Accelerator mass spectrometry

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Accelerator mass spectrometry
1 MV accelerator mass spectrometer.jpg
Accelerator mass spectrometer at Lawrence Livermore National Laboratory
AcronymAMS
Classification Mass spectrometry
Analytes Organic molecules
Biomolecules
Other techniques
Related Particle accelerator

Accelerator mass spectrometry (AMS) is a form of mass spectrometry that accelerates ions to extraordinarily high kinetic energies before mass analysis. The special strength of AMS among the different methods of mass spectrometry is its ability to separate a rare isotope from an abundant neighboring mass ("abundance sensitivity", e.g. 14C from 12C). [1] The method suppresses molecular isobars completely and in many cases can also separate atomic isobars (e.g. 14N from 14C). 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.)

Contents

AMS can outperform the competing technique of decay counting for all isotopes where the half-life is long enough. [2] Other advantages of AMS include its short measuring time as well as its ability to detect atoms in extremely small samples. [3]

Method

Generally, negative ions are created (atoms are ionized) in an ion source. 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 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. [4] [5] 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 centre 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 the second stage of mass spectrometer, the fragments from the molecules are separated from the ions of interest. This spectrometer may consist of magnetic or electric sectors, and so-called velocity selectors, which utilizes both electric fields and magnetic fields. After this stage, 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.

Generalizations

Schematic of an accelerator mass spectrometer 12929 2008 Article 54 Fig1 HTML.jpg
Schematic of an accelerator mass spectrometer

The above is just one 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.

History

L.W. Alvarez and Robert Cornog of the United States first used an accelerator as a mass spectrometer in 1939 when they employed a cyclotron to demonstrate that 3He was stable; from this observation, they immediately and correctly concluded that the other mass-3 isotope, tritium (3H), was radioactive. In 1977, inspired by this early work, Richard A. Muller at the Lawrence Berkeley Laboratory recognised 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 [7] 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. 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, since it was more sensitive by a factor of about 1,000, virtually supplanted the older "decay counting" methods for these and other radioisotopes. In 1982, AMS labs began processing archaeological samples for radiocarbon dating [8]

Applications

There are many applications for AMS throughout a variety of disciplines. AMS is most often employed to determine the concentration of 14C, e.g. by archaeologists for radiocarbon dating. Compared to other radiocarbon dating methods, AMS requires smaller sample sizes (about 50 mg), while yielding extensive chronologies. MS technology has expanded the scope of radiocarbon dating. Samples ranging from 50,000 years old to 100 years old can be successfully dated through utilizing AMS. [9] An accelerator mass spectrometer is required over other forms of mass spectrometry due to their insufficient suppression of molecular isobars to resolve 13CH and 12CH2 from radiocarbon. Because of the long half-life of 14C decay counting requires significantly larger samples. 10Be, 26Al, and 36Cl are used for surface exposure dating in geology. [10] 3H, 14C, 36Cl, and 129I are used as hydrological tracers.

Accelerator mass spectrometry is widely used in biomedical research. [11] [12] [13] In particular, 41Ca has been used to measure bone resorption in postmenopausal women.

See also

Related Research Articles

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Radiocarbon dating is a method for determining the age of an object containing organic material by using the properties of radiocarbon, a radioactive isotope of carbon.

Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed. The method compares the abundance of a naturally occurring radioactive isotope within the material to the abundance of its decay products, which form at a known constant rate of decay. The use of radiometric dating was first published in 1907 by Bertram Boltwood and is now the principal source of information about the absolute age of rocks and other geological features, including the age of fossilized life forms or the age of Earth itself, and can also be used to date a wide range of natural and man-made materials.

<span class="mw-page-title-main">Carbon-14</span> Isotope of carbon

Carbon-14, C-14, 14
C or radiocarbon, is a radioactive isotope of carbon with an atomic 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 (1949) to date archaeological, geological and hydrogeological samples. Carbon-14 was discovered on February 27, 1940, by Martin Kamen and Sam Ruben at the University of California Radiation Laboratory in Berkeley, California. Its existence had been suggested by Franz Kurie in 1934.

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Beryllium-10 (10Be) is a radioactive isotope of beryllium. It is formed in the Earth's atmosphere mainly by cosmic ray spallation of nitrogen and oxygen. Beryllium-10 has a half-life of 1.39 × 106 years, and decays by beta decay to stable boron-10 with a maximum energy of 556.2 keV. It decays through the reaction 10Be→10B + e−. Light elements in the atmosphere react with high energy galactic cosmic ray particles. The spallation of the reaction products is the source of 10Be (t, u particles like n or p):

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<span class="mw-page-title-main">Bomb pulse</span> Sudden increase of carbon-14 in the Earths atmosphere due to nuclear bomb tests

