THE short history of accelerator mass spectrometry (AMS) for biomedical research at Livermore has been sweet indeed. Just 10 years ago, Livermore scientists first used AMS to determine how low doses of a suspected carcinogen affect the DNA of mice. A remarkably sensitive measuring technique, AMS can seek out one carbon-14 isotope from among a quadrillion other carbon atoms. It achieved a tenfold improvement in detecting damaged DNA over the best methods then available, thus enabling studies to be conducted directly with humans. Ever since, Livermore has been expanding the development of AMS for biomedical and pharmaceutical applications and is today a recognized leader in the field. (See box below for information on how AMS works.) |
Accelerator Mass Spectrometry at Livermore
Livermore's Center for Accelerator Mass Spectrometry (CAMS) is home to the most versatile and productive AMS facility in the world. AMS is an exceptionally sensitive technique for measuring concentrations of isotopes in small samples, typically less than 1 milligram, and the relative abundance of isotopes at low levels. It can, for example, find one carbon-14 isotope among a quadrillion other carbon atoms.
Mass spectrometry has been used since early in the 1900s to study the chemical makeup of substances. A sample is put into a mass spectrometer, which ionizes it and analyzes the motion of the various ions in an electric field to sort them out by their mass-to-charge ratios. The basic principle is that isotopes of different masses move differently in a given electromagnetic field.
In accelerator mass spectrometry, the same principle applies but the process is different. Negative ions made in an ion source are accelerated in a field of millions of volts. The accelerated ions smash through a thin carbon foil or gas that destroys all molecular species. After passing through a high-energy mass spectrometer and various filters, the ions finally slow to a stop in a gas ionization detector. (See the image above of the large AMS machine at Livermore.) The identity of individual ions can be determined from how the ions slow down. For example, carbon-14 slows down more slowly than nitrogen-14, so those ions of the same mass can be distinguished from one another. Once the charges are determined, the detector can tell to which element each ion belongs and counts the desired isotope as a ratio of a more abundant isotope-carbon-14 as a ratio of carbon-13, for example.
The two "tricks" that make AMS work so well are the molecular dissociation process that occurs in the accelerator and the ion detection at the end. The resulting sensitivity is typically a million times greater than that of conventional isotopic detection.
For biological studies, AMS has been used primarily for counting carbon-14 because carbon is present in most molecules of biological interest and carbon-14 is relatively rare in the biosphere. Tritium (hydrogen-3) has also been used extensively as a tracer in biological research. The use of tritium in AMS is new and holds great promise, because many molecules are easier to tag with tritium than with carbon-14. Other isotopes are measured by AMS as well, including plutonium-239, calcium-41, beryllium-10, chlorine-36, and iodine-129.
All over the world, AMS is still used primarily to count carbon-14 in archaeological and geologic samples for dating purposes. In the 1980s, it replaced the traditional method of scintillation counting for precise radiocarbon dating, which was time-consuming and required relatively large samples. Livermore performs radiocarbon dating and many other forms of AMS 24 hours a day, 7 days a week for its own research and collaborations as well as for others on a fee-for-service basis.
![]() ![]() An overwhelming case was made for the need for an AMS resource and a number of outstanding collaborative projects have already been initiated. . . . At the present time, the LLNL Resource is clearly the most advanced site in the U.S. to explore the use of AMS in biomedical research.
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Research Opportunities Grow with New and Better Equipment
A continuing thrust of the Center for Accelerator Mass Spectrometry (CAMS) at Livermore has been the development and application of new AMS technologies as well as the improvement of existing ones. Three entirely new systems for measuring carbon-14, tritium (hydrogen-3), and plutonium-239 have come on line in recent weeks and months. Tweaking of the 15-meter-long, multipurpose AMS unit, in place since the inception of CAMS in 1989, never stops. After 11 years of development, the unit's cesium ion source is the most powerful in the world. Sample preparation methods and experimental protocols evolve and are standardized as Livermore's capabilities grow and as the demands of the biological research community expand. The focus now is to robotize the sample preparation process as much as possible.
