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Deuterium

Deuterium

Deuterium, also called heavy hydrogen, is a stable isotope of hydrogen with a natural abundance of one atom in 6500 of hydrogen. The nucleus of deuterium, called a deuteron, contains one proton and one neutron, whereas a normal hydrogen nucleus just has one proton. The chemical symbol ²H identifies deuterium. The unofficial symbol D is also often used, even though deuterium is not a chemical element in its own right. It occurs naturally as deuterium gas, written ²H₂ or D₂. When bonded with a typical ¹H atom, the gas is called hydrogen deuteride. Deuterium behaves chemically identically to ordinary hydrogen, although, because of the greater atomic mass, reactions involving deuterium tend to occur at a somewhat slower reaction rate than the corresponding reactions involving ordinary hydrogen. The two isotopes can be distinguished physically by using mass spectrometry. Deuterium can replace the normal hydrogen in water molecules to form heavy water (D₂O). Although not strictly toxic, consumption of heavy water could nevertheless pose a health threat. The existence of deuterium in stars is an important datum in cosmology. Stellar fusion destroys deuterium, and there are no known natural processes, other than the Big Bang nucleosynthesis, which produce deuterium. Thus it is one of the arguments in favour of the Big Bang theory over the steady state theory of the universe. The world's leading producer of deuterium is Canada, in the form of heavy water as neutron moderator for the operation of the CANDU reactor.

Applications

Deuterium is useful in nuclear fusion reactions, especially in combination with tritium, because of the large reaction rate (or cross section) and high energy yield of the D-T reaction. In chemistry and biochemistry, deuterium is used in tracer molecules to study chemical reactions and metabolic pathways because chemically it behaves identically to ordinary hydrogen, but it can be distinguished from ordinary hydrogen by its mass using mass spectrometry.

History

Deuterium was predicted in 1926 by Walter Russell, using his "spiral" periodic table, and first detected in 1931 by Harold Clayton Urey, a chemist at Columbia University. Urey earned the 1934 Nobel Prize in Chemistry for this work. Gilbert Newton Lewis isolated the first sample of pure heavy water in 1933. During World War II, Germany was known to be conducting experiments using heavy water as moderator for a nuclear reactor. This was a source of concern because it might allow them to produce plutonium for an atomic bomb. Ultimately it led to a seemingly important Allied operation, the Norwegian heavy water sabotage, to destroy the Vemork deuterium production facility in Norway. Unfortuately it turned out that Germany was was not putting any serious efforts into the program, and only had a small experimental reactor hidden away. In reality the Germans did not even have half the amount of heavy water needed to run the reactor, partially due to the Norwegian Heavy Water Sabotage operation.

Data

- density: 0.180 kg/m³ at STP (0 °C, 101.325 kPa).
- atomic weight: 2.01355321270. Data at approximately 18 K for D₂ (triple point):
- density: :
- solid: 195 kg/m³ :
- gas: 0.452 kg/m³
- viscosity: 1.3 µPa·s
- specific heat capacity at constant pressure c_p: :
- solid: 2950 J/(kg·K) :
- gas: 5200 J/(kg·K)

Anti-deuterium

An antideuteron is the antiparticle of the nucleus of deuterium, consisting of an antiproton and an antineutron. The antideuteron was first produced at CERN and the Brookhaven National Laboratory in 1965. A complete atom, with a positron orbiting the nucleus, would be called antideuterium, but as of 2005 antideuterium has not yet been created.

References

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- [http://atom.kaeri.re.kr/ Nuclear Data Evaluation Lab]
- [http://www.newscientist.com/article.ns?id=dn7315 Desktop nuclear fusion demonstrated with deuterium gas] (New Scientist) Category:Isotopes Category:Nuclear materials ko:중수소 ms:Deuterium ja:重水素 th:ดิวเทอเรียม

Stable isotope

Stable isotopes are chemical isotopes that are not radioactive. Stable isotopes of the same element have the same chemical characteristics and therefore behave almost identically. The mass differences, due to a difference in the number of neutrons, result in partial separation of the light from heavy isotopes during chemical reactions (isotope fractionation). For example, the difference in mass between the two stable isotopes of hydrogen, ¹H (1 proton, no neutron, also known as protium) and ²H (1 proton, 1 neutron, also known as deuterium) is almost 100%. Therefore, a significant fractionation will occur. Commonly analysed stable isotopes include oxygen, carbon, nitrogen, hydrogen and sulfur. These isotope systems have been under investigation for many years as they are relatively simple to measure. Recent advances in mass spectrometry (ie. muliple-collector inductively coupled plasma mass spectrometry) now enable the measurement of heavier stable isotopes, such as iron, copper, zinc, molybdenum, etc.

Stable isotope fractionation

There are three types of isotope fractionation:
- equilibrium fractionation
- kinetic fractionation
- mass-independent fractionation

Hydrogen

|- | Critical temperature || 32.19 K |- | Critical pressure || 1.315 MPa |- | Critical density || 30.12 g/L (Bohr radius) Hydrogen (Latin: hydrogenium, from Greek: hydro: water, genes: forming) is a chemical element in the periodic table that has the symbol H and atomic number 1. At standard temperature and pressure it is a colorless, odorless, nonmetallic, univalent, highly flammable diatomic gas. Hydrogen is the lightest and most abundant element in the universe. It is present in water, all organic compounds (rare exceptions exist, like buckminsterfullerene) and in all living organisms. Hydrogen is able to react chemically with most other elements. Stars in their main sequence are overwhelmingly composed of hydrogen in its plasma state. The element is used in ammonia production, as a lifting gas, as an alternative fuel, and more recently as a power source of fuel cells. Despite its ubiquity in the universe, hydrogen is surprisingly hard to produce in large quantities on the Earth. In the laboratory, the element is prepared by the reaction of acids on metals such as zinc. The electrolysis of water is a simple method of producing hydrogen, but is economically inefficient for mass production. Large-scale production is usually achieved by steam reforming natural gas. Scientists are now researching new methods for hydrogen production; if they succeed in developing a cost-efficient method of large-scale production, hydrogen may become a viable alternative to greenhouse-gas-producing fossil fuels. One of the methods under investigation involves use of green algae; another promising method involves the conversion of biomass derivatives such as glucose or sorbitol at low temperatures using a catalyst. Yet another method is the "steaming" of Carbon, whereby hydrocarbons are broken down with heat to release hydrogen.

Basic features

Hydrogen is the lightest chemical element; its most common isotope comprises just one negatively charged electron, distributed around a positively charged proton (the nucleus of the atom). The electron is bound to the proton by the Coulomb force, the electrical force that one stationary, electrically charged nanoparticle exerts on another. The hydrogen atom has special significance in quantum mechanics as a simple physical system for which there is an exact solution to the Schrödinger equation; from that equation, the experimentally observed frequencies and intensities of the hydrogen's spectral lines can be calculated. Spectral lines are dark or bright lines in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range, compared with the nearby frequencies. At standard temperature and pressure, hydrogen forms a diatomic gas, H₂, with a boiling point of only 20.27 K and a melting point of 14.02 K. Under extreme pressures, such as those at the center of gas giants, the molecules lose their identity and the hydrogen becomes a liquid metal. Under the extremely low pressure in space—virtually a vacuum—the element tends to exist as individual atoms, simply because there is no way for them to combine. However, clouds of H₂ and singular hydrogen atoms are said to form in H I and H II regions and are associated with star formation, however the existence of singular hydrogen atoms is disputed.. Hydrogen plays a vital role in powering stars through the proton–proton and carbon–nitrogen cycle. These are nuclear fusion processes, which release huge amounts of energy in stars and other hot celestial bodies as hydrogen atoms combine into helium atoms. H₂ is highly soluble in water, alcohol, and ether. It has a high capacity for adsorption, in which it is attached to and held to the surface of some substances. It is an odorless, tasteless, colorless, and highly flammable gas that burns at concentrations as low as 4% H₂ in air. It reacts violently with chlorine and fluorine, forming hydrohalic acids that can damage the lungs and other tissues. When mixed with oxygen, hydrogen explodes on ignition. A unique property of hydrogen is that its flame is completely invisible in air. This makes it difficult to tell if a leak is burning, and carries the added risk that it is easy to walk into a hydrogen fire inadvertently. See also: hydrogen atom.

