HR: 1330h
AN: C43A-13    [Abstracts]
TI: Determination of Delta 34S and S Concentration in Environmental Samples by Multi-Detector-Thermal Ionization Mass Spectrometry (MC-TIMS) Using a 33S-36S Double Spike
AU: * Mann, J L
EM: jlmann@geol.umd.edu
AF: University of Maryland, Department of Geology, College Park, MD 20742 United States
AU: * Mann, J L
EM: jlmann@geol.umd.edu
AF: National Institute of Standards and Technology, Analytical Chemistry Division, Chemical Science and Technology Laboratory, Gaithersburg, MD 20899-8391 United States
AU: Kelly, W R
EM: william.kelly@nist.gov
AF: National Institute of Standards and Technology, Analytical Chemistry Division, Chemical Science and Technology Laboratory, Gaithersburg, MD 20899-8391 United States
AB: The variability of sulfur isotopes, caused by mass fractionation during biogeochemical processing, is commonly used for tracing the various sources of sulfur and for understanding the sulfur cycle. Snow and ice preserve a continuous chronological record of the sources, sinks, and geochemical processing of sulfur through time. The ability to decipher this record has been limited by the analytical capability, which requires > 1 μmole of S for precise isotopic measurements by gas source mass spectrometers. A new analytical technique that is both highly accurate and precise has been developed for the simultaneous determination of sulfur isotope composition and concentration of low concentration samples based on production of AsS⁺ ions by thermal ionization using silica gel as an emitter. The technique combines multi-collector thermal ionization mass spectrometry (TIMS) with a ³³S/³⁶S internal standard and has been applied to three international sulfur standards (IAEA-S-1, IAEA-S-2, and IAEA-S-3) and snow and firn samples from the Inilchek Glacier, Kyrgyzstan and Summit, Greenland. Using a well characterized ³³S/³⁶S double spike, calibrated relative to the internationally accepted consensus absolute value for the standard IAEA-S-1 (δ³⁴S = -0.3 ‰), a fractionation factor (α) can be determined that corrects for the instrumental fractionation (changing ratio) inherent to the TIMS technique. This correction yields the absolute true ³²S/³⁴S ratio and the δ³⁴S values relative to the VCDT scale. The ³³S/³⁶S double spike also allows small concentration samples to be measured for both concentration and isotope composition, because it acts as a chemical carrier by adding to the total sulfur mass in the sample. The δ³⁴S values determined (reported relative to VCDT (δ³⁴S = -0.3 ‰) were -0.32 ‰ ± 0.04 ‰ (1s, n = 4), 22.65 ‰ ± 0.04 ‰ (1s, n = 7), and -32.47 ‰ ± 0.07 ‰ (1s, n = 8) for IAEA-S-1, IAEA-S-2, and IAEA-S-3, respectively. The amount of natural sample used for these analyses ranged from 0.39 to 1.98 μmoles, with precisions on the S concentration measurements being typically better then 0.2 % (rsd). The uncertainties reported for sulfur isotopic composition of these standards are comparable to or better then those obtained by isotope ratio mass spectrometers (IRMS). The δ³⁴S values for the Inilchek Glacier samples ranged from 2.3 to 7.6 ‰ with uncertainties of 0.10 to 0.35 ‰ (1s) on sample sizes ranging from 0.1 to 1.8 μmoles. The uncertainties on the S concentration measurements ranged from 0.35 to 2.24% at the 95% confidence level. The δ³⁴S values for the Greenland samples ranged from 3.5 to 11.4 ‰ with uncertainties of 0.62 to 0.70 ‰ (1s) on sample sizes ranging from 0.04 to 0.29 μ moles. The uncertainties on the S concentration averaged 13% at the 95% confidence level. For both the Inilchek and Summit samples the uncertainties are dominated by blank corrections and not by measurement uncertainty. The smaller sample requirements for this technique may make it possible to access to the higher-resolution sulfur isotope record of snow and ice.
DE: 1040 Isotopic composition/chemistry
DE: 1827 Glaciology (1863)
DE: 1863 Snow and ice (1827)
DE: 1894 Instruments and techniques
DE: 9315 Arctic region
SC: Cryosphere [C]
MN: 2005 Joint Assembly