups.

see also Analytical instrumentation; Fourier transform infrared spectrophotometer (FTIR).

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Spectroscopy

Spectroscopy


Spectroscopy is the measurement of interactions between electromagnetic radiation and matter. Electromagnetic radiation, which includes light, has characteristics of waves and particles. Each "particle" of light, or "photon ," has a discrete amount of energy that can be transferred to a molecule. The transverse waves of electromagnetic radiation possess a constantly changing magnitude of electric and magnetic fields in directions that are perpendicular to the direction in which the wave is traveling. These changes in the electric and magnetic fields can cause changes in molecules. Electromagnetic radiation can be transmitted, absorbed, or reflected by matter, and each region of the spectrum can be used to investigate different aspects of the structure and properties of molecules, depending on the amount of energy imparted to the molecule. The absorption of radiation by matter is a quantized process, in that a molecule will only absorb radiation of certain discrete frequencies. These frequencies are determined by well-defined spacings of energy levels in the molecule under investigation. The absorption of photons of high energy cause large changes (often irreversible) in the molecules and correlate to moving electrons to higher energies.

Ultraviolet-Visible Radiation

Electromagnetic radiation is often characterized by its wavelengththe distance between successive peaks in the wave. Radiation with wavelengths between 100 and 700 nanometers (3.94 × 106 and 2.76 × 105 inches) is termed ultraviolet-visible radiation. The transmittance (T) of a sample is the amount of light transmitted (P) by a sample divided by the amount of light transmitted by a blank (P0)T = P/P0. The absorbance (A) of a solution is the negative logarithm of the transmittanceA = LOG(T). The

absorbance of a sample is proportional to the length of the sample that is in the path of the probe radiation and the number of the molecules in the beam path. A plot of the absorbance or transmittance versus the wavelength of radiation is called a spectrum.

When molecules absorb photons in the ultraviolet and visible regions of the spectrumcorresponding to waves with wavelengths between 190 and 1,100 nanometers (7.48 × 106 and 4.33 × 105 inches)electrons are promoted to higher electronic energy levels. Since molecules absorb photons with energies that match the difference in energy between their electronic energy levels, only a portion of white light is absorbed by a given molecule, giving it color. The color of light absorbed by a molecule is subtracted from white light, and the remaining light will be the complement of the light absorbed. Ultraviolet-visible spectra show the relative spacing of energy levels in molecules. Generally, molecular energy levels are stabilized when a molecule possesses alternating double bonds and the energy of the photons that these molecules absorbed shifts to lower wavelengths. This phenomenon explains the observation that ethylene, possessing one C=C bond, absorbs light of 180 nanometers (7.09 × 106 inches) and is colorless, while beta-carotene, possessing eleven alternating C=C bonds, absorbs at 450 nanometers (1.77 × 105 inches) and appears orange in color. Light absorbance in the ultraviolet and visible regions is used to determine the concentration of molecules in solution and of atoms in the gas phase . Chemists can determine the concentration of lead in drinking water with absorption spectroscopy.

Nuclear Magnetic Resonance Spectroscopy

Radio waves can be used to probe the electronic environment of the nuclei of atoms. The nuclei of atoms spin in a clockwise or counterclockwise fashion and create a magnetic field. This field can have the same or opposite field as a superconducting magnet surrounding the sample. When radio waves of a particular frequency are applied to the sample, the spin of these nuclei will change. The frequency of radiation absorbed by molecules in a magnetic field is determined by the types of bonds and the way these bonds are connected. Chemists measure the absorption of radio waves by molecules using a technique called nuclear magnetic resonance spectroscopy. This type of spectroscopy can also be used to determine areas of the body that are diseased, through a technique called magnetic resonance imaging.

Microwave Spectroscopy

Microwaves with long wavelengths cause molecules to rotate faster when they are absorbed. Polar bonds in molecules like water and sugar act as handles for the microwave radiation to grab on to, and the rotational energy can be greatly increased by short exposures to microwaves. This fact explains why polar molecules heat up quickly in microwave ovens. By measuring the wavelength of absorption through microwave spectroscopy, researchers can determine the size of the molecule.

