Accelerator mass spectrometry
Accelerator mass spectrometry (AMS) differs from other forms of mass spectrometry
Mass spectrometry
Mass spectrometry is an analytical technique that measures the mass-to-charge ratio of charged particles.It is used for determining masses of particles, for determining the elemental composition of a sample or molecule, and for elucidating the chemical structures of molecules, such as peptides and...

 in that it accelerates ions to extraordinarily high kinetic energies
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...

 before mass analysis. The special strength of AMS among the mass spectrometric methods is its power to separate a rare isotope from an abundant neighboring mass ("abundance sensitivity", e.g. 14C from 12C). The method suppresses molecular isobars completely and in many cases can separate atomic isobars (e.g. 14N from 14C) also. This makes possible the detection of naturally occurring, long-lived radio-isotopes such as 10Be, 36Cl, 26Al and 14C. Their typical isotopic abundance ranges from 10−12 to 10−18. AMS can outperform the competing technique of decay counting for all isotopes where the half-life is long enough.

The method

Generally, negative ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

s are created (atoms are ionized) in an ion source
Ion source
An ion source is an electro-magnetic device that is used to create charged particles. These are used primarily to form ions for mass spectrometers, optical emission spectrometers, particle accelerators, ion implanters and ion engines.- Electron ionization :...

. In fortunate cases this already allows the suppression of an unwanted isobar, which does not form negative ions (as 14N in the case of 14C measurements). The pre-accelerated ions are usually separated by a first mass spectrometer of sector-field type and enter an electrostatic "tandem accelerator". This is a large nuclear particle accelerator based on the principle of a Tandem van de Graaff Accelerator
Van de Graaff generator
A Van de Graaff generator is an electrostatic generator which uses a moving belt to accumulate very high voltages on a hollow metal globe on the top of the stand. It was invented in 1929 by American physicist Robert J. Van de Graaff. The potential differences achieved in modern Van de Graaff...

 operating at 0.2 to many million volts with two stages operating in tandem to accelerate the particles. At the connecting point between the two stages, the ions change charge from negative to positive by passing through a thin layer of matter ("stripping", either gas or a thin carbon foil). Molecules will break apart in this stripping stage. The complete suppression of molecular isobars (e.g. 13CH in the case of 14C measurements) is one reason for the exceptional abundance sensitivity of AMS. Additionally, the impact strips off several of the ion's electrons, converting it into a positively charged ion. In the second half of the accelerator the now positively charged ion is accelerated away from the highly positive center of the electrostatic accelerator which previously attracted the negative ion. When the ions leave the accelerator they are positively charged and are moving at several percent of the speed of light. In a second stage of mass spectrometer, the fragments from the molecules are separated from the ions of interest. This spectrometer may exist of magnetic or electric sectors
Sector instrument
A sector instrument is a general term for a class of mass spectrometer that uses a static electric or magnetic sector or some combination of the two as a mass analyzer. A popular combination of these sectors has been the BEB...

, and so called velocity selector
Velocity selector
A velocity selector is used in accelerator mass spectrometry to select particles based on their speed. The velocity selector is composed of orthogonal electric and magnetic fields, such that particles with the correct charge to mass ratio and speed will be unaffected, and other particles will be...

s, which utilizes both electric fields and magnetic fields. After this state, no background is left, unless a stable (atomic) isobar forming negative ions exists (e.g. 36S if measuring 36Cl), which is not suppressed at all by the setup described so far. Thanks to the high energy of the ions, these can be separated by methods borrowed from nuclear physics, like degrader foils and gas-filled magnets. Individual ions are finally detected by single-ion counting (with silicon surface-barrier detectors, ionization chambers, and/or time-of-flight telescopes). Thanks to the high energy of the ions, these detectors can provide additional identification of background isobars by nuclear-charge determination.


The above is an example. There are other ways in which AMS is achieved; however, they all work based on improving mass selectivity and specificity by creating high kinetic energies before molecule destruction by stripping, followed by single-ion counting.


L.W. Alvarez and Robert Cornog
Robert Cornog
Robert Alden Cornog , was a physicist and engineer who helped develop the atomic bomb and missile systems from the Snark to the Minuteman....

 of the United States first used an accelerator as a mass spectrometer in 1939 when they employed a cyclotron
In technology, a cyclotron is a type of particle accelerator. In physics, the cyclotron frequency or gyrofrequency is the frequency of a charged particle moving perpendicularly to the direction of a uniform magnetic field, i.e. a magnetic field of constant magnitude and direction...

 to demonstrate that 3He was stable; from this observation, they immediately (and correctly) concluded that the other mass-3 isotope tritium
Tritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons...

 was radioactive. In 1977, inspired by this early work, Richard A. Muller
Richard A. Muller
Richard A. Muller is a noted American professor of physics at the University of California, Berkeley. He is also a faculty senior scientist at the Lawrence Berkeley National Laboratory.-Career:...

 at the Lawrence Berkeley Laboratory recognized that modern accelerators could accelerate radioactive particles to an energy where the background interferences could be separated using particle identification techniques. He published the seminal paper in Science
Science (journal)
Science is the academic journal of the American Association for the Advancement of Science and is one of the world's top scientific journals....

showing how accelerators (cyclotrons and linear) could be used for detection of tritium, radiocarbon (14C), and several other isotopes of scientific interest including 10Be; he also reported the first successful radioisotope date experimentally obtained using tritium (3H). His paper was the direct inspiration for other groups using cyclotrons (G. Raisbeck and F. Yiou, in France) and tandem linear accelerators (D. Nelson, R. Korteling, W. Stott at McMaster). K. Purser and colleagues also published the successful detection of radiocarbon using their tandem at Rochester. Soon afterwards the Berkeley and French teams reported the successful detection of 10Be, an isotope widely used in geology. Soon the accelerator technique, because it was about a factor of 1000 more sensitive, virtually supplanted the older “decay counting” methods for these and other radioisotopes.


The applications are many. AMS is most often employed to determine the concentration of 14C
Carbon-14, 14C, or radiocarbon, is a radioactive isotope of carbon with a nucleus containing 6 protons and 8 neutrons. Its presence in organic materials is the basis of the radiocarbon dating method pioneered by Willard Libby and colleagues , to date archaeological, geological, and hydrogeological...

, e.g. by archaeologists for radiocarbon dating
Radiocarbon dating
Radiocarbon dating is a radiometric dating method that uses the naturally occurring radioisotope carbon-14 to estimate the age of carbon-bearing materials up to about 58,000 to 62,000 years. Raw, i.e. uncalibrated, radiocarbon ages are usually reported in radiocarbon years "Before Present" ,...

. An accelerator mass spectrometer is required, over other forms of mass spectrometry, because of their insufficient abundance sensitivity, and to resolve stable nitrogen-14 from radiocarbon. Due to the long half-life of 14C, decay counting requires significantly larger samples. 10Be, 26Al, and 36Cl are used for surface exposure dating
Surface exposure dating
Surface exposure dating is a collection of geochronological techniques for estimating the length of time that a rock has been exposed at or near Earth's surface. Surface exposure dating is used to date glacial advances and retreats, erosion history, lava flows, meteorite impacts, rock slides, fault...

 in geology. 3H, 14C, 36Cl, and 129I are used as hydrological tracer.

Accelerator mass spectrometry is widely used in biomedical research. In particular 41Ca has been used to measure bone resorption in postmenopausal women.

See also

External links

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