Applications

Large quantities of hydrogen are needed in the chemical and petrolium industries, notably in the Haber process for the production of ammonia, which by mass ranks as the world's fifth most highly produced industrial compound. Hydrogen is used in the hydrogenation of fats and oils (into items such as margarine), and in the production of methanol. Hydrogen is used in hydrodealkylation, hydrodesulfurization, and hydrocracking. The element has several other important uses.
- The element is used in the manufacture of hydrochloric acid, in welding processes, and in the reduction of metallic ores.
- It is an ingredient in rocket fuels.
- It is used as the rotor coolant in electrical generators at power stations, because it has the highest thermal conductivity of any gas.
- Liquid hydrogen is used in cryogenic research, including superconductivity studies.
- Since hydrogen is 14.5 times lighter than air, it was once widely used as a lifting agent in balloons and airships. However, this use was curtailed when the Hindenburg disaster convinced the public that the gas was too dangerous for this purpose.
- Deuterium, an isotope of hydrogen (hydrogen-2), is used in nuclear fission applications as a moderator to slow neutrons, and in nuclear fusion reactions. Deuterium compounds have applications in chemistry and biology in studies of reaction isotope effects.
- Tritium (hydrogen-3), produced in nuclear reactors, is used in the production of hydrogen bombs, as an isotopic label in the biosciences, and as a radiation source in luminous paints. There are no "hydrogen wells" or "hydrogen mines" on Earth, so hydrogen cannot be considered a primary energy source like fossil fuels or uranium. Hydrogen can however be burned in internal combustion engines, an approach advocated by BMW's experimental hydrogen car. However, it is currently difficult and dangerous to store and handle in sufficient quantity for motor fuel use. Hydrogen fuel cells are being investigated as mobile power sources with lower emissions than hydrogen-burning internal combustion engines. The low emissions of hydrogen in internal combustion engines and fuel cells are currently offset by the pollution created by hydrogen production. This may change if the substantial amounts of electricity required for water electrolysis can be generated primarily from low pollution sources such as nuclear energy or wind. Research is being conducted on hydrogen as a replacement for fossil fuels. It could become the link between a range of energy sources, carriers and storage. Hydrogen can be converted to and from electricity (solving the electricity storage and transport issues), from bio-fuels, and from and into natural gas and diesel fuel. All of this can theoretically be achieved with zero emissions of CO₂ and toxic pollutants.

History

Hydrogen was first produced by Theophratus Bombastus von Hohenheim (1493–1541)—also known as Paracelsus—by mixing metals with acids. He was unaware that the explosive gas produced by this chemical reaction was hydrogen. In 1671, Robert Boyle described the reaction between two iron fillings and dilute acids, which results in the production of gaseous hydrogen. In 1766, Henry Cavendish was the first to recognize hydrogen as a discrete substance, by identifying the gas from this reaction as "inflammable" and finding that the gas produces water when burned in air. Cavendish stumbled on hydrogen when experimenting with acids and mercury. Although he wrongly assumed that hydrogen was a compound of mercury—and not of the acid—he was still able to accurately describe several key properties of hydrogen. Antoine Lavoisier gave the element its name and proved that water is composed of hydrogen and oxygen. One of the first uses of the element was for balloons. The hydrogen was obtained by mixing sulfuric acid and iron. Harold C. Urey discovered Deuterium, an isotope of hydrogen, by repeated distilling the same sample of water. For this discovery, Urey received the Nobel prize for in 1934. In the same year, the third isotope, tritium, was discovered. Because of its relatively simple structure, hydrogen has often been used in models of how an atom works.

Electron energy levels

The ground state energy level of the electron in a Hydrogen atom is 13.6 eV, which is equivalent to an ultraviolet photon of roughly 92 nm. With the Bohr Model the energy levels of Hydrogen can be calculated fairly accurately. This is done by modeling the electron as revolving around the proton, much like the earth revolving around the sun. Except the sun holds earth in orbit with the force of gravity, but the proton holds the electron in orbit with the force of electromagnetism. Another difference between the Earth-Sun system and the Electron-Proton system is that, in this model, due to quantum mechanics the electron is allowed to only be at very specific distances from the proton. Modeling the hydrogen atom in this fashion yields the correct energy levels and spectrum.

Occurrence

quantum mechanics.]] Hydrogen is the most abundant element in the universe, making up 75% of normal matter by mass and over 90% by number of atoms. This element is found in great abundance in stars and gas giant planets. It is very rare in the Earth's atmosphere (1 ppm by volume), because being the lightest gas causes it to escape Earth's gravity, though when compounds are considered, it is the tenth most abundant element on Earth. The most common source for this element on Earth is water, which is composed two parts hydrogen to one part oxygen (H₂O). Other sources include most forms of organic matter (currently all known life forms) including coal, natural gas, and other fossil fuels. Methane (CH₄) is an increasingly important source of hydrogen. Throughout the Universe, hydrogen is mostly found in the plasma state whose properties are quite different to molecular hydrogen. As a plasma, hydrogen's electron and proton are not bound together, resulting in very high electrical conductivity, even when the gas is only partially ionised. The charged particles are highly influenced by magnetic and electric fields, for example, in the Solar Wind they interact with the Earth's magnetosphere giving rise to Birkeland currents and the aurora. Hydrogen can be prepared in several different ways: steam on heated carbon, hydrocarbon decomposition with heat, reaction of a strong base in an aqueous solution with aluminium, water electrolysis, or displacement from acids with certain metals. Commercial bulk hydrogen is usually produced by the steam reforming of natural gas. At high temperatures (700–1100 °C), steam reacts with methane to yield carbon monoxide and hydrogen. :CH₄ + H₂O → CO + 3 H₂ Additional hydrogen can be recovered from the carbon monoxide through the water-gas shift reaction: :CO + H₂O → CO₂ + H₂

Compounds

The lightest of all gases, hydrogen combines with most other elements to form compounds. Hydrogen has an electronegativity of 2.2, so it forms compounds where it is the more nonmetallic and where it is the more metallic element. The former are called hydrides, where hydrogen either exists as H^- ions or just as a solute within the other element (as in palladium hydride). The latter tend to be covalent, since the H⁺ ion would be a bare nucleus and so has a strong tendency to pull electrons to itself. These both form acids. Thus even in an acidic solution one sees ions like hydronium (H₃O⁺) as the protons latch on to something. Although exotic on earth, one of the most common ions in the universe is the H₃⁺ ion. Hydrogen combines with oxygen to form water, H₂O, and releases a lot of energy in doing so, burning explosively in air. Deuterium oxide, or D₂O, is commonly referred to as heavy water. Hydrogen also forms a vast array of compounds with carbon. Because of their association with living things, these compounds are called organic compounds, and the study of the properties of these compounds is called organic chemistry. organic chemistry

Forms

Under normal conditions, hydrogen gas is a mix of two different kinds of molecules which differ from one another by the relative spin of the nuclei. These two forms are known as ortho- and para-hydrogen (this is different from isotopes, see below). In ortho-hydrogen the nuclear spins are parallel (form a triplet), while in para they are antiparallel (form a singlet). At standard conditions hydrogen is composed of about 25% of the para form and 75% of the ortho form (the so-called "normal" form). The equilibrium ratio of these two forms depends on temperature, but since the ortho form has higher energy (is an excited state), it cannot be stable in its pure form. In low temperatures (around boiling point), the equilibrium state is comprised almost entirely of the para form. The conversion process between the forms is slow, and if hydrogen is cooled down and condensed rapidly, it contains large quantities of the ortho form. It is important in preparation and storage of liquid hydrogen, since the ortho-para conversion produces more heat than the heat of its evaporation, and a lot of hydrogen can be lost by evaporation in this way during several days after liquefying. Therefore, some catalysts of the ortho-para conversion process are used during hydrogen cooling. The two forms have also slightly different physical properties. For example, the melting and boiling points of parahydrogen are about 0.1 K lower than of the "normal" form.