Infrared Radiation

The absorption of infrared radiation (1 to 1,000 micrometers, or 0.0000394 to 0.0394 inches) causes bonds in molecules to vibrate. A bond in the molecule must undergo a change in the dipole moment when the infrared radiation is absorbed. The stiffer the bond, the more energy is required to cause the bond to stretch. Therefore the frequency required to cause CN, C=N, and CN bonds to stretch increases from left to right. Often the infrared spectrum is considered to be a fingerprint of the molecule. Matching a sample's spectrum with a standard spectrum can positively identify the sample. This technique is used to measure emissions in automobile exhaust.

Fluorescence

Fluorescence is the process by which molecules emit light. When an electron moves to a level of lower energy, light can be given off with a frequency that matches the spacing between the original and final levels. The electron must initially be placed in a higher energy level by the absorption of light at short wavelengths. In fluorescence, the molecule loses some of this excess energy by emitting light at longer wavelengths instantaneously. This process is observed in sodium streetlights, where sodium atoms in the gas phase have been excited by an applied voltage and the electrons relax to lower energy levels and give off yellow light at 589 nanometers (2.32 × 105 inches). Fluorescent dye molecules on clothing are excited by ultraviolet light, and these molecules give off energy of longer wavelengths, as electrons in the molecules relax to lower energy levels. The fluorescence of molecules is very sensitive to the polarity, temperature, and viscosity that the molecule resides in. Unlike absorbance, fluorescence is not measured on a background and can quantitate very low amounts of materials. Richard Mathies and coworkers have detected single molecules in solution by fluorescence spectroscopy.

X Rays and Gamma Rays

X rays and gamma rays have enough energy to remove electrons from atoms and molecules and thereby ionize them. The wavelengths of x rays that are absorbed are determined by the distance that an electron is from the nucleus. Furthermore, the regular spacing of atoms in a molecule can create a diffraction pattern of x rays. By examining the diffraction pattern, researchers can accurately determine the arrangement of atoms in a molecule.

Applications

Many scientists use spectroscopy on a daily basis to gain insight into the structure of molecules or the concentration of atoms or molecules in a sample. The chemist uses radio waves and infrared radiation to determine the structure of a new molecule. The geologist uses ultraviolet radiation to determine the concentration of a particular element in a rock or mineral. The microbiologist uses fluorescence measurements to determine the concentration of bacteria in solution.

see also Rydberg, Johannes.

G. Brent Dawson

Bibliography

Harris, Daniel C. (1999). Quantitative Chemical Analysis, 5th edition. New York: W. H. Freeman.

Ingle, James D., Jr., and Crouch, Stanley R. (1988). Spectrochemical Analysis. Englewood Cliffs, NJ: Prentice Hall.

Peck, K.; Stryer, L.; Glazer, A. N.; and Mathies, R. A. (1989). Proceedings of the National Academy of Sciences of the United States of America 86: 4,0874,091.

Skoog, Douglas A.; Holler, F. James; and Nieman, Timothy A. (1998). Principles of Instrumental Analysis, 5th edition. New York: Brooks/Cole.

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Spectroscopy

Spectroscopy

Geoscientists utilize a number of different spectroscopy techniques in the study of Earth materials. The absorption, emission, or scattering of electromagnetic radiation by atoms or molecules is referred to as spectroscopy. A transition from a lower energy level to a higher level with transfer of electromagnetic energy to the atom or molecule is called absorption; a transition from a higher energy level to a lower level is called emission (if energy is transferred to the electromagnetic field); and the redirection of light as a result of its interaction with matter is called scattering.

When atoms or molecules absorb electromagnetic energy, the incoming energy transfers the quantized atomic or molecular system to a higher energy level. Electrons are promoted to higher orbitals by ultraviolet or visible light; vibrations are excited by infrared light, and rotations are excited by microwaves. Atomic-absorption spectroscopy measures the concentration of an element in a sample, whereas atomic-emission spectroscopy aims at measuring the concentration of elements in samples.

Infrared spectroscopy has been widely used in the study of surfaces. The most frequently used portion of the infrared spectrum is the region where molecular vibrational frequencies occur. This technique was first applied around the turn of the twentieth century in an attempt to distinguish water of crystallization from water of constitution in solids.

Ultraviolet spectroscopy takes advantage of the selective absorbance of ultraviolet radiation by various substances. Ultraviolet instruments have also been used to monitor air and water pollution , to analyze petroleum fractions, and to analyze pesticide residues. Ultraviolet photoelectron spectroscopy, a technique that is analogous to x-ray photoelectron spectroscopy, has been used to study valence electrons in gases.

