The Technique of Accelerator Mass Spectrometry

AMS is a technique for measuring isotope ratios with high selectivity, sensitivity, and precision. In general, AMS separates a rare radioisotope from stable isotopes and molecular ions of the same mass using a variety of nuclear physics techniques. In the case of carbon, ¹⁴C ions are separated and counted as particles relative to ¹³C or ¹²C that are measured as an electrical current. The key steps of AMS allowing quantitative and specific measurement of isotopes are the production of negative ions from the sample to be analyzed, a molecular disassociation step to convert the negatively charged molecular ions to positively charged nuclei and the use of high energies (MeV) which allow for the identification of ions with high selectivity.

Radiocarbon AMS analysis requires samples be converted to a form that retains the isotopic ratio from the original sample and that provides chemical and physical equivalence for all carbon atoms. Consequently, ¹⁴C AMS currently uses mg sized graphite aliquots derived from the CO₂ of oxidized samples as the interface between biological samples and AMS measurement of ¹⁴C. These samples are bombarded by a Cs⁺ beam forming negative elemental or molecular ions. The production of negative ions removes the primary isobaric interference for radiocarbon, ¹⁴N, because nitrogen does not form a stable negative ion. Ions are subsequently selected at single atomic mass units through a low energy magnetic analyzer by switching the electrostatic potential on the magnets vacuum chamber. ¹²C is separated from mass 13 and 14 ions and can be quantified in a Faraday cup. This low energy mass spectrometer cannot resolve the small differences among the rare isotope and the nuclear and molecular isobars. Hence negative ions and molecules are accelerated to MeV energies in the first stage of a tandem accelerator (Figure 1 shows a schematic diagram of our current 10 MV tandem accelerator AMS system). At the end of this first acceleration stage these ions pass through a stripper. The stripper, which consists of a thin carbon foil or gas, strips electrons from ions and destroys molecular isobars, such as ¹³CH. These positive ions are further accelerated to energies of up to several tens of MeV in the second stage of the tandem accelerator. Acceleration of the ions to high energies makes possible the unique identification of ions based on energy loss and total energy.

Figure 1: ¹⁴C 10 MV tandem accelerator AMS system at LLNL

Positive nuclei from the accelerator are focused by a quadruple lens, and then separated on the basis of momentum by a magnetic analyzer. ¹³C is measured after this first high energy magnetic analyzer in a Faraday cup. Nuclei of the correct rigidity (momentum/charge) are then passed through a second magnetic analyzer to reject scattered ions of incorrect mass/charge, refocused by a second quadrupole lens, passed through a Wein filter to select for velocity, and finally counted in a multi-anode gas-ionization detector. Importantly AMS counts ¹⁴C as individual nuclei and is independent of decay. Around 1% of the ¹⁴C in a sample is counted which is 1000-times more efficient than decay counting. Sensitivities approaching ¹⁴C/C of ~ 2 x 10^-15 or 10 attomoles of ¹⁴C can be achieved in mg sized samples.

For information about this page contact:
Karolyn Burkhart-Schultz, burkhartschultz1@llnl.gov
UCRL-MI-140299