Isotopes

Hydrogen is the only element that has different names for its isotopes. (During the early study of radioactivity, various heavy radioactive isotopes were given names, but such names are no longer used, although one element, radon, has a name that originally applied to only one of its isotopes.) The symbols D and T (instead of ²H and ³H) are sometimes used for deuterium and tritium, although this is not officially sanctioned. (The symbol P is already in use for phosphorus and is not available for protium.) ;¹H The most common isotope of hydrogen, this stable isotope has a nucleus consisting of a single proton; hence the descriptive, although rarely used, name protium. The spin of a protium atom is 1/2+. ;²H The other stable isotope is deuterium, with an extra neutron in the nucleus. Deuterium comprises 0.0184%–0.0082% of all hydrogen (IUPAC); ratios of deuterium to protium are reported relative to the VSMOW standard reference water. The spin of a deuterium atom is 1+. ;³H The third naturally occurring hydrogen isotope is the radioactive tritium. The tritium nucleus contains two neutrons in addition to the proton. It decays through beta decay and has a half-life of 12.32 years. Tritium occurs naturally due to cosmic rays interacting with atmospheric gases. Like ordinary hydrogen, tritium reacts with the oxygen in the atmosphere to form T₂O. This radioactive "water" molecule constantly enters the Earth's seas and lakes in the form of slightly radioactive rain, but its half-life is short enough to prevent a buildup of hazardous radioactivity. The spin of a tritium atom is 1/2+. ;⁴H Hydrogen-4 was synthesized by bombarding tritium with fast-moving deuterium nuclei. It decays through neutron emission and has a half-life of 9.93696x10^-23 seconds. The spin of a hydrogen-4 atom is 2-. ;⁵H In 2001 scientists detected hydrogen-5 by bombarding a hydrogen target with heavy ions. It decays through neutron emission and has a half-life of 8.01930x10^-23 seconds. ;⁶H Hydrogen-6 decays through triple neutron emission and has a half-life of 3.26500^-22 seconds. ;⁷H In 2003 hydrogen-7 was created ([http://physicsweb.org/articles/news/7/3/3 article]) at the RIKEN laboratory in Japan by colliding a high-energy beam of helium-8 atoms with a cryogenic hydrogen target and detecting tritons—the nuclei of tritium atoms—and neutrons from the breakup of hydrogen-7, the same method used to produce and detect hydrogen-5.

References

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- [http://www.riken.go.jp/engn/r-world/research/lab/wako/ribeam/ RIKEN Beam Science Laboratory, Japan - Heavy hydrogen research]
- [http://chartofthenuclides.com/default.html Nuclides and Isotopes] Fourteenth Edition: Chart of the Nuclides, General Electric Company, 1989 ;Book references:
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External links

- [http://www.hydropole.ch/Hydropole/Intro/Phasediag.gif Hydrogen phase diagram.]
- [http://www.compchemwiki.org/index.php?title=Hydrogen Computational Chemistry Wiki] Category:Nonmetals Category:Fuels Category:Chemical elements ko:수소 ms:Hidrogen ja:水素 simple:Hydrogen th:ไฮโดรเจน

Natural abundance

Natural abundance refers to the prevalence of different isotopes of an element as found in nature. The weighted (by natural abundance) average mass of these isotopes is the atomic weight listed for the element in the periodic table. The abundance of an element varies from planet to planet. Percent natural abundances refer to the relative proportions, expressed as percentages by number, in which the isotopes of an element are found in natural sources. Category:Chemical properties ja:天然存在比

Atomic nucleus

The nucleus of an atom is the very dense region in its center consisting of protons and neutrons. The size of the nucleus is much smaller than the size of the atom itself, and almost all of the mass in an atom is made up from the protons and neutrons with almost no contribution from the electrons.

Nuclear Makeup

The nucleus of an atom is made up of very tightly bound protons and neutrons. The electromagnetic force which causes like charges to repel prevents protons from binding together without neutrons (it would blow such a nucleus apart). When neutrons and protons are in very close proximity they are held together by the strong nuclear force. The strong force is much much stronger than gravity or the electromagnetic force, but because it only works over very short distances (as opposed to gravity and electromagnetism which have infinite range) we don't usually notice it in everyday life. The element hydrogen is the only element which exists whose nuclei doesn't need neutrons to hold it together, and this is because the hydrogen nucleus consists of only a single proton. The stable form of helium, the next lightest element, has two protons and two neutrons. Most of the light elements are stable when they have roughly even numbers of protons and neutrons, but as elements get heavier and heavier they need more neutrons to stay together.

Isotopes

The isotope of an atom is determined by the number of neutrons in the nucleus. Different isotopes of the same element have very similar chemical properties because chemical reactions depend almost entirely on the number of electrons that an atom has. Different isotopes in a sample of a particular chemical can be separated by using a centrifuge or by using a spectrometer. The first method is used in producing enriched uranium from a sample of regular uranium, and the second is used in carbon dating. The number of protons and neutrons together determine the nuclide (type of nucleus). Protons and neutrons have nearly equal masses, and their combined number, the mass number, is approximately equal to the atomic mass of an atom. The combined mass of the electrons is very small in comparison to the mass of the nucleus, since protons and neutrons weigh roughtly 2000 times more than electrons.

Nuclear Decay

If a nucleus has too few or too many neutrons it may be unstable, and will decay after some period of time. For example, nitrogen atoms with 16 neutrons (nitrogen-16) beta decays to oxygen-16 within a few seconds of being created. In this decay a neutron in the nitrogen nucleus is turned into a proton and an electron by the weak nuclear force. The element of the atom changes because while it previously had seven protons (which makes it nitrogen) it now has eight (which makes it oxygen). Many elements have multiple isotopes which are stable for weeks, years, or even billions of decades duh.

Nucleus Size

The radius of a nucleon (neutron or proton) is of the order of 1 fm (femtometre = 10^-15 m). The nuclear radius, which can be approximated by the cubic root of the mass number times 1.2 fm, is less than 0.01 % of the radius of the atom. Thus the density of the nucleus is more than a trillion times that of the atom as a whole. One solid cubic millimetre of nuclear material, if compressed together, would have a mass of around 200,000 tonnes. Neutron stars are composed of such material.