The bomb pulse is the sudden increase of carbon-14 (14C) in the Earth's atmosphere due to the hundreds of aboveground nuclear bombs tests that started in 1945 and intensified after 1950 until 1963, when the Limited Test Ban Treaty was signed by the United States, the Soviet Union and the United Kingdom. These hundreds of blasts were followed by a doubling of the relative concentration of 14C in the atmosphere. The reason for the term “relative concentration”, is because the measurements of 14C levels by mass spectrometers are most accurately made by comparison to another carbon isotope, often the common isotope 12C. Isotope abundance ratios are not only more easily measured, they are what 14C carbon daters want, since it is the fraction of carbon in a sample that is 14C, not the absolute concentration, that is of interest in dating measurements. The figure shows how the fraction of carbon in the atmosphere that is 14C, of order only a part per trillion, has changed over the past several decades following the bomb tests. Because 12C concentration has increased by about 30% over the past fifty years, the fact that “pMC”, measuring the isotope ratio, has returned (almost) to its 1955 value, means that 14C concentration in the atmosphere remains some 30% higher than it once was. Carbon-14, the radioisotope of carbon, is naturally developed in trace amounts in the atmosphere and it can be detected in all living organisms. Carbon of all types is continually used to form the molecules of the cells of organisms. Doubling of the concentration of 14C in the atmosphere is reflected in the tissues and cells of all organisms that lived around the period of nuclear testing. This property has many applications in the fields of biology and forensics.

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References

  1. ↑ McNaught, A. D.; Wilkinson, A., eds. (1997). "Abundance sensitivity (in mass spectrometry)". Compendium of Chemical Terminology (2nd ed.). IUPAC. ISBN   978-0-86542-684-9.
  2. ↑ Budzikiewicz, H.; Grigsby, R. D. (2006). "Mass spectrometry and isotopes: A century of research and discussion". Mass Spectrometry Reviews . 25 (1): 146–157. Bibcode:2006MSRv...25..146B. doi:10.1002/mas.20061. PMID   16134128.
  3. ↑ Hellborg, Ragnar; Skog, Göran (September 2008). "Accelerator mass spectrometry". Mass Spectrometry Reviews. 27 (5): 398–427. Bibcode:2008MSRv...27..398H. doi: 10.1002/mas.20172 . ISSN   0277-7037. PMID   18470926.
  4. ↑ Litherland, A. E. (1980). "Ultrasensitive mass spectrometry with accelerators". Annual Review of Nuclear and Particle Science . 30: 437–473. Bibcode:1980ARNPS..30..437L. doi: 10.1146/annurev.ns.30.120180.002253 .
  5. ↑ de Laeter, J. R. (1998). "Mass spectrometry and geochronology". Mass Spectrometry Reviews . 17 (2): 97–125. Bibcode:1998MSRv...17...97D. doi:10.1002/(SICI)1098-2787(1998)17:2<97::AID-MAS2>3.0.CO;2-J.
  6. ↑ Hah, Sang (2009). "Recent advances in biomedical applications of accelerator mass spectrometry". Journal of Biomedical Science. 16 (1): 54. doi: 10.1186/1423-0127-16-54 . ISSN   1423-0127. PMC   2712465 . PMID   19534792.
  7. ↑ Muller, R. A. (1977). "Radioisotope Dating with a Cyclotron". Science . 196 (4289): 489–494. Bibcode:1977Sci...196..489M. doi:10.1126/science.196.4289.489. PMID   17837065. S2CID   21813292.
  8. ↑ Harris, D.R (August 25, 1987). "The impact on archaeology of radiocarbon dating by accelerator mass spectrometry". Royal Society. 323 (1569): 23–43. Bibcode:1987RSPTA.323...23H. doi:10.1098/rsta.1987.0070. S2CID   91488734 . Retrieved July 12, 2022.
  9. ↑ Morlan, Richard. "Radiocarbon Dating Principles". Canadian Archaeology. Canadian Archaeological Radiocarbon Database. Retrieved July 12, 2022.
  10. ↑ Schaefer, Joerg M.; Codilean, Alexandru T.; Willenbring, Jane K.; Lu, Zheng-Tian; Keisling, Benjamin; Fülöp, Réka-H.; Val, Pedro (2022-03-10). "Cosmogenic nuclide techniques". Nature Reviews Methods Primers. 2 (1): 1–22. doi:10.1038/s43586-022-00096-9. ISSN   2662-8449. S2CID   247396585.
  11. ↑ Brown, K.; Dingley, K. H.; Turteltaub, K. W. (2005). "Accelerator Mass Spectrometry for Biomedical Research". Biological Mass Spectrometry. Methods in Enzymology. Vol. 402. pp. 423–443. doi:10.1016/S0076-6879(05)02014-8. ISBN   9780121828073. PMID   16401518.
  12. ↑ Vogel, J. S. (2005). "Accelerator mass spectrometry for quantitative in vivo tracing". BioTechniques . 38 (S6): S25–S29. doi: 10.2144/05386SU04 . PMID   16528913.
  13. ↑ Palmblad, M.; Buchholz, B. A.; Hillegonds, D. J.; Vogel, J. S. (2005). "Neuroscience and accelerator mass spectrometry". Journal of Mass Spectrometry . 40 (2): 154–159. Bibcode:2005JMSp...40..154P. doi: 10.1002/jms.734 . PMID   15706618.

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