Microwave spectroscopy, or molecular rotational resonance spectroscopy, addresses the microwave region and the absorption of energy by molecules as they undergo transitions between rotational energy levels. From these spectra, it is possible to obtain information about molecular structure, including bond distances and bond angles. One example of the application of this technique is in the distinction of trans and gauche rotational isomers. It is also possible to determine dipole moments and molecular collision rates from these spectra.

Although there are many other forms of spectroscopy (e.g., UV-VIS absorption spectroscopy, molecular fluorescence spectroscopy, etc.) many modern advances in inorganic and organic based studies have resulted from the development of nuclear magnetic resonance (NMR) technology. In NMR, resonant energy is transferred between a radio-frequency alternating magnetic field and a nucleus placed in a field sufficiently strong to decouple the nuclear spin from the influence of atomic electrons. Transitions induced between substates correspond to different quantized orientations of the nuclear spin relative to the direction of the magnetic field. Nuclear magnetic resonance spectroscopy has two subfields: broadline NMR and high resolution NMR. High resolution NMR has been used in inorganic and organic chemistry to measure subtle electronic effects, to determine structure, to study chemical reactions, and to follow the motion of molecules or groups of atoms within molecules.

Electron paramagnetic resonance is a spectroscopic technique similar to nuclear magnetic resonance except that microwave radiation is employed instead of radio frequencies. Electron paramagnetic resonance has been used extensively to study paramagnetic species present on various solid surfaces. These species may be metal ions, surface defects, or absorbed molecules or ions with one or more unpaired electrons. This technique also provides a basis for determining the bonding characteristics and orientation of a surface complex. Because the technique can be used with low concentrations of active sites, it has proven valuable in studies of oxidation states.

Atoms or molecules that have been excited to high energy levels can decay to lower levels by emitting radiation. For atoms excited by light energy, the emission is referred to as atomic fluorescence; for atoms excited by higher energies, the emission is called atomic or optical emission. In the case of molecules, the emission is called fluorescence if the transition occurs between states of the same spin, and phosphorescence if the transition takes place between states of different spin.

In x-ray fluorescence, the term refers to the characteristic x rays emitted as a result of absorption of x rays of higher frequency. In electron fluorescence, the emission of electromagnetic radiation occurs as a consequence of the absorption of energy from radiation (either electromagnetic or particulate), provided the emission continues only as long as the stimulus producing it is maintained.

The effects governing x-ray photoelectron spectroscopy were first explained by German-American physicist Albert Einstein (18791955) in 1905, who showed that the energy of an electron ejected in photoemission was equal to the difference between the photon and the binding energy of the electron in the target.

When electromagnetic radiation passes through matter, most of the radiation continues along its original path, but a tiny amount is scattered in other directions. Light that is scattered without a change in energy is called Rayleigh scattering; light that is scattered in transparent solids with a transfer of energy to the solid is called Brillouin scattering. Light scattering accompanied by vibrations in molecules or in the optical region in solids is called Raman scattering.

See also Astronomy; Atmospheric chemistry; Focused Ion Beam (FIB); Geochemistry; Mineralogy

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Spectroscopy

Spectroscopy

Spectroscopy is a science concerned with the analysis of the composition of matter based on the kind of radiation emitted by that matter. For example, suppose that a piece of iron is heated until it begins to glow. The light given off by the iron is a characteristic property of iron. That is, the light is different from light produced by any other metal such as copper, tin, lead, uranium, or aluminum.

The spectroscope

In heated samples of iron, copper, tin, and other metals, the light produced does not look very different from one metal to the next. The differences that do exist in these cases can be detected only by using a special instrument known as a spectroscope. The structure of one type of spectroscope is shown in Figure 1.

Light produced from some source (such as a heated metal) is first passed through a narrow slit. The slit causes the light to spread out, forming

a set of diverging rays. Those rays are caused to fall on a lens (L1 in the figure), which makes them parallel to each other. The parallel rays then fall on a grating, a piece of glass or plastic into which hundreds or thousands of very narrow parallel grooves are etched.