History

The discovery of the electron was the first indication that the atom had internal structure. At the turn of the 20th century the accepted model of the atom was JJ Thomson's "plum pudding" model in which the atom was a large positively charged ball with small negatively charged electrons embedded inside of it. By the turn of the century physicists had also discovered three types of radiation coming from atoms, which they named alpha, beta, and gamma radiation. Experiments in 1911 by Lise Meitner and Otto Hahn, and by James Chadwick in 1914 discovered that the beta decay spectrum was continuous rather than discrete. That is, electrons were ejected from the atom with a range of energies, rather than the discrete amounts of energies that were observed in gamma and alpha decays. This was a problem for nuclear physics at the time, because it indicated that energy was not conserved in these decays. Around the same time that this was happening (1911) Ernest Rutherford performed a remarkable experiment in which he fired alpha particles (helium nuclei) at a thin film of gold foil. The plum pudding model predicted that the alpha particles should come out of the foil with their trajectories being at most slightly bent. He was shocked to discover that many particles were scattered through large angles, even completely backwards in some cases. The discovery led to the Rutherford model of the atom, in which the atom has a very small, very dense nucleus made up of heavy positivly charged particles with embedded electrons in order to balance out the charge. As an example, in this model nitrogen-14 was made up of a nucleus with 14 protons and 7 electrons, and the nucleus was surrounded by 7 more orbiting electrons. The Rutherford model worked quite well until studies of nuclear spin were carried out by Franco Rasetti at the California Institute of Technology in 1929. By 1925 it was known that protons and electrons had a spin of 1/2, and in the Rutherford model of nitrogen-14 the 14 protons and six of the electrons should have paired up to cancel each others spin, and the final electron should have left the nucleus with a spin of 1/2. Rasetti discovered, however, that nitrogen-14 has a spin of one. In 1930 Wolfgang Pauli was unable to attend a meeting in Tubingen, and instead sent a famous letter with the classic introduction "Dear Radioactive Ladies and Genlemen". In his letter Pauli suggested that perhaps there was a third particle in the nucleus which he named the neutron. He suggested that it was very light (lighter than an electron), had no charge, and that it did not readily interact with matter (which is why it hadn't yet been detected). This desparate way out solved both the problem of energy conservation and the spin of nitrogen-14, the first because the neutron was carrying away the extra energy and the second because an extra neutron paired off with the elecrton in the nitrogen-14 nucleus giving it spin one. Pauli's neutron was renamed the neutrino (italian for little neutral one) by Enrico Fermi in 1931, and after about thirty years it was finally demonstrated that a neutrino really is emitted during beta decay. In 1932 Chadwick realized that radiation that had been observed by Walther Bothe, Herbert Becker, Irene and Frédéric Joliot-Curie was actually due to a massive particle that he called the neutron. In the same year Dmitrij Iwanenko suggested that neutrons were in fact spin 1/2 particles and that the nucleus contained neutrons and that there were no electrons in it, and Francis Perrin suggested that neutrinos were not nuclear particles but were created during beta decay. To cap the year off Fermi submitted a theory of the neutrino to Nature (which the editors rejected for being "too remote from reality"). Fermi continued working on his theory and published a paper in 1934 which placed the neutrino on solid theoretical footing. In the same year Hideki Yukawa proposed the first significant theory of the strong force to explain how the nucleus holds together. With Fermi and Yukawa's papers the modern model of the atom was complete. The center of the atom contains a tight ball of neutrons and protons, which is held together by the strong nuclear force. Unstable nuclei may undergo alpha decay, in which they emit an energetic helium nuclei, beta decay, in which they eject an electron (or positron). After one of these decays the resultant nucleus may be left in an excited state, and in this case it decays to its ground state by emitting high energy photons (gamma decay). The study of the strong and weak nuclear forces led physicists to collide nuclei and electrons at ever higher energies. This research became the science of particle physics, the crown jewel of which is the standard model of particle physics which unifies the strong, weak, and electromagnetic forces.

Nuclear Fusion

When two light nuclei come into very close contact with each other it is possible for the strong force to fuse the two together. It takes a great deal of energy to push the nuclei close enough together for the strong force to have an effect, so the process of nuclear fusion can only take place at very high temperatures or high densities. Once the nuclei are close enough together the strong force overcomes their electromagnetic repulsion and squishes them into a new nucleus. A very large amount of energy is released when light nuclei fuse together because the binding energy per nucleon increases with atomic number up until iron. Stars like our sun are powered by the fusion of four protons into a helium nucleus, two positrons, and two neutrinos. The fusion of hydrogen into helium is also the source of energy for thermonuclear weapons.

Nuclear Fission

After iron the binding energy per nucleon begins decreasing, so it is possible for energy to be released if a heavy nucleus breaks apart into two lighter ones. This splitting of atoms is known as nuclear fission. This is the source of energy for nuclear power plants and conventional nuclear bombs like the two that the United States used to destroy the buildings and civilians of Hiroshima and Nagasaki. Nuclear reactions occur naturally on Earth, and are in fact quite common. These include alpha decay and beta decay, and heavy nuclei such as uranium may also undergo spontanious fission. There is even one known example of a naturally occurring fission reactor, which was active in Oklo, Gabon, Africa over 1.5 billion years ago. [http://www.ans.org/pi/np/oklo/]

Production of Heavy Elements

As the Universe cooled after the big bang it eventually became possible for particles as we know them to exist. The most common particles created in the big bang which are still easily observable to us today were protons (hydrogen) and electrons (in equal numbers). Some heavier elements were created as the protons colided with each other, but most of the heavy elements we see today were created inside of stars during a series of fusion stages, such as the proton-proton chain, the CNO cycle and the triple-alpha process. Progressively heavier elements are created during the evolution of a star. Since the binding energy per nucleon peaks around iron, energy is only released in fusion processes occurring below this point. Since the creation of heavier nuclei by fusion costs energy, nature resorts to the process of neutron capture. Neutrons (due to their lack of charge) are readily absorbed by a nucleus. The heavy elements are created by either a slow neutron capture process (the so-called s process) or by the rapid, or r process. The s process occurs in thermally pulsing stars (called AGB, or asymptotic giant branch stars) and takes hundreds to thousands of years to reach the heaviest elements of lead and bismuth. The r process is thought to occur in supernova explosions due to the fact that the conditions of high temperature, high neutron flux and ejected matter are present. These stellar conditions make the successive neutron captures very fast, involving very neutron-rich species which then beta-decay to heavier elements, especially at the so-called waiting points that correspond to more stable nuclides with closed neutron shells (magic numbers). The r process duration is typically in the range of a few seconds.

Nuclear Physics

A heavy nucleus can contain hundreds of nucleons (neutrons and protons), which means that to some approximation it can be treated as a classical system, rather than a quantum-mechanical one. In the resulting liquid-drop model, the nucleus has an energy which arises partly from surface tension and partly from electrical repulsion of the protons. The liquid-drop model is able to reproduce many features of nuclei, including the general trend of binding energy with respect to mass number, as well as the phenomenon of nuclear fission. Superimposed on this classical picture, however, are quantum-mechanical effects, which can be described using the nuclear shell model, developed in large part by Maria Goeppert-Mayer. Nuclei with certain numbers of neutrons and protons (the magic numbers 2, 8, 20, 50, 82, 126, ...) are particularly stable, because their shells are filled. Much of current research in nuclear physics relates to the study of nuclei under extreme conditions such as high spin and excitation energy. Nuclei may also have extreme shapes (similar to that of american footballs) or extreme neutron-to-proton ratios. Experimenters can create such nuclei using artificially induced fusion or nucleon transfer reactions, employing ion beams from a accelerator. Beams with even higher energies can be used to create nuclei at very high temperatures, and there are signs that these experiments have produced a phase transition from normal nuclear matter to a new state, the quark-gluon plasma, in which the quarks mingle with one another, rather than being segregated in triplets as they are in neutrons and protons.

External links

- [http://www.sckcen.be/ SCK.CEN Belgian Nuclear Research Centre] Mol, Belgium Category:Nuclear chemistry Category:Nuclear physics Category:Subatomic particles ko:원자핵 ja:原子核 th:นิวเคลียสอะตอม

Proton

:For alternative meanings see proton (disambiguation). In physics, the proton (Greek proton = first) is a subatomic particle with an electric charge of one positive fundamental unit (1.602 × 10⁻¹⁹ coulomb) and a mass of 938.3 MeV/c² (1.6726 × 10⁻²⁷ kg), or about 1836 times the mass of an electron. The proton is observed to be stable, with a lower limit on its half-life of about 10³⁵ years, although some theories predict that the proton may decay. The proton and neutron are both nucleons. The nucleus of the most common isotope (called protium) of the hydrogen atom is a single proton. The nuclei of other atoms are composed of protons and neutrons held together by the strong nuclear force. The number of protons in the nucleus determines the chemical properties of the atom and which chemical element it is. Protons are classified as baryons and are composed of two up quarks and one down quark, which are also held together by the strong nuclear force, mediated by gluons. The proton's antimatter equivalent is the antiproton, which has the same magnitude charge as the proton but the opposite sign. In chemistry and biochemistry, the term proton may refer to the hydrogen ion, H⁺. In this context, a proton donor is an acid and a proton acceptor a base (see acid-base reaction theories).

