TY - CONF T1 - Microscale AMS (super 14) C measurement at NOSAMS Y1 - 1998 A1 - Pearson, Ann A1 - McNichol, Ann P. A1 - Schneider, Robert J. A1 - von Reden, Karl F. A1 - Zheng, Yan KW - 03 KW - absolute age KW - accelerator mass spectroscopy KW - accuracy KW - C-14 KW - C-14/C-12 KW - carbon KW - Carbon dioxide KW - combustion KW - data processing KW - Geochronology KW - graphite KW - isotope fractionation KW - isotopes KW - mass spectroscopy KW - mathematical methods KW - methods KW - native elements KW - radioactive isotopes KW - sample preparation KW - size KW - spectroscopy KW - Stable isotopes AB - Techniques for making precise and accurate radiocarbon accelerator mass spectrometry (AMS) measurements on samples containing less than a few hundred micrograms of carbon are being developed at the NOSAMS facility. A detailed examination of all aspects of the sample preparation and data analysis process shows encouraging results. Small quantities of CO (sub 2) are reduced to graphite over cobalt catalyst at an optimal temperature of 605 degrees C. Measured (super 14) C/ (super 12) C ratios of the resulting targets are affected by machine-induced isotopic fractionation, which appears directly related to the decrease in ion current generated by the smaller sample sizes. It is possible to compensate effectively for this fractionation by measuring samples relative to small standards of identical size. Examination of the various potential sources of background (super 14) C contamination indicates that the sample combustion process is the largest contributor, adding ca. 1 mu g of carbon with a less-than-modern (super 14) C concentration. PB - University of Arizona, Department of Geosciences, Tucson, AZ, United States (USA) CY - United States (USA) VL - 40 UR - http://hdl.handle.net/10150/653650 N1 - id: 2237; Source type: conferencepapers&proceedings; Object type: Article; Object type: Conference Paper; Copyright: GeoRef, Copyright 2007, American Geological Institute.; CSAUnique: 1999-044438; AccNum: 1999-044438; ISSN: 0033-8222; CODEN: RACAAT ER - TY - CONF T1 - (super 14) C AMS measurements of Y1 - 1998 A1 - von Reden, Karl F. A1 - McNichol, Ann P. A1 - Pearson, Ann A1 - Schneider, Robert J. KW - 03 KW - absolute age KW - accelerator mass spectroscopy KW - C-14 KW - carbon KW - dilution KW - Geochronology KW - ions KW - isotopes KW - mass spectroscopy KW - precision KW - radioactive isotopes KW - sample preparation KW - spectroscopy KW - volume AB - The NOSAMS facility at Woods Hole Oceanographic Institution has started to develop and apply techniques for measuring very small samples on a standard Tandetron accelerator mass spectrometry (AMS) system with high-current hemispherical Cs sputter ion sources. Over the past year, results on samples ranging from 7 to 160 mu g C showed both the feasibility of such analyses and the present limitations on reducing the size of solid carbon samples. One of the main factors affecting the AMS results is the dependence of a number of the beam optics parameters on the extracted ion beam current. The extracted currents range from 0.5 to 10 mu A of (super 12) C (super -) for the sample sizes given above. We here discuss the setup of the AMS system and methods for reliable small-sample measurements and give the AMS-related limits to sample size and the measurement uncertainties. PB - University of Arizona, Department of Geosciences, Tucson, AZ, United States (USA) CY - United States (USA) VL - 40 N1 - id: 2239; Source type: conferencepapers&proceedings; Object type: Article; Object type: Conference Paper; Copyright: GeoRef, Copyright 2007, American Geological Institute.; CSAUnique: 1999-044459; AccNum: 1999-044459; ISSN: 0033-8222; CODEN: RACAAT ER -