The grating acts like a glass prism, causing the light to break apart into a whole range of colors. If the light coming from the source were pure white light, the grating would break it up into a continuous spectrum, a rainbowlike array containing every color from violet to red.

Finally, the spectrumthe spread of colored lightcan be viewed through a small telescope (L2 and L3 in the figure). Or it can be recorded on a piece of photographic film on a device known as a spectrograph for later study. In many cases, the actual wavelengths present in a spectrumand the intensity of each onecan be recorded by means of an instrument known as a spectrometer.

Words to Know

Absorption spectrum: The spectrum formed when light passes through a cool gas.

Diffraction grating: A device consisting of a surface into which are etched very fine, closely spaced grooves that cause different wavelengths of light to reflect or refract (bend) by different amounts.

Emission spectrum: The colors of light emitted by a heated gas.

Spectrograph: An instrument for recording spectra.

Spectrometer: An instrument that records the wavelengths (or frequencies) and intensities of radiation emitted or absorbed by a sample.

Emission and absorption spectra

A continuous spectrum is produced only when a great many different elements and compounds are heated together all at the same time. When a single element or a single compound is heated all by itself, a different kind of spectruma line spectrumis produced. A line spectrum consists of a number of lines located at various specific angles in the range from blue to red. For example, hydrogen produces two lines in the blue region of the spectrum, another line in the green region, and a fourth line in the red region of the spectrum. In contrast, sodium produces only two lines, both in the yellow region of the spectrum.

These lines are called emission spectra because they are produced when an element gives off light. Every element has a distinctive emission spectrum, like those described for hydrogen and sodium. If a scientist views the emission spectrum produced by some unknown material, he or she can refer to a chart of emission spectra of the elements. The spectrum from the unknown material can be compared to those in the chart, allowing the scientist to identify the unknown.

One can also study absorption spectra. Imagine an experiment in which a white light is shined through a gas. The light passing through the gas is then studied by means of a spectrometer. In this case, the light that is recorded consists of all of the white light that came from the original source less any light absorbed by the gas through which it passed.

Again, the spectrum observed in this experiment is a line spectrum. In this case, however, the lines observed are those that are not absorbed by the gas between the light source and the observer.

Types of spectroscopy

As described above, all elements have distinctive line spectra. It happens that all compounds have distinctive spectra as well. If one were to heat a sample of iron oxide instead of pure iron metal, the same experiment as the one described above could be conducted. In this case, the line spectrum produced would be that of the compound iron metal rather than the element iron.

Light spectroscopy, the technique described so far, has its limitations. When some elements and compounds are heated, they produce spectra that lie outside the visible spectrum. In some cases, lines are produced in the infrared, ultraviolet, or even X-ray region of the electromagnetic spectrum. Special techniques and instruments have been developed to analyze spectra produced in all of these ranges.

[See also Diffraction; Electromagnetic spectrum; Qualitative analysis ]

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Raman effect

Raman effect (rä´mən), appearance of additional lines in the spectrum of monochromatic light that has been scattered by a transparent material medium. The effect was discovered by C. V. Raman in 1928. The energy and thus the frequency and wavelength of the scattered light is changed as the light either imparts rotational or vibrational energy to the scattering molecules or takes energy away. The line spectrum of the scattered light will have one prominent line corresponding to the original wavelength of the incident radiation, plus additional lines to each side of it corresponding to the shorter or longer wavelengths of the altered portion of the light. This Raman spectrum is characteristic of the transmitting substance. Raman spectrometry is a useful technique in physical and chemical research, particularly for the characterization of materials.

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spectroscopy

spectroscopy Branch of optics dealing with the measurement of the wavelength and intensity of a spectrum. The main tool is the spectroscope. It produces a spectrum and a spectrograph photographs it. An analysis of the spectrogram can reveal the substances causing the spectrum by the position of emission and absorption lines and bands. Analyses are used for determining the composition and motion of stars and other celestial bodies. A spectrometer is a calibrated spectroscope capable of precise measurements.

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spectroscopy

spectroscopy The study of methods of producing and analysing spectra using spectroscopes and other instruments. Spectroscopy is employed in a wide range of biological research areas, such as biochemistry and toxicology, for the identification of metabolites and other compounds of biological significance. See mass spectroscopy.

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spectrograph

spectrograph Analytical instrument used mainly for elemental analysis.

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spectrograph

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