History

Ernest Rutherford is generally credited with the discovery of the proton. In 1918 Rutherford noticed that when alpha particles were shot into nitrogen gas, his scintillation detectors showed the signatures of hydrogen nuclei. Rutherford determined that the only place this hydrogen could have come from was the nitrogen, and therefore nitrogen must contain hydrogen nuclei. He thus suggested that the hydrogen nucleus, which was known to have an atomic number of 1, was an elementary particle. Prior to Rutherford, Eugene Goldstein had observed canal rays, which were composed of positively charged ions.

Technological applications

Protons can exist in spin states. This property is exploited by nuclear magnetic resonance spectroscopy. In NMR spectroscopy, a magnetic field is applied to a substance in order to detect the shielding around the protons in the nuclei of that substance, which is provided by the surrounding electron clouds. Scientists can use this information to then construct the molecular structure of the molecule under study.

Antiproton

The antiproton is the antiparticle of the proton. It was discovered in the year 1955 by Emilio Segre and Owen Chamberlain, for which they were awarded a 1959 Nobel Prize in Physics. CPT-symmetry puts strong constraints on the relative properties of particles and antiparticles and, therefore, is open to stringent tests. For example, the charges of the proton and antiproton must sum to exactly zero. This equality has been tested to one part in 10^-8. The equality of their masses is also tested to better than one part in 10^-8. By holding antiprotons in a Penning trap, the equality of the charge to mass ratio of the proton and the antiproton has been tested to 1 part in 9×10^-11. The magnetic moment of the antiproton has been found with error of 8×10^-3 nuclear Bohr magnetons, and is found to be equal and opposite to that of the proton.

External links

- [http://pdg.lbl.gov/ Particle Data Group] Category:Nucleon ko:양성자 ms:Proton ja:陽子 simple:Proton th:โปรตอน

Chemical symbol

:See also chemical formula. A chemical symbol is an abbreviation or short representation of the name of a chemical element. Natural elements all have symbols of one or two letters; some man-made elements have temporary symbols of three letters. Chemical symbols are listed in the periodic table and are used as shorthand and in chemical equations, e.g., :

2H_2 + O_2\ \rightarrow\ 2H_2O

. Because chemical symbols are often derived from the Latin or Greek name of the element, they may not bear much similar to the common English name, e.g., Na for sodium (Latin natrium) and Au for gold (Latin aurum). In China, each chemical element is assigned an ideograph as its symbol; most of them have been explicitly created for this purpose (see Chinese characters for chemical elements). Chemical symbols may also be changed to show if one particular isotope of an atom that is specified, as well as to show other attributes such as ionization and oxidation state of a chemical compound. For complete listings of the chemical elements and their symbols, see:
- List of elements by symbol
- List of elements by name
- List of elements by number
- Periodic table of the elements
- Category:Symbols ko:원소 기호 ja:元素記号

Chemical element

A chemical element, often called simply element, is a chemical substance that canot be divided or changed into other chemical substances by any ordinary chemical technique. The smallest unit of this kind of chemical substances is an atom. An element is a class of substances that contain the same number of protons in all its atoms.

Chemistry terminology

Earlier an element or pure element was defined as a substance which "cannot be further broken down into another compound with different chemical properties" -- which should be taken to mean it consists of atoms of one element. However, due to allotropy, the isotope effect, and the confusion with the more useful term referring to the general class of atoms (irrespective of what compound it may be in), this usage is in disfavor amongst contemporary chemists, and sees restricted, mostly historical, use. This definition was motivated by the observation that these elements could not be dissociated by chemical means into other compounds. For example, water could be converted into hydrogen and oxygen, but hydrogen and oxygen could not be further decomposed, thus "elemental". There are also many counterexamples (for example "elemental oxygen" (O₂) can be decomposed by solely chemical means into oxygen ions and atoms which have drastically different chemical properties). The remainder of this article will concern itself with the first definition.

Description

The atomic number of an element, Z, is equal to the number of protons which defines the element. For example, all carbon atoms contain 6 protons in their nucleus, so for carbon Z=6. These atoms may have different amounts of neutrons, and are known as isotopes of the element. The atomic mass of an element, A, is measured in unified atomic mass units (u) is the average mass of all the atoms of the element in an environment of interest (usually the earth's crust and atmosphere). Since electrons are light, and neutrons are barely more than the mass of the proton, this usually corresponds to the sum of the protons and neutrons in the nucleus of the most abundant isotope, though this is not always the case (notably chlorine, which is about three-quarters ³⁵Cl and a quarter ³⁷Cl). Some isotopes are radioactive and decay into other elements upon radiating an alpha or beta particle. Some elements have no nonradioactive isotopes, in particular all elements with Z >= 84. The lightest elements are hydrogen and helium. Hydrogen is thought to be the first element to appear after the Big Bang. All the heavier elements, are made naturally and artificially through various methods of nucleosynthesis. As of 2005, there are 116 known elements: 93 occur naturally on earth (including technetium and plutonium), and 94 (including promethium) have been detected so far in the universe. The 23 elements not found on earth are derived artificially; the first purportedly synthesized element was technetium, in 1937, although the trace amounts of naturally occurring technetium were not known then. All artificially derived elements are radioactive with short half-lives so that any such atoms that were present at the formation of Earth are extremely likely to have already decayed. Lists of the elements by name, by symbol, by atomic number, by density, by melting point and by boiling point are available. The most convenient presentation of the elements is in the periodic table, which groups elements with similar chemical properties together.

Nomenclature

The naming of elements precedes the atomic theory of matter, although at the time it was not known which chemicals were elements and which compounds. When it was learned, existing names (e.g., gold, mercury, iron) were kept in most countries, and national differences emerged over the names of elements either for convenience, linguistic niceties, or nationalism. For example, the Germans use "Wasserstoff" for "hydrogen" and "Sauerstoff" for "oxygen," while some romance languages use "natrium" for "sodium" and "kalium" for "potassium," and the French prefer the obsolete but historic term "azote" for "nitrogen." But for international trade, the official names of the chemical elements both ancient and recent are decided by the International Union of Pure and Applied Chemistry, which has decided on a sort of international English language. That organization has recently prescribed that "aluminium" and "caesium" take the place of the US spellings "aluminum" and "cesium," while the US "sulfur" takes the place of the British "sulphur." But chemicals which are practicable to be sold in bulk within many countries, however, still have national names, and those which do not use the Latin alphabet cannot be expected to use the IUPAC name. According to IUPAC, the full name of an element is not capitalized, even if it is derived from a proper noun (unless it would be capitalized by some other rule, for instance if it begins a sentence). And in the second half of the twentieth century physics laboratories became able to produce nuclei of chemical elements that have too quick a decay rate to ever be sold in bulk. These are also named by IUPAC, which generally adopts the name chosen by the discoverer. This can lead to the controversial question of which research group actually discovered an element, a question which delayed the naming of elements with atomic number of 104 and higher for a considerable time. (See element naming controversy). Precursors of such controversies involved the nationalistic namings of elements in the late nineteenth century (e.g., as "lutetium" refers to Paris, France, the Germans were reticent about relinquishing naming rights to the French, often calling it "cassiopeium"). And notably, the British discoverer of "niobium" originally named it "columbium," after the New World, though this did not catch on in Europe. The Americans had to accept the international name just when it was becoming an economically important material late in the twentieth century.

Chemical symbols

Specific chemical elements

Before chemistry became a science, alchemists had designed arcane symbols for both metals and common compounds. These were however used as abbreviations in diagrams or procedures; there was no concept of one atoms combining to form molecules. With his advances in the atomic theory of matter, John Dalton devised his own simpler symbols, based on circles, which were to be used to depict molecules. These were superseded by the current typographical system in which chemical symbols are not used as mere abbreviations though each consists letters of the Latin alphabet - they are symbols intended to be used by peoples of all languages and alphabets. The first of these symbols were intended to be fully international, for they were based on the Latin abbreviations of the names of metals: Fe comes from Ferrum; Ag from Argentum. The symbols were not followed by a period (full stop) as abbreviations were. Besides a name, later chemical elements are also given a unique chemical symbol, based on the name of the element, not necessarily derived from the colloquial English name. (e.g., sodium has chemical symbol 'Na' after the Latin natrium). The same applies to "W" (wolframium) for Tungsten , "Hg" (Hydrargyrum) for mercury and "K" for potassium. Stricly taken, a symbol like Tu for tungsten or M or Me for mercury seems to be more logical. Chemical symbols are understood internationally when element names might need to be translated. There are sometimes differences; for example, the Germans have used "J" instead of "I" for iodine, so the character would not be confused with a roman numeral. The first letter of a chemical symbol is always capitalized, as in the preceding examples, and the subsequent letters, if any, are always minuscule (small letters).

