HR: 1340h
AN: PP23B-1440    [Abstracts]
TI: High precision measurement of Mg/Ca and Sr/Ca ratios in carbonates by cool plasma quadrupole inductively coupled plasma mass spectrometry
AU: * Shen, C
EM: river@ntu.edu.tw
AF: Department of Geosciences, National Taiwan University, No 1, Sec. 4, Roosevelt Road, Taipei, 106 Taiwan
AU: Chui, H
EM: r91224204@ntu.edu.tw
AF: Department of Geosciences, National Taiwan University, No 1, Sec. 4, Roosevelt Road, Taipei, 106 Taiwan
AU: Chu, M
EM: f88224109@ntu.edu.tw
AF: Department of Geosciences, National Taiwan University, No 1, Sec. 4, Roosevelt Road, Taipei, 106 Taiwan
AU: Chen, M
EM: mtchen@mail.ntou.edu.tw
AF: Institute of Applied Geophysics, National Taiwan Ocean University, 2, Pei-Ning Road, Keelung, 20224 Taiwan
AU: Wei, K
EM: weiky@ntu.edu.tw
AF: Department of Geosciences, National Taiwan University, No 1, Sec. 4, Roosevelt Road, Taipei, 106 Taiwan
AU: Steinke, S
EM: ssteinke@uni-bremen.de
AF: Research Center Ocean Margins, Department of Geosciences, University of Bremen, Postfach 33 04 40, Bremen, D-28334 Germany
AB: We have developed a high-precision analytical technique using an quadrupole inductively coupled plasma mass spectrometer Agilent 7500s, for simultaneously measuring Mg/Ca and Sr/Ca ratios in carbonates, which are broadly used as environmental and climatic proxies in the Earth Sciences. Cool plasma technique with 650-W RF power was employed to diminish spectral interference. A desolvation introduction system was used to enhance sensitivity, improve detection limit and reduce sample size. Intensities of isotopes, including $^{24}$Mg, $^{43}$Ca and $^{86}$Sr, were all detected in pulse-counting mode. Mg/Ca and Sr/Ca ratios were calculated directly from intensity ratios, $^{24}$Mg/$^{43}$Ca and $^{86}$Sr/$^{43}$Ca, using external matrix-matched standards to correct for instrumental mass discrimination and low-frequency ratio drift. For Mg/Ca analysis, matrix effect of Ca level is 0.018 Mg/Ca (mmol/mol) / [Ca] (ppm). This problem can be overcome by holding [Ca] from 6-8 ppm in sample solution or correcting measured ratio with an empirical formula. Different Sr/Ca also slightly affects Mg/Ca determination with a slope of 0.017 Mg/Ca (mmol/mol) / Sr/Ca (mmol/mol). It is mainly caused by the formation of doubly-charge ion of $^{86}$Sr, which biases intensity measurement of $^{43}$Ca$^{+}$ ion beam. This effect brings about only a trivial offset less than 0.1% on Mg/Ca determination for Quaternary foraminiferal and coral samples. Internal precision given by our method ranges 0.1-0.2% (2sd). Replicate measurements made on standards and samples show that external uncertainties of 0.5% and 0.4% (2sd) can be achieved for Mg/Ca and Sr/Ca, respectively. Requirement of a small sample size of only 3.5 microgram carbonate even allows investigating characteristics of single foraminiferal chamber.
DE: 1050 Marine geochemistry (4835, 4850)
DE: 1065 Trace elements (3670)
DE: 1094 Instruments and techniques
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting