Trace Element Accelerator Mass Spectrometry

Abstract

AbstractAccelerator mass spectrometry is a relatively new analytical technique that is being used in over 40 laboratories worldwide for the measurements of cosmogenic long‐lived radioisotopes, as a tracer in biomedical applications, and for trace element analysis of stable isotopes. The use of particle accelerators, along with mass spectrometric methods, has allowed low‐concentration measurements of small‐volume samples. The use of AMS to directly count ions, rather than measure radiation from slow radioactive decay processes in larger samples, has resulted in sensitivities of one part in 1015. The ions are accelerated to mega‐electron‐volt energies and can be detected with 100% efficiency in particle detectors. The improved sensitivities and smaller sample sizes have resulted in a wide variety of applications in anthropology, archaeology, astrophysics, biomedical sciences, climatology, ecology, geology, glaciology, hydrology, materials science, nuclear physics, oceanography, sedimentology, terrestrial studies, and volcanology, among others. Accelerator mass spectrometry has opened new areas of research in the characterization of trace elements in materials.This article will focus is on the technique of trace element accelerator mass spectrometry (TEAMS) and its applications in the analysis of stable isotopes in materials.Trace element AMS employs a tandem electrostatic accelerator with the center terminal at a positive potential.Sensitivities for TEAMS measurements vary for different elements. These sensitivities depend on (1) the ability to form a negative atomic ion or a negatively charged molecule that can be accelerated to the terminal and then broken apart; (2) the charge state used for the measurements since the charge state (≥3+) must be chosen to remove molecular interferences; and (3) transmission of the ion from the sample through the accelerator and beamlines to the detector. By calibrating the secondary‐ion yield against reference standards and by measuring the depth of the sputtered crater, a quantified depth profile can be obtained. The sensitivity or detection limit depends on the amount or volume of material available to analyze. The sensitivity can be much better for bulk measurements, where the element of interest is uniformly dispersed throughout the material. Then, the sample can be run almost indefinitely until good statistics have been obtained. Typically, sensitivities of 0.1 part per billion (ppb) can be obtained in the depth‐profiling mode. Because sensitivities are so high, it is sometimes difficult to obtain suitable reference standards.