General chemical symbols

There are also symbols for series of chemical elements, for comparative formulas. These are one capital letter in length, and the letters are reserved so they are not permitted to be given for the names of specific elements. For example, an "X" is used to indicate a variable group amongst a class of compounds (though usually a halogen), while "R" is used for a radical (not to be confused with radical_(chemistry), meaning a compound structure such as a hydrocarbon chain. The letter "Q" is reserved for "heat" in a chemical reaction. "Y" is also often used as a general chemical symbol, although it is also the symbol of Yttrium. "Z" is also frequently used as a general variable group. "L" is used to represent a general ligand in inorganic and organometallic chemistry. "M" is also often used in place of a general metal.

Nonelement symbols

Nonelements, especially in organic and organometallic chemistry, often acquire symbols which are inspired by the elemental symbols. A few examples: Cy - cyclohexyl; Ph - phenyl; Bz - benzoyl; Bn - benzyl; Cp - Cyclopentadiene; Pr - propyl; Me - methyl; Et - ethyl; Tf - triflate; Ts - tosyl.

External links

- [http://www.vanderkrogt.net/elements/ Elementymology & Elements Multidict] word history and language dictionary

Chemical information

- [http://www.webelements.com/ WebElements]
- [http://www.vcs.ethz.ch/chemglobe/ptoe/ ChemGlobe]
- [http://pearl1.lanl.gov/periodic/default.htm Los Alamos National Laboratory]
- [http://www.chemicalelements.com/ ChemicalElements] ko:화학 원소 ms:Unsur kimia ja:元素 simple:Element th:ธาตุเคมี

Mass spectrometry

Mass spectrometry is an analytical technique which determines the mass-to-charge (m/z) ratio of ions. It is most generally used to find the composition of a physical sample by generating a mass spectrum representing the masses of the components of a sample. It has several broad applications: # Identifying unknown compounds by the mass of the compound and/or fragments thereof. # Determining the isotopic composition of one or more elements in a compound. # Determining the structure of compounds by observing the fragmentation of the compound. # Quantitating the amount of a compound in a sample using carefully designed methods (mass spectrometry is not inherently quantitative). # Studying the fundamentals of gas phase ion chemistry (the chemistry of ions and neutrals in vacuum). # Determining other physical, chemical or even biological properties of compounds with a variety of other approaches. A mass spectrometer is a device used for mass spectrometry, and produces a mass spectrum of a sample to find its composition. This is normally achieved by ionizing the sample and separating ions of differing masses and recording their relative abundance by measuring intensities of ion flux. A typical mass spectrometer comprises three parts: an ion source, a mass analyzer, and a detector.

How it works in layman terms

Different molecules have different masses, and this fact is used in a mass spectrometer to determine what molecules are present in a sample. For example, table salt (Na Cl), is vaporized (turned into gas) and broken down (ionized) into electrically charged particles, called ions, in the first part of the mass spectometer. The sodium ions and chloride ions have specific molecular weights. They also have a charge, which means that they will be moved under the influence of an electric field. These ions are then sent into an ion acceleration chamber and passed through a slit in a metal sheet. A magnetic field is applied to the chamber, which pulls on each ion equally and deflects them (makes them curve instead of travelling straight) onto a detector. The lighter ions deflect further than the heavy ions because the force on each ion is equal but their masses are not (this is derived from the equation

F=ma

which states that if the force remains the same, the mass and acceleration are inversely proportional). The detector measures exactly how far each ion has been deflected, and from this measurement, the ion's 'mass to charge ratio' can be worked out. From this information it is possible to determine with a high level of certainty what the chemical composition of the original sample was. This example was of a sector instrument, however there are many types of mass spectrometers that not only analyze the ions differently but produce different types of ions; however they all use electric and magnetic fields to change the path of ions in some way.

Instrumentation

Ion source

The ion source is the part of the mass spectrometer that ionizes the material under analysis (the analyte). The ions are then transported by magnetic or electrical fields to the mass analyzer. Techniques for ionization have been key to determining what types of samples can be analyzed by mass spectrometry. Electron ionization and chemical ionization are used for gases and vapors. In chemical ionization sources, the analyte is ionized by chemical ion-molecule reactions during collisions in the source. Two techniques often used with liquid and solid biological samples include electrospray ionization (due to John Fenn) and matrix-assisted laser desorption/ionization (MALDI, due to M. Karas and F. Hillenkamp). Inductively coupled plasma sources are used primarily for metal analysis on a wide array of samples types. Others include fast atom bombardment (FAB), thermospray, atmospheric pressure chemical ionization (APCI), secondary ion mass spectrometry (SIMS) and thermal ionisation.

Mass analyzer

Mass analyzer separate the ions according to their mass per charge (m/z). There are many types of mass analyzers. Usually they are categorized based on the principles of operation. Sector MS: It uses an electric and/or magnetic field to affect the path and/or velocity of the charged particles in some way. The force exerted by electric and magnetic fields are defined by the Lorentz force law: :

\mathbf
  = q (\mathbf + \mathbf \times \mathbf),

where E is the electric field strength, B is the magnetic field induction, q is the charge of the particle, v is its current velocity (expressed as a vector), and × is the cross product. All mass analyzers use the Lorentz forces in some way either statically or dynamically in mass-to-charge determination. As shown above, sector instruments change the direction of ions that are flying through the mass analyzer. The ions enter a magnetic or electric field which bends the ion paths depending on their mass-to-charge ratios (m/z), deflecting the more charged and faster-moving, lighter ions more. The ions eventually reach the detector and their relative abundances are measured. The analyzer can used to select a narrow range of m/z's or to scan through a range of m/z's to catalog the ions present. Besides the original magnetic-sector analyzers, several other types of analyzer are now more common, including time-of-flight, quadrupole ion trap, quadrupole and Fourier transform ion cyclotron resonance mass analyzers. TOFMS: Perhaps the easiest to understand is the Time-of-flight (TOF) analyzer. It boosts ions to the same kinetic energy by passage through an electric field and measures the times they take to reach the detector. Although the kinetic energy is the same, the velocity is different so the lighter more highly charged ion will reach the detector first. QMS: Quadrupole mass analyzers use oscillating electrical fields to selectively stabilize or destabilize ions passing through a RF quadrupole field. QIT: The quadrupole ion trap works on the same physical principles as the QMS, but the ions are traped and sequentially ejected. Ions are created and trapped in a mainly quadrupole RF potential and separated by m/z, non-destructively or destructively. There are many mass/charge separation and isolation methods but most commonly used is the mass instability mode in which the RF potential is ramped so that the orbit of ions with a mass

a>b

are stable while ions with mass b become unstable and are ejected on the z-axis onto a detector. The cylindrical ion trap mass spectrometer is a derivative of the quadrupole ion trap mass spectrometer. : See also the main article on quadrupole ion trap mass spectrometer Linear QIT: In the linear quadrupole ion trap the ions are trapped in a 2D quadrupole filed instead of the 3D quadrupole field of the QIT. FTMS: Fourier transform mass spectrometry measures mass by detecting the image current produced by ions cyclotroning in the presence of a magnetic field. Instead of measuring the deflection of ions with a detector such as a electron multiplier, the ions are injected into a Penning trap (a static electric/magnetic ion trap) where they effectively form part of a circuit. Detectors at fixed positions in space measure the electrical signal of ions which pass near them over time producing cyclical signal. Since the frequency of the ions' cycling is determined by its mass to charge ratio, this can be deconvoluted by performing a Fourier transform on the signal. FTMS has the advantage of improved sensitivity (since each ion is 'counted' more than once) as well as much higher resolution and thus precision. : See also the main article on Fourier transform ion cyclotron resonance Each analyzer type has its strengths and weaknesses. In addition, there are many more less common mass analyzers. Many mass spectrometers use two or more mass analyzers for tandem mass spectrometry (MS/MS).

