HR: 0800h
AN: A51C-0801    [Abstracts]
TI: Multiply-Substituted Isotopologues of Molecular Gases: Instrumentation, Methods, and Illustrative applications
AU: * Eiler, J M
EM: eiler@gps.caltech.edu
AF: California Institute of Technology, Division of Geological and Planetary Sciences, Pasadena, CA 91125 United States
AU: Afek, H
EM: hagit@caltech.edu
AF: California Institute of Technology, Division of Geological and Planetary Sciences, Pasadena, CA 91125 United States
AU: Cina, S
EM: cinawp@caltech.edu
AF: California Institute of Technology, Division of Geological and Planetary Sciences, Pasadena, CA 91125 United States
AU: Ghosh, P
EM: pghosh@gps.caltech.edu
AF: California Institute of Technology, Division of Geological and Planetary Sciences, Pasadena, CA 91125 United States
AU: Schauble, E
EM: schauble@ucla.edu
AF: UCLA, Department of Earth and Space Sciences, Los Angeles, CA 90095 United States
AB: Molecules containing two or more heavy, rare stable isotopes (e.g., D$_{2}$; $^{15}$N$_{2}$) undergo distinctive physical, chemical and photochemical fractionations and could provide new, independent constraints on many geochemical problems. Until recently, these species have been un-analyzable at their low natural abundances. We configured a Finnigan Mat 253 gas-source mass spectrometer to measure the doubly-substituted isotopologues of CO$_{2}$, N$_{2}$O, O$_{2}$, N$_{2}$, and CO, with the aim of exploring their uses in geochemistry, including atmospheric chemistry. We describe this instrument, relevant methods and illustrative applications, focusing on measurements of the mass-47 CO$_{2}$ isotopologue, $^{13}$C$^{18}$O$^{16}$O. Accompanying abstracts by Afek and Eiler, Ghosh et al., Schauble et al. and Wang et al. present related theoretical and applied studies. The collection system of our instrument includes three faraday cups for masses 44, 45 and 46 registered through 10$^{8}$ to 10$^{11}$ Ohm resistors and three faraday cups for masses 47, 48 and 49 registered through 10$^{12}$ Ohm resistors. Duel-inlet measurements of $\sim$50 $\mu$mole samples produce $\sim$2 pA ion currents for mass 47. External precision for measurements of R$^{47}$ ($\sim$[$^{13}$C$^{18}$O$^{16}$O]/[$^{12}$C$^{16}$O$_{2}$]) varies with protocol, but is typically $\pm$0.03 to 0.02 $\permil$, 1$\sigma$. We report data for R$^{47}$ using a reference frame in which all C and O isotopes are randomly distributed among all possible isotopologues. We define the variable, $\Delta$$_{47}$, as the difference in per mil between the measured value of R$^{47}$ for a given sample and the value of R$^{47}$ expected for a random distribution in that sample. Values of $\Delta$$_{47}$ are standardized by comparison with an intra-laboratory standard that has a known bulk isotopic composition and that has been heated to make it take on the random distribution. External precision of $\Delta$$_{47}$ values for repeat measurements of purified CO$_{2}$ average $\pm$0.011 $\permil$, $1\sigma$. We infer that $\Delta$$_{47}$ values are more reproducible than R$^{47}$ values because analytical errors in R$^{45}$, R$^{46}$ and R$^{47}$ are correlated with one another (much as measurements of $\Delta$$^{17}$O are more precise than measurements of $\delta$$^{18}$O and $\delta$$^{17}$O). Sample contamination, particularly by hydrocarbons, is a pernicious problem; we will review experiments demonstrating their effects and methods for their removal. Multiply-substituted isotopologues generally have lower zero-point energies than their isotopically normal and singly-substituted relatives. Therefore, reactions such as: $^{13}$C$^{16}$O$_{2}$ + $^{12}$C$^{18}$O$^{16}$O = $^{13}$C$^{18}$O$^{16}$O + $^{12}$C$^{16}$O$_{2}$ generally are driven toward the right, so that a population of isotopologues at equilibrium generally has greater abundances of multiply-substituted isotopologues than predicted by the stochastic distribution. This effect is subtle (typically permil), but the external precision of our measurements indicates it could be used as a thermometer with temperature sensitivity as good as $\sim$1.5 $\deg$C. We will discuss further examples of isotopic variations observed or expected to arise from thermodynamics, (e.g., air-sea exchange) classical physical processes (e.g., diffusion; thermogravitation), and photochemistry (e.g., methane photolysis; ozone chemistry).
DE: 1040 Isotopic composition/chemistry
DE: 1094 Instruments and techniques
DE: 0322 Constituent sources and sinks
DE: 0330 Geochemical cycles
DE: 0394 Instruments and techniques
SC: Atmospheric Sciences [A]
MN: 2004 AGU Fall Meeting