For several years, researchers at Livermore have been working to develop a smaller, less expensive spectrometer dedicated to carbon-14 biological research. Biological AMS experiments can be run more quickly than most other AMS applications such as radiocarbon dating. A dedicated bioresearch unit would free the larger machine for other AMS work.
Testing for the first such small unit began in June (see photo at top). One-tenth the size of the larger machine, the small spectrometer will provide higher throughput for biological samples. It also will serve as a testbed for new sample preparation and delivery technologies.
John Knezovich, director of CAMS, says, "These smaller units are just beginning to appear. Livermore is one of four institutions in the world using one. Now, virtually every AMS unit, of whatever size, is custom-built. But when they have been developed to the point where the cost for one is below $1 million, then we can expect to see a proliferation of small AMS machines operating in universities and pharmaceutical companies, making significant scientific advances."
Livermore's new tritium system (below left) is the result of a collaboration with AccSys Technology of Pleasanton, California, and was funded by a National Institutes of Health/National Cancer Institute Small Business Innovation Research grant. The system was specifically designed to be small, simple, and inexpensive, making the technique of tritium AMS more broadly accessible.
Physicist Mark Roberts was primarily responsible for the system's design and for bringing it on line. Biochemist Karen Dingley, who performed the first AMS experiments using tritium as a biological tracer, has been developing experimental methods, including sample preparation and handling protocols. One of the system's first uses is in experiments to improve current methods to measure the rate at which cells divide. Cancer is a disease in which cells divide uncontrollably. Researchers want to know if exposure to cancer-causing chemicals results in an increased rate of cell division before a tumor appears. Normally, experiments such as this would require large doses of toxic chemicals or radioactive isotopes. With AMS, only a very small amount of tritium in drinking water does the trick. Then, the tritium in newly synthesized DNA can be measured. The level of tritium incorporation in DNA indicates the approximate rate at which new cells are synthesized and thus the rate of cell division.
Physicist Jeff McAnich has led the development of a new heavy-isotope spectrometer (below right) that makes it easier to run plutonium samples on a routine basis. The initial focus of this work is the measurement of plutonium concentrations and isotope ratios in urine bioassays of Marshall Islanders who are being resettled to Bikini Atoll and other islands used in the 1950s for weapons testing. The unit will be able to measure plutonium-239/240 as well as plutonium-241. Other heavy isotopes such as neptunium-237 may also be studied in the future.
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Sensitivity Is the Key![]() ![]() ![]() ![]() ![]() ![]() |
Grant Recognize Achievement![]() ![]() ![]() |
Measuring Damage to Molecules![]() ![]() ![]() ![]() |
From Ants to Elephants![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
In 1998, a University of California at Riverside doctoral candidate won the annual award given by the International Journal of Mass Spectrometry and Ion Processes for best student paper. Her fundamental work in protein chemistry netted her a trip to Amsterdam as well as a cash prize. She could not have won without Livermore's biological AMS facilities because her subject, nematodes, supplied only the smallest of samples.
Science in Revolution ![]() ![]() ![]() ![]() —Katie Walter |
Key Words: biomedical research, carbon-14, Center for Accelerator Mass Spectrometry (CAMS), Marshall Islands, MeIQx, National Institutes of Health (NIH), nutrition, pesticides, PhIP, plutonium, radioisotope tracing, tritium.
For more information contact John Knezovich (925) 422-0925 (knezovich1@llnl.gov).
ABOUT THE SCIENTIST
John Knezovich has been the director of the Center for Accelerator Mass Spectrometry since May 1998. He received his B.A. in biology from the University of the Pacific in 1977 and his Ph.D. in chemical ecology from the University of California at Davis in 1983. He was previously a group leader for Environmental Chemistry and Toxicology. Knezovich is an environmental chemist with extensive experience in the design and application of experimental approaches for determining the fate, transport, and toxicity of contaminants in the environment. He originally came to Lawrence Livermore as a student guest in 1977 and returned as a postdoctoral scientist in 1983. Knezovich serves on University of California, state, and federal advisory panels that oversee research on toxic substances. He has written more than 60 scholarly articles and publications.