Accelerator mass spectrometry
Accelerator mass spectrometry (AMS) is an analytical method for analysing the isotope content of a specific element in a sample with high accuracy and sensitivity. In AMS, the sample is ionized and accelerated through several magnetic and electrical fields in order to separate the isotopes and measure their concentrations. AMS has many applications for example in geology, archaeology, environmental research and product safety, probably the most well-known of them being the radiocarbon dating.
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What is accelerator mass spectrometry?
Accelerator mass spectrometry (AMS) is an ultra-sensitive analytical technique used for measuring the concentration of a single isotope in a sample. Different isotopes of the same element have the same number of protons but different numbers of neutrons in their nuclei. Some of the isotopes are radioactive while others are stable.
AMS is based on the use of an ion accelerator as a powerful mass spectrometer. Ion sources, large magnets and detectors are used together with the accelerator to separate different isotopes and count single atoms. After the ionization of the sample, the magnetic and electric fields of the accelerator system filter out additional isotopes from the ion beam by deflecting them from their original direction. The high speed of the ion beam enables molecules to be destroyed and removed from the measurement background. Finally the ion beam is isotopically analysed: the magnetic and electrostatic analyzers measure the amount of the isotope of interest and ion detectors identify selected isotopes and count them individually. As a result, even concentrations of one atom in aroud 1000 atoms can be measured.
How does accelerator mass spectrometry work?
First, the element of interest is chemically extracted from the sample and loaded into the ion source of the tandem accelerator. The ion source creates a beam of ions from the element by sputtering its atoms with the help of cesium ions. The cesium ions are produced on an ionizer and targeted at a small spot of the sample. This creates negative ions on the sample’s surface which are then extracted from the ion source and accelerated towards the first magnet.
The first magnet is an injector magnet, which bends the negative ion beam by 90° in order to separate the desired radioisotope of the element. The injector magnet selects the mass and therefore the route of the isotope in question and rejects the neighboring stable isotopes. After this, the negative ions continue traveling down the tube through an electrostatic tandem accelerator. The tandem accelerator has a positively charged center called the terminal towards which the negative ions are attracted. At the terminal the negative ions pass through an electron stripper (a column of gas or a very thin carbon foil) and emerge as positive ions. Due to high velocities, molecules are destroyed in the center of the accelerator and the remaining positive ions repel from the positive terminal accelerating again.
After the tandem accelerator, there are still lots of molecules and isobars (isotopes of neighboring elements having the same mass as the isotope of interest), which must be removed by more magnets. The analyzing and switching magnets work similarly to the injector magnet and therefore further reduce the amount of neighboring stable isotopes. In addition, they eliminate all highly charged molecules by selecting only the charged ions that are produced in the terminal of the tandem accelerator. Isotope ratios are measured by alternately selecting the stable and radioisotopes with the injector and analyzing magnets.
Lastly, the electrostatic analyzer deflects the ion beam by 20° with the help of a high voltage. Thus, the ions are selected based on their energy: the ones that happen to receive the wrong energy from the accelerator are removed. The concentration of the isotope of interest is then measured with a gas ionization detector. The detector counts ions one at a time by slowing them down and storing them in propane gas. As the ions stop, the electrons of the gas atoms come free. The atoms are then collected and the computer determines the rate of energy loss for each atom. Then the atomic number of the element in the sample is deduced from the data in order to distinguish interfering isobars.
What is accelerator mass spectrometry used for?
Many rare isotopes, such as Be-10, C-14, Al-26, Cl-36, Ca-41, I-129 and several isotopes of Uranium and Plutonium can be analysed with AMS. One of the most common applications of the method is radiocarbon dating. The technique is widely used for determining the age of carbon-containing samples by measuring the amount of radioactive C-14 isotope in them. In addition to archaeology and historical research, AMS is also commonly used for determining the bioportion (the proportion of biobased content in a sample) of different kinds of fuels.
The isotopes analysed with AMS have a wide range of dating applications and they are used in a variation of chronometers and tracers. Therefore, AMS is utilized in many disciplines, such as geological and planetary sciences, geomorphology, quaternary science, environmental and atmospheric research, archaeology, historical research, global climate change control, nuclear safeguards and biomedicine.
Sample requirements and preparation
Solid, liquid and gaseous samples can be analysed with AMS. For example archaeological samples (e.g. fossils, pottery), geological samples (e.g. rocks, sediments), fuels, flue gases and samples from nature, such as rainwater are suitable for the analysis.
Because AMS is a highly sensitive method, only a small amount of sample is needed for the test. Usually, a few milligrams of solid material is enough for the analysis.
Before the AMS analysis, the element of interest (e.g. carbon, beryllium, aluminium, iodine, uranium or plutonium) has to be isolated and purified from the sample. Chemical methods are used for separating very small amounts of the element and for preparing the appropriate target material for the measurement in the accelerator.
Need an AMS analysis?
Measurlabs offers AMS analyses of high quality with fast results and affordable prices. If you have any questions about your sample or it’s suitability for the method, our experts are always happy to help. You can contact us through the form below or by emailing us at firstname.lastname@example.org.
Suitable sample matrices
- Fuels (gaseous, liquid and solid)
- Geological and environmental samples (for example rocks, meteorites, sediments, soil and water)
- Archaeological samples (for example fossils, seeds and bones)
- Historical samples (for example wood, fabrics and pottery)
- Flue gases
- Radioactive samples
- Biomedical samples
Ideal uses of AMS
- Radiocarbon dating
- Bioportion determination (carbon content in fuels and flue gases)
- Analysing the concentrations of different isotopes in rocks and sediments in geology and geomorphology
- Measuring the concentrations of long-lived radioisotopes from the environment in environmental and atmospheric research
- Analysing the contents of meteorites in planetary sciences
- Product development and quality control of chronometers and tracers
- Nuclear safeguards
- Actinide and heavy ion isotopic analysis
- Cosmogenic isotope dating
Frequently asked questions
Measurlabs offers a variety of laboratory analyses for product developers and quality managers. We perform some of the analyses in our own lab, but mostly we outsource them to carefully selected partner laboratories. This way we can send each sample to the lab that is best suited for the purpose, and offer high-quality analyses with more than a thousand different methods to our clients.
When you contact us through our contact form or by email, one of our specialists will take ownership of your case and answer your query. You get an offer with all the necessary details about the analysis, and can send your samples to the indicated address. We will then take care of sending your samples to the correct laboratories and write a clear report on the results for you.
Samples are usually delivered to our laboratory via courier. Contact us for further details before sending samples.
Accelerator mass spectrometry (AMS) is an analytical method for analysing the isotope content of a specific element in a sample with high accuracy and sensitivity. AMS has many applications for example in geology, archaeology, environmental research and product safety, probably the most well-known of them being the radiocarbon dating.