Detector

The final element of the mass spectrometer is the detector. The detector records the charge induced or current produced when an ion passes by or hits a surface. In a scanning instrument the signal produced in the detector during the course of the scan versus where the instrument is in the scan (at what m/z) will produce a mass spectrum, a record of how many ions of each m/z are present. Typically, some types of electron multiplier is used, though other detectors (such as Faraday cups) have been used. Because the number of ions leaving the mass analyzer at a particular instant is typically quite small, significant amplification is often necessary to get a signal. Microchannel Plate Detectors are commonly used in modern commercial instruments. In FTMS, the detector consists of a pair of metal plates within the mass analyzer region which the ions only pass near. No DC current is produced, only a weak AC image current is produced in a circuit between the plates.

Hyphenated MS

Gas chromatography/MS

: See also the main article on Gas chromatography-mass spectrometry A common form of mass spectrometry is gas chromatography-mass spectrometry (GC/MS or GC-MS). In this technique, a gas chromatograph is used to separate compounds. This stream of separated compounds is fed on-line into the ion source, a metallic filament to which voltage is applied. This filament emits electrons which ionize the compounds. The ions can then further fragment, yielding predictable patterns. Intact ions and fragments pass into the mass spectrometer's analyser and are eventually detected.

Liquid chromatography/MS

: See also the main article on Liquid chromatography-mass spectrometry Similar to gas chromatography MS (GC/MS), liquid chromatography mass spectrometry (LC/MS or LC-MS) separates compounds chromatographically before they are introduced to the ion source and mass spectrometer. It differs from GC/MS in that the mobile phase is liquid, usually a combination of water and organic solvents, instead of gas. Most commonly, an electrospray ionization source is used in LC/MS.

IMS/MS

Ion mobility spectrometry/mass spectrometry is a technique where ions are first separated by drift time through some pressure of neutral gas given an electrical potential gradient before being introduced into a mass spectrometer. The drift time is a measure of the radius relative to the charge of the ion. The duty cycle of IMS (time over which the experiment takes place) is longer than most mass spectrometers such that the mass spectrometer can sample along the course of the IMS separation. This produces data about the IMS separation and the mass-to-charge ratio of the ions in a manner similar to LC/MS. Note, however, that the duty cycle of IMS is short relative to liquid chromatography or gas chromatography separations and can thus be coupled to such techniques producing triply hyphenated techniques such as LC/IMS/MS.

Tandem MS (MS/MS)

Tandem mass spectrometry involves multiple steps of mass selection or analysis, usually separated by some form of fragmentation. A tandem mass spectrometer is one capable of multiple rounds of mass spectrometry. For example, one mass analyzer can isolate one peptide from many entering a mass spectrometer. A second mass analyzer then stabilizes the peptide ions while they collide with a gas, causing them to fragment by collision-induced dissociation (CID). A third mass analyzer then catalogs the fragments produced from the peptides. Tandem MS can also be done in a single mass analyzer over time as in a quadrupole ion trap. There are various methods for fragmenting molecules for tandem MS, including collision-induced dissociation (CID), electron capture dissociation (ECD), infrared multiphoton dissociation (IRMPD) and blackbody infrared radiative dissociation (BIRD).

Applications

Isotope ratio MS

Mass spectrometry is also used to determine the isotopic composition of elements within a sample. Differences in mass among isotopes of an element are very small, and the less abundant isotopes of an element are typically very rare, so a very sensitive instrument is required. These instruments, sometimes referred to as isotope ratio mass spectrometers (IR-MS), usually use a single magnet to bend a beam of ionized particles towards a series of Faraday cups which convert particle impacts to electric current. A fast on-line analysis of deuterium content of water can be done using Flowing afterglow mass spectrometry, FA-MS. Probably the most sensitive and accurate mass spectrometer for this purpose is the accelerator mass spectrometer (AMS). Isotope ratios are important markers of a variety of processes. Some isotope ratios are used to determine the age of materials for example as in carbon dating.

Trace Gas Analysis

Several techniques use ions created in a dedicated ion source injected into a flow tube or a drift tube: selected ion flow tube (SIFT-MS), and proton transfer reaction (PTR-MS), are variants of chemical ionization dedicated for trace gas analysis of air, breath or liquid headspace using well defined reaction time allowing calculations of analyte concentrations from the known reaction kinetics without the need for internal standard or calibration.

Pharmcokinetics

Pharmacokinetics is often studied using mass spectrometry due to the complex nature of the matrix (often blood or urine) and the need for high sensitivity to observe low dose and long time point data. The most common instrumentation used in this application is LC-MS with a triple quadrupole mass spectrometer. Tandem mass spectrometry is usually employed for added specificity. Standard curves and internal standards are used for quantitation of usually a single pharmaceutical in the samples. The samples represent different time points as a pharmaceutical is administered and then metabolized or cleared from the body. Blank or t=0 samples taken before administration are important in determining background and insuring data integrity with such complex sample matrices. Much attention is paid to the linearity of the standard curve however it is not uncommon to use curve fitting with more complex functions such as quadratics since the response of most mass spectrometers is less than linear across large concentration ranges.

Mass spectrometry of proteins

Mass spectrometry is an important emerging method for the characterization of proteins. The two primary methods for ionization of whole proteins are electrospray ionization and matrix-assisted laser desorption ionization (MALDI). In keeping with the performance and mass range of available mass spectrometers, two approaches are used for characterizing proteins. In the first, intact proteins are ionized by either of the two techniques described above, and then introduced to a mass analyser. In the second, proteins are enzymatically digested into smaller peptides using an agent such as trypsin or pepsin. Other proteolytic digest agents are also used. The collection of peptide products are then introduced to the mass analyser. This is often referred to as the "bottom-up" approach of protein analysis. Whole protein mass analysis is primarily conducted using either time-of-flight (TOF) MS, or Fourier transform ion cyclotron resonance. These two types of instrument are preferable here because of their wide mass range, and in the case of FT-ICR, its high mass accuracy. Mass analysis of proteolytic peptides is a much more popular method of protein characterization, as cheaper instrument designs can be used for characterization. Additionally, sample preparation is easier once whole proteins have been digested into smaller peptide fragments. The most widely used instrument for peptide mass analysis is the quadrupole ion trap. Multiple stage quadrupole-time-of-flight and MALDI time-of-flight instruments also find use in this application.

Protein and peptide fractionation coupled with mass spectrometry

Proteins of interest to biological researchers are usually part of a very complex mixture of other proteins and molecules that co-exist in the biological medium. This presents two significant problems. First, the two ionization techniques used for large molecules only work well when the mixture contains roughly equal amounts of constituents, while in biological samples, different proteins tend to be present in widely differing amounts. If such a mixture is ionized using electrospray or MALDI, the more abundant species have a tendency to "drown" signals from less abundant ones. The second problem is that the mass spectrum from a complex mixture is very difficult to interpret due to the overwhelming number of mixture components. This is exacerbated by the fact that enzymatic digestion of a protein gives rise to a large number of peptide products. To contend with this problem, two methods are widely used to fractionate proteins, or their peptide products from an enzymatic digestion. The first method fractionates whole proteins and is called two-dimensional gel electrophoresis. The second method, high performance liquid chromatography is used to fractionate peptides after enzymatic digestion. In some situations, it may be necessary to combine both of these techniques. Gel spots identified on a 2D Gel are usually attributable to one protein. If the identity of the protein is desired, the gel spot can be excised, and digested proteolytically. The peptide masses resulting from the digestion can be determined by mass spectrometry using peptide mass fingerprinting. If this information does not allow unequivocal identification of the protein, its peptides can be subject to tandem mass spectrometry. Characterization of protein mixtures using HPLC/MS is also called shotgun proteomics and mudpit. A peptide mixture that results from digestion of a protein mixture is fractionated by one or two steps of liquid chromatography. The eluent from the chromatography stage can be either directly introduced to the mass spectrometer through electrospray ionization, or laid down on a series of small spots for later mass analysis using MALDI.

