HR: 0800h
AN: V51A-0509    [Abstracts]
TI: Insights Into Mass-Dependant Fractionation During Desolvation of Analyte Solutions and its Importance for Accurate Isotope Ratio Measurements by MC-ICP-MS
AU: * Kamenov, G
EM: kamenov@ufl.edu
AF: University of Florida, Dept. of Geological Sciences, Gainesville, FL 32611
AU: Mueller, P
EM: mueller@geology.ufl.edu
AF: University of Florida, Dept. of Geological Sciences, Gainesville, FL 32611
AU: Perfit, M
EM: mperfit@geology.ufl.edu
AF: University of Florida, Dept. of Geological Sciences, Gainesville, FL 32611
AB: A series of experiments with mixed Pb-Tl solutions revealed that variable photoxidation of Tl+ to Tl3+ occurs in the presence of Pb and solar UV radiation. The Tl3+ species behave distinctly from Tl+ and Pb2+ when using a membrane-based desolvation system and yield less precise and less accurate results. For example, epsilon 205Tl variations from -4 to +30, equivalent to the observed natural variations, were measured in the same Tl standard solution when thallium was in the Tl3+ form. When analyses were restricted to Pb-Tl solutions, in which thallium was in the Tl+ state, highly precise isotopic ratios were obtained for lead (206Pb/204Pb=16.9373 (+/-0.0011, 2s), 207Pb/204Pb=15.4907 (+/-0.0012, 2s), and 208Pb/204Pb=36.6935 (+/-0.0039, 2s)) and for thallium (e205Tl=1.5 (+/-0.8, 2s)). Qualitative scans of the DSN waste product, produced during analyses of mixed Pb-Tl solutions reveals presence of Pb and Tl, indicating that a portion of the sample solution passes through the DSN membrane. The isotopic variations observed during desolvation of the solutions are most likely related to one or more processes occurring during desolvation including: 1) Fractionation may occur during transport of the analyte through the porous fluorocarbon membrane tube as a result of differential diffusion across the membrane due to differential gas pressure and/or differences in volatility of the chemical species in the analyte. During this process, commercially utilized in uranium enrichment plants, the lighter isotopes diffuse faster than the heavier isotopes through the porous membrane. 2) Fractionation may occur during the vaporization of solvent, which leads to the formation of compounds and/or complex molecules that may preferentially incorporate heavier isotopes. In addition, the so formed compounds, molecules, and/or hydrous ions may have different vapor pressures, and consequently, one species (e.g., Tl) may be lost preferentially to another (e.g., Pb) and also isotopically fractionated. The issue for isotope ratio mass spectrometry is to evaluate the mechanism by which analyte ions are fractionated during desolvation, the extent to which they are fractionated elementally and isotopically, and to determine how appropriate corrections can be applied to the data. In the best-case scenario, fractionation during desolvation would be sufficiently small so that overall fractionation (i.e. sum of fractionations occurring in the desolvator, the plasma, and the plasma interface) can be adequately described by a single mass bias equation (e.g., "exponential law"). The extent to which a single correction factor can be used to correct isotope ratio data sets, however, must be evaluated on a case-by-case basis. For example, we have shown that Pb and Tl+ have very similar fractionation profiles during desolvation and in the mass-spectrometer, and a single correction factor can be used to correct Pb or Tl isotope ratio data. On the other hand, the behavior of Pb+2 and Tl+3 is such that the use of a single fractionation correction will not yield precise data. Because the extent of fractionation occurring during desolvation is sensitive to the operating conditions of the desolvator and the chemical species present in the analyte, it is critical to independently evaluate the extent of fractionation for each analyte solution in order to obtain the most precise isotope ratios.
DE: 1040 Isotopic composition/chemistry
DE: 1094 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting