Accelerator Mass Spectrometry (AMS) is an ultra-sensitive analytical technique used to measure long-lived radionuclides at extremely low concentrations, often at the level of parts per trillion or lower. It is a form of mass spectrometry that uses a particle accelerator to separate rare isotopes from abundant ones and from molecular or isobaric interferences.
AMS is most commonly applied in radiocarbon dating (¹⁴C), but it is also used for other isotopes such as ¹⁰Be, ²⁶Al, ³⁶Cl, ¹²⁹I, and ²³⁶U. Applications include archaeology, geology, environmental science, biomedicine, and nuclear forensics.
HOW IT WORKS
AMS begins with the conversion of a sample into a suitable form, often a graphite or metal oxide, followed by negative ionization using a cesium sputter ion source. The negatively charged ions are then accelerated through a tandem accelerator.
At the center of the accelerator, the ions pass through a stripping stage, where electrons are removed, turning the ions into positively charged particles. This process breaks apart molecular ions that could interfere with the measurement of the rare isotopes.
After acceleration and stripping, the ions are further separated by magnetic and electrostatic analyzers based on their mass-to-charge ratio and energy. Finally, the ions of interest are detected using a particle detector (e.g., silicon detector or time-of-flight system), which measures both the identity and quantity of the rare isotopes.
Because AMS counts individual atoms, rather than waiting for radioactive decay, it provides much faster and more sensitive results than traditional radiometric techniques.
ADVANTAGES
Extremely High Sensitivity: Can detect isotopic ratios as low as 1 part in 10¹⁵, making it ideal for ultra-trace analysis.
Very Small Sample Size: Requires milligram or even microgram quantities, valuable for rare or precious materials.
Direct Isotope Counting: Measures rare isotopes independently of their decay rate, allowing faster analysis of long-lived radionuclides.
High Precision and Accuracy: Enables accurate quantification of isotopic ratios, critical in dating and tracer studies.
Wide Application Range: Used in radiocarbon dating, cosmogenic nuclide dating, climate science, drug metabolism studies, and nuclear safeguards.
CHALLENGES AND LIMITATIONS
Complex and Expensive Equipment: Requires a particle accelerator and high-vacuum system, making it costly to install and operate.
Specialized Sample Preparation: Requires chemical conversion to specific forms, such as graphite for carbon analysis, with strict contamination control.
Limited Accessibility: Due to the size and complexity of the equipment, AMS facilities are relatively few and centralized.
Interference Management: Although molecular interferences are removed by stripping, isobaric interferences (e.g., ¹⁴N for ¹⁴C) may still require careful correction.
Long Setup Times: Instrument tuning, calibration, and sample loading can be time-consuming compared to other forms of mass spectrometry.