Accelerator mass spectrometry

Accelerator mass spectrometry
Name	Accelerator mass spectrometry
Acronym	AMS
Classification	Mass spectrometry
Manufacturer	National Electrostatics Corporation, High Voltage Engineering Europa, EAG Laboratories
Related	Radiocarbon dating, Isotope ratio mass spectrometry

Accelerator mass spectrometry is an ultrasensitive analytical technique used to measure extremely low concentrations of long-lived radioisotopes and stable isotopes. It separates ions based on their mass-to-charge ratio after accelerating them to high energies, typically using a tandem accelerator. This method is most renowned for its application in radiocarbon dating, where it can analyze samples containing as few as a million carbon atoms, far surpassing the sensitivity of conventional decay counting.

Principle of operation

The core principle involves creating negative ions from a sample, such as carbon from archaeological artifacts, within a caesium sputter ion source. These ions are initially accelerated towards a high positive potential in a tandem accelerator. At the terminal, they pass through a stripper canal, often containing argon gas or a carbon foil, which removes several electrons, converting them to positive ions and destroying molecular isobars. The now positively charged ions are repelled from the same high voltage terminal and accelerated again to even higher energies. Subsequent magnetic and electrostatic analyzers, like those at the University of Arizona's NSF-funded facility, separate the ions by mass and energy, allowing individual isotopes such as carbon-14 to be counted directly with detectors like a gas ionization chamber or silicon surface barrier detector.

Instrumentation

A typical system includes a high-intensity ion source, a tandem electrostatic accelerator capable of millions of volts, a high-energy analysis system, and a specialized detection system. Key components are manufactured by companies such as National Electrostatics Corporation and High Voltage Engineering Europa. Facilities dedicated to this technique are found at major research institutions worldwide, including the Lawrence Livermore National Laboratory, the University of Oxford's Radiocarbon Accelerator Unit, and the Australian Nuclear Science and Technology Organisation. The Center for Accelerator Mass Spectrometry at Lawrence Livermore National Laboratory operates one of the highest-throughput systems, while compact systems like the MICADAS developed at ETH Zurich have expanded accessibility.

Applications

Its primary application is in radiocarbon dating for archaeology, paleoclimatology, and Quaternary research, used on materials like those from Ötzi the Iceman or Dead Sea Scrolls. In geology, it measures isotopes like beryllium-10 and aluminium-26 for surface exposure dating and erosion studies. Biomedical research employs it for ultrasensitive pharmacokinetic tracing using carbon-14 and calcium-41, as pioneered at Lawrence Livermore National Laboratory. It is also crucial in nuclear safeguards for detecting minute quantities of plutonium and in environmental science for tracing fallout from events like the Chernobyl disaster and Fukushima Daiichi nuclear disaster.

History and development

The technique originated in the late 1970s from separate experiments at University of Rochester and McMaster University, with key early work by researchers like Kurt Marti and Richard A. Muller. A seminal 1977 paper in Science (journal) by a group from University of Rochester, General Ionex Corporation, and University of Toronto demonstrated its feasibility for carbon-14 detection. Rapid development followed at institutions like the University of Oxford and University of Arizona. The development of smaller, dedicated systems, such as those by National Electrostatics Corporation, transformed it from a nuclear physics tool into a mainstream analytical method for the Earth sciences and archaeology.

Comparison with other techniques

Compared to conventional decay counting methods like liquid scintillation counting, it offers vastly superior sensitivity and requires much smaller sample sizes, reducing destruction of precious artifacts from sites like Pompeii. Unlike standard isotope ratio mass spectrometry, which measures stable isotopes, it can isolate and count rare radioisotopes directly, free from molecular interferences. While inductively coupled plasma mass spectrometry is excellent for elemental analysis, it typically cannot match the isotopic selectivity for specific radionuclides like iodine-129 without significant isobaric interference.

Limitations and challenges

The technique requires large, expensive instrumentation and specialized facilities, limiting its widespread use. Sample preparation is complex and requires careful chemistry to avoid contamination, as highlighted in protocols from the University of Georgia's Center for Applied Isotope Studies. While excellent for long-lived isotopes, it is generally not suitable for short-lived radionuclides. The process can also suffer from subtle effects like isotopic fractionation during ion source operation, requiring careful calibration against standards such as those provided by the National Institute of Standards and Technology.

Category:Mass spectrometry Category:Analytical chemistry