Protein identification

There are two main ways MS is used to identify proteins. Peptide mass fingerprinting (mentioned in the previous section) uses the masses of proteolytic peptides as input to a search of a database of predicted masses that would arise from digestion of a list of known proteins. If a protein sequence in the reference list gives rise to a significant number of predicted masses that match the experimental values, there is some evidence that this protein was present in the original sample. Tandem MS is becoming a more popular experimental method for identifying proteins. Collision-induced dissociation is used in mainstream applications to generate a set of fragments from a specific peptide ion. The fragmentation process primarily gives rise to cleavage products that break along peptide bonds. Because of this simplicity in fragmentation, it is possible to use the observed fragment masses to match with a database of predicted masses for one of many given peptide sequences. Tandem MS of whole protein ions has been investigated recently using electron capture dissociation and has demonstrated extensive sequence information in principle but is not in common practice. This is sometimes referred to as the "top-down" approach in that it involves starting with the whole mass and then pulling it apart rather than starting with pieces (proteolytic fragments) and piecing the protein back together using De novo repeat detection (bottom-up).

History

The first mass spectrography technique was described in an 1899 article by English scientist J.J. Thomson. The processes that more directly gave rise to the modern version were devised by Arthur Jeffrey Dempster and F.W. Aston in 1918 and 1919 respectively. In 2002, the Nobel Prize in Chemistry was received by John Fenn for the development of electrospray ionization (ESI) and Koichi Tanaka for the development of soft laser desorption (SLD) in 1987. An improved SLD method, matrix-assisted laser desorption/ionization (MALDI), was developed by Franz Hillenkamp and Michael Karas in 1988. The choice of Tanaka to recieve the nobel prize for this work over Hillenkamp and Karas is a contentious issue to some people in the field. The two methodologies are remarkably similiar yet significantly different. The work of Hillenkamp and Karas is fundamentally the same as the current implementation of matrix-assisted laser desorption/ionization which is now ubiqitous in mass spectrometry. Hillenkamp and Karas also demonstrated exceptionally well the importance of this new technique. On the other hand the work of Tanaka is similar and published significantly earlier such that the work of Hillenkamp and Karas could in theory be a derivative improvement. Yet, the work of Tanaka on SLD may have never come to prominence or become particularly useful without futher improvement.

External links

- [http://www.asms.org/ American Society for Mass Spectrometry]
- [http://www.latrobe.edu.au/anzsms/ Australian and New Zealand Society for Mass Spectrometry]
- [http://www.bmss.org.uk/ British Mass Spectrometry Society]
- [http://www.csms.inter.ab.ca/ Canadian Society for Mass Spectrometry]
- [http://www.imss.nl/ International Mass Spectrometry Society]
- [http://masspec.scripps.edu/information/history/ A History of Mass Spectrometry (Scripps)]
- [http://www.vias.org/simulations/simusoft_msscope.html Mass spectrometer simulation] An interactive application simulating the console of a mass spectrometer
- [http://www.msterms.com/wiki/ Mass spectrometry terms wiki]
- [http://www.chem.arizona.edu/quizplease/msintro/aldehyd/aldehyd.htm#begin Self-test]

References

- McLafferty, F. W. and Turecek, F., Interpretation of Mass Spectra, University Science Books; 4th edition (May, 1993) ISBN 0935702253 Category:Mass spectrometry Category:Measuring instruments ja:質量分析法

Big Bang nucleosynthesis

In cosmology, Big Bang nucleosynthesis (or primordial nucleosynthesis) refers to the production of nuclei other than H-1, the normal, light hydrogen, during the early phases of the universe, shortly after the Big Bang. It is believed to be responsible for the formation of hydrogen (H-1 or H), its isotope deuterium (H-2 or D), the helium isotopes He-3 and He-4, and the lithium isotope Li-7.

Characteristic of Big Bang nucleosynthesis

There are two important characteristics of Big Bang nucleosynthesis (BBN):
- It only lasted for about three minutes; after that, the temperature and density of the universe fell below that which is required for nuclear fusion. The brevity of BBN is important because it prevented elements heavier than beryllium from forming while at the same time allowing unburned light elements, such as deuterium, to exist.
- It was widespread, encompassing the entire universe. The key parameter which allows one to calculate the effects of BBN is the number of photons per baryon. This parameter corresponds to the temperature and density of the early universe and allows one to determine the conditions under which nuclear fusion occurs. From this we can derive elemental abundances. Although the baryon per photon ratio is important in determining elemental abundances, the precise value makes little difference to the overall picture. Without major changes to the Big Bang theory itself, BBN will result in 25% helium-4; about 1% of deuterium; trace amounts of lithium and beryllium; and no other heavy elements, leaving about 74% of H-1. That the observed abundances in the universe are consistent with these numbers is considered strong evidence for the Big Bang theory.

Sequence of BBN

Big Bang nucleosynthesis begins about one minute after the Big Bang, when the universe has cooled enough to form stable protons and neutrons, after baryogenesis. From simple thermodynamical arguments, one can calculate the fraction of protons and neutrons based on the temperature at this point. This fraction is in favour of protons, because the higher mass of the neutron results in a spontaneous decay of neutrons to protons with a half-life of about 15 minutes. One feature of BBN is that the physical laws and constants that govern the behavior of matter at these energies is very well understood, and hence BBN lacks some of the speculative uncertainties that characterize earlier periods in the life of the universe. Another feature is that the process of nucleosynthesis is determined by conditions at the start of this phase of the life of the universe, making what happens before irrelevant. As the universe expands it cools. Free neutrons and protons are less stable than helium nuclei, and the protons and neutrons have a strong tendency to form helium-4. However, forming helium-4 requires the intermediate step of forming deuterium. At the time at which nucleosynthesis occurs, the temperature is high enough for the mean energy per particle to be greater than the binding energy of deuterium; therefore any deuterium that is formed is immediately destroyed (a situation known as the deuterium bottleneck). Hence, the formation of helium-4 is delayed until the universe becomes cool enough to form deuterium (at about T = 0.1 MeV), when there is a sudden burst of element formation. Shortly thereafter, at three minutes after the Big Bang, the universe becomes too cool for any nuclear fusion to occur. At this point, the elemental abundances are fixed, and only change as some of the radioactive products of BBN (such as tritium) decay.

History of Big Bang nucleosynthesis

The history of Big Bang nucleosynthesis began with the calculations of Ralph Alpher and George Gamow in the 1940s. During the 1970s, there was a major puzzle in that the density of baryons as calculated by Big Bang nucleosynthesis was much less than the observed mass of the universe based on calculations of the expansion rate. This puzzle was resolved in large part by postulating the existence of dark matter.

Heavy elements

Big Bang nucleosyntheis produces no elements heavier than beryllium. There is no stable nucleus with 8 nucleons, so there was a bottleneck in the nucleosynthesis that stopped the process there. In stars, the bottleneck is passed by triple collisions of helium-4 nuclei (the triple-alpha process). However, the triple alpha process takes tens of thousands of years to convert a significant amount of helium to carbon, and therefore was unable to convert any significant amount of helium in the minutes after the Big Bang.

Helium-4

Big Bang nucleosynthesis predicts about 25% helium-4, and this number is extremely insensitive to the initial conditions of the universe. The reason for this is that helium-4 is very stable and so almost all of the neutrons will combine with protons to form helium-4. In addition, two helium-4 atoms cannot combine to form