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Detecting zeptomoles of 14C in biological samples When Mehran Salehpour of Uppsala searchers solved these issues empirically ples of 50-fold, A schematic of the AMS procedure. The carbon in a biological sample is which organs in the body mainly by miniaturizing the converted into graphite and placed in a secondary ion MS (SIMS) source. can produce new cells? reactor that turns a sample’s Ions are separated, and the various isotopes of carbon are measured by AMS can help answer this carbon into graphite. For single-particle counting or current measurements. From these data, the question by analyzing carthis proof-of-principle study, carbon isotopic composition of the sample is calculated. bon isotope ratios in small the researchers used a 14CDNA samples; from these labeled pharmaceutical compound with After the graphite is prepared, it is data, the age of a cell can be calculated well-defined specific activity and varyplaced in a sputter ion source. The reto within ~1 year. ing concentrations in human blood. sulting negatively charged carbon ions Salehpour’s group is also collaboratWhen traditional AMS is performed, enter an electrostatic accelerator, which ing with neuroscientists who want to the sample is placed in a combustion increases their kinetic energy. Halfway date obstructive tissues in multiple scletube under vacuum and heated with an along the accelerator, they pass through rosis or lesions in Alzheimer’s disease to oxidizing agent to 900 °C. This procea cell in which high-energy collisions see whether these pathologies signifidure converts the carbon (14C, 13C, and tear molecules apart and strip eleccantly predate outward symptoms. Such 12C) to CO , which is then cryogenically trons from the constituent atoms. The experiments require very sensitive meth2 transferred into a graphitization reactor. positive ions are then filtered through ods because brain samples are minute. At 525 °C, graphite aggregates form on a mass analyzer. After the carbon iso“We are very close to being able to an iron powder in the presence of a retopes are detected, current measurestudy a whole range of new samples that ducing agent. This process becomes less ment is used to quantify 12C and 13C; could not be studied before because of efficient for small samples, however, besingle-particle counting determines the the limited amount available for analycause the low CO2 partial pressure may amount of 14C. The ratio of 14C to 12C sis,” Salehpour says. These tiny samples not be sufficient to drive graphitization can then be calculated, with a nominal could eventually provide huge amounts and because smaller samples are more accuracy of 0.4–0.8%. of information. a susceptible to contamination. The reThe researchers analyzed blood sam—Linda Sage J u n e 1 , 2 0 0 8 / A n a ly t i c a l C h e m i s t r y
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