Accelerator mass spectrometry

Accelerator mass spectrometry (AMS) differs from other forms of mass spectrometry in that it accelerates ions to extraordinarily high kinetic energies before mass analysis. AMS is exceptional in its ability to sensitively and accurately analyze elemental and isotopic compositions.^[1]

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Introduction

In this case "tandem accelerator" means a large nuclear particle accelerator called a Tandem van de Graaff Accelerator operating at several million volts with two stages operating in tandem to accelerate the particles. At the connecting point between the two stages, the ion charge is changed from negative to positive by a stripping stage. Since molecules will break apart in this stripping stage, this system is particularly useful for isotope ratio analysis of very rare cosmogenic isotopes such as Be-10, Cl-36, Al-26 and C-14. It is the discrimination power of AMS which gives it exceptional sensitivity such that trace amounts of these rare elements may be observed in complex samples. ^[2][3]

How it Works (An Example)

Generally negative ions are created (atoms are ionized) in an ion source. It is preferable, but not necessary that the charges be the same for each atom. These ions are introduced to the gas phase and they enter an electrostatic accelerator that accelerates them to very high kinetic energy by presenting ever more positive electrical potentials. Half-way through the accelerator they impact a sheet of carbon. The impact strips off many 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 very fast. Next, the exact ion velocities must be filtered such that only a narrow selection of ion velocities are allowed to pass to allow for proper mass analysis. This is most frequently accomplished by a device called a velocity selector, which utilizes both electric fields and magnetic fields to allow only ions of a specific charge and kinetic energy to pass. The ions then pass through at least one mass analyzer, most often a magnetic or electric sector. For example with a magnetic sector, the atom, at its known velocity (relative to mass) and charge is released into a magnetic field perpendicular to it velocity. This field causes the particle's path to curve in a circular arc. The radius of this circular arc is related to the mass-to-charge ratio of the particle. The ions are then detected by dedicated detectors for each isotope or element.

Generalizations

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 very high kinetic energies before the analyte ions enter the mass analyzer.

History

L.W. Alvarez and Robert Cornog of the United States first used an accelerator as a mass spectrometry in 1939 when they employed a cyclotron to demonstrate that tritium was radioactive, while ³He was stable. Commonly, the year 1977 is regarded as the starting year of modern AMS, when it was shown by two groups at the tandem accelerators of McMaster University in Hamilton and of the University of Rochester that ¹⁴C can be detected at the extremely low, natural isotope ratio of ¹⁴C/¹²C = 10⁻¹². This opened the well established field of Radiocarbon dating to accelerators, which served as a 'killer application' for the new method.

Applications

This technique is most often employed to determine the concentration of carbon-14. Carbon-14 concentrations can tell us dates of historic events, because its rate of decay is very predictable. Utilizing accelerator mass spectroscopy, the carbon-14 isotope can be separated from its stable form carbon-12 due to the difference in mass. An accelerator mass spectrometer is required, over other forms of mass spectrometry, to resolve stable nitrogen-14 contaminants from radiocarbon. Radiologically labeled molecules can easily be detected this way with some processing, making this a useful and powerful analytic technique.

Accelerator mass spectrometry is widely used in biomedical research.^[4][5][6]

References

Bibliography