Accelerator mass spectrometry sample masses

A more recently developed technique, known as the accelerator mass spectrometry (AMS) radiocarbon dating technique, based on counting, in a mass spectrometer, the relative amount of radiocarbon to stable carbon isotopes in a sample (see Textbox 10). 

The impact of this new technique, which was called Accelerator Mass Spectrometry (AMS), on the radiocarbon and archaeologist communities, was immediate and revolutionary. The introduction of AMS is indeed recognized by some as the third revolution in radiocarbon dating[22,23] and it has provided the opportunity to date very precious finds by collecting very small samples. The interest in developing the technique of AMS was so evident that, just few years after the measurements cited above, a first dedicated AMS system (based on a tandem accelerator) was designed and built [24] then, the first dedicated... 

Principal characteristics of small sample liquid scintillation counting (lsc), gas proportional low-level counting (11c) and atom counting by accelerator mass spectrometry (AMS) are summarized in Table 1, and systems we have used are shown in figure 1. The most important differences (apart from cost and availability)... 

In order to provide AMS analyses to the broad ocean sciences research community, the National Ocean Sciences Accelerator Mass Spectrometry Facility (NOSAMS) was established at Woods Hole Oceanographic Institution (Massachusetts) in 1989. Studies performed there include identification of sources of carbon-bearing materials in the water column and sediment, dating of sedimentary samples, investigations of paleocirculation patterns (e.g., from observations of differences in 14C relative abundances in planktonic and benthic foraminifera, and coral cores and cross sections), as well as studies of modern oceanic carbon cycling and circulation. In fact, much that is known about advective and diffusive processes in the ocean comes from measurements of chemical tracers, such as 14C, rather than from direct measurements of water mass flow. 

Ann McNichol is a Research Specialist at the National Ocean Sciences Accelerator Mass Spectrometry Facility at the Woods Hole Oceanographic Institution, which produces high-precision 14C measurements from small-volume samples. Dr. McNichol s research interests include the study and use of carbon, nitrogen, and oxygen isotope techniques to quantify bio-geochemical processes, the study of the fate of organic matter (both natu-... 

Secondary ion sources (using primary ion beams to sputter solid sample surfaces) are applied not only in SIMS and SNMS, but also for the formation of negative ions in accelerator mass spectrometry (AMS). 

In 1986, researchers at the Research Laboratory for Archaeology and the History of Art, University of Oxford, reported on how the radioactive carbon-14 isotope can be separated from other atoms in a sample by use of accelerator mass spectrometry, thus making it possible to derive more accurate chronologies from much smaller archaeological or anthropological specimens. For details, consult Hedges/Gowlett reference listed. 

Ognibene, T. J., Bench, G., Vogel, J. S., Peaslee, G. F., and Murov, S. (2003). A high-through-put method for the conversion of CO2 obtained from biochemical samples to graphite in septa-sealed vials for quantification of 14C via accelerator mass spectrometry. Anal. Chem. 75 2192-2196. 

Ognibene, T. J., and Vogel, J. S. (2004). Highly sensitive 14C and 3H quantification of biochemical samples using accelerator mass spectrometry. In Synthesis and Applications of Isotopically Labelled Compounds (Dean, D. C., Filer, C. N., and McCarthy, K. E., Eds.). Wiley, Hoboken, NJ, pp. 293-295. 

The ratio of a sample is measured in relation to a standard to improve the accuracy and precision of accelerator mass spectrometry measurements (Elmore and Phillips, 1987). Multiplying the ratio by 1000 results in the delta (del) values having units of parts per thousand, also know as per mil (%o). For standards, it is necessary to use wood from trees harvested before about 1850 pre-industrial, to avoid the Suess effects. The standard value for pre-industrialized atmospheric CO2 is 13.56 dpm g-1 or 14C/C equals 1.176 x 10-12 (Broecker and Peng, 1982). A correction term involving the effects of isotopic fractionation (IF) are also subtracted out of this equation. Isotopes are fractionated due to physical and chemical reactions (more details in the following section), thereby making the abundance of carbon isotopes (12C, 13C, and 14C) different in plants (Faure, 1986). The National Bureau of Standards currently provides an oxalic acid 14C standard that is used for this correction however, there have been many problems associated with development of this standard (Craig, 1954, 1961 Stuiver and Polach, 1977). 

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