| This thesis presents procedures for the precise measurement of variations in the isotopic composition of Fe using multiple collector inductively-coupled plasma mass spectrometry (MC-ICP-MS), and data on the fractionation of Fe isotopes during anion exchange chromatography. The results of this research are reported in Anbar et al. (2000) and Roe et al. (in press).; Procedures were developed to minimize and correct for ArO+ and ArN+ interferences at masses 56 and 54 using a desolvating nebulizer. To correct for instrumental mass fractionation, an “element spike” was used. This procedure was adapted from MC-ICP-MS methods for Cu isotope analysis developed prior to this study and re-examined here (Marechal et al., 1999). Variations in 56Fe/54Fe and 57Fe/54Fe can be measured with a 2σ external precision of ±0.3‰ and 0.6‰, respectively.; In chromatographic experiments, heavier isotopes eluted more rapidly than lighter isotopes. Variations in 56Fe/54Fe of 1–10 parts per thousand were obtained using a 2 cm3 column. Variations in 57Fe/54Fe were larger by a factor of ∼1.5, as expected for mass-dependent fractionation. The extent of isotope separation was found to increase significantly as elution flow rate decreased, demonstrating that the magnitude of Fe isotope fractionation increases as more time is allowed for equilibration. This provides evidence that Fe isotope fractionation results from an equilibrium isotope effect, while also demonstrating that expression of this effect can be inhibited if the flow rate exceeds the equilibration rate. This conclusion is consistent with other experimental findings, including 54Fe tracer experiments that constrain the equilibration rate.; The isotope separation factor, 54KD/ 56KD, ranged from 1.0001 to 1.001. A small, but possibly significant, increase in 54KD/56K D was found with increasing elution acid strength. Fractionation is probably due to an equilibrium effect during speciation between Fe chloro-aquo complexes, particularly tetrahedral FeCl4− which binds to the anion-exchange resin, and six-fold coordinated FeCl 3(H2O)3o and FeCl2(H 2O)4+ species. Theoretical estimates by other workers of the isotope effect expected during equilibration of these species compare favorably with the values reported here (Schauble et al., 2001).; These results demonstrate that nonbiological chemistry can generate substantial Fe isotope variations comparable to those produced by Fe-reducing bacteria. Fe isotopic variations can arise from equilibrium isotope effects. The existence of such effects must be considered Fe isotope variations are used in “biosignature” applications (Beard et al., 1999). |
