Recent advances in biomedical applications of accelerator mass spectrometry

Accelerator mass spectrometry (AMS; see Figure 1 for AMS schematic diagram) is an extremely sensitive nuclear physics technique for detection of very low-abundant, stable and long-lived isotopes, initially developed in the mid-70's as a method of determining isotope ratios for geochronology and archaeological research [1,2]. The technique utilizes a tandem van de Graaff accelerator in order to generate the potential energy, allowing for separation of elemental isotopes at the single atom level. Therefore, AMS can be applied to quantitating the concentrations of long-lived radioisotopes, such as 14C, for which decay counting is an inefficient method of quantitation because of its relatively long half-life of 5760 years [3]. Much of this discussion concerns 14C because it is the predominant isotope for biomedical/bioanalytical probe analysis. However, equivalent discussions refer equally well with changes in chemistry [4], to several other long-lived isotopes that can be quantitated by AMS: 3H, 10Be, 16Al, 36Cl, 41Ca, 56Ni, 99Tc, 129I, and 239Pu.The sensitivity of AMS for radiocarbon can be emphasized by its use in geochronology or carbon dating of historical artefacts [1,2]. As widely known, small amounts of 14C are constantly being formed from 14N by bombardment with cosmic radiation in the upper atmosphere, and this formation gives rise to 14CO2, and maintains the atmosphere at a nearly constant radiocarbon concentration of about 1.2 × 10-10% 14C (primarily as CO2) [5-7]. Plants fix atmospheric 14CO2, animals eat the plants and thus all higher living organisms contain 14C in equilibrium. When an organism dies, there is no longer any carbon exchange, and 14C decays over time. Thus, the 12C:14C ratio can be correlated with the amount of time elapsed after an organism's death, which is the basis of carbon dating. Carbon dating now extends beyond 50,000 years back in time [3].The most conventional method for the measurement of radioactivity for biomedical applications