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Table 3. Change in HFSE concentration in IF-G iron-formation stored in 0.5 M HCl over a period of ~11 months. mg/kg IF-G 27-Oct-06* IF-G 16-Sep-07 % difference 2007/2006 Ti 23.9 23.5 -1.88 Zr 0.903 0.865 -4.24 Nb 0.914 0.876 -4.12 Mo 0.562 0.604 7.50 Hf 0.0246 0.0227 -7.57 Ta 0.168 0.157 -6.71 W 251 248 -1.35 * date of ICPMS analysis. longer time scales, as measured HFSE values in the IF-G geostandard change by less than 8% over a period of ~11 months (Table 3). 4.3. Data collection Unknown samples that are analyzed within an ICPMS run (i.e., in a single day, as part of one or more sample batches that are analyzed consecutively), are generally measured once, whereas blanks, calibration standards, and CRMs are measured repeatedly. Prior to beginning any ICPMS measurements, the instrument is prepared by performing three consecutive analyses of the CRM solution to be used during that ICPMS run. This has the primary effect of conditioning the cones located in the interface region between the plasma and the mass spectrometer. These cones are instrumental in shaping the ion beam and must be periodically cleaned, and this pre-treatment significantly stabilizes signal intensities. A rinse solution is pumped through the sample introduction system before the measurement of any solution, be it a blank, standard, or sample. Nitric acid (0.5 M) is the typical rinsing agent, and a six minute rinsing step has been found effective for minimizing memory effects. An ICPMS measurement of any single solution consists of 60 individual scans through the mass range 45 Sc to 238 U using peak hopping mode and a peak dwell time of 50 ms. For any individual element, these 60 scans are then averaged to produce the raw ICPMS data in cps. Scanning time for a single solution is approximately 3.5 minutes, and when rinsing time, the time required for the autosampler to move to a different 13
solution, etc., are considered, the total amount of time necessary to measure any single solution is ~10 minutes. 4.4. Internal standardization After an ICPMS analysis, the raw data in cps are transferred to an Excel spreadsheet for the application of three separate mathematical corrections. These corrections are applied to all analytical solutions (blanks, calibration standards, samples) in the following order: 1) internal standard corrections; 2) corrections for interfering polyatomic species; and 3) blank corrections. The internal standard (IS) correction using 101 Ru, 187 Re, and 209 Bi follows the method of Doherty (1989). The general application of the internal standard correction method is illustrated by the following equation, where Ru is used to correct the signal intensity of Sr: I ⎛ ⎞ ⎜ I Ru, std = ∗ ⎟ , sample I Sr, sample (2) ⎜ ⎟ ⎝ I Ru, sample ⎠ corr Sr where I is raw intensities in cps, I Ru,std is the measured intensity for the second 10 μg/kg calibration standard within an ICPMS sample batch (see Fig. 3), and I corr is the corrected intensity. The term in parentheses in (2) is the IS correction factor. This approach assumes that any matrix or instrument drift effects that suppress or enhance the signal intensity for Sr proportionally affect the Ru signal intensity, and internal standard corrections are most effective when the element of interest and the IS are of similar mass (Thompson and Houk, 1987). Therefore, 101 Ru is used as an internal standard for all elements with atomic masses lower than 101 (Sc, Ti, Co, Ni, Rb, Sr, Y, Zr, Nb, and Mo), and 209 Bi is used for elements with masses greater than 209 (Th and U). For analytes with masses between Ru and Re, the IS correction factor term in (2) is expanded to a mass-dependent linear function between 101 Ru and 187 Re: IS correction factor = ⎛ ⎜ I ⎜ ⎝ I Re, std Re, sample Ru, sample ( M analyte − M Ru ) I Ru, std + ( M Re − M Ru ) I Ru, sample Ru, std − * (3) I I ⎞ ⎟ ⎟ ⎠ where M is the atomic mass number for Ru, Re, and the analyte of interest. This treatment provides an IS correction factor that varies smoothly as a function of mass between 101 Ru and 187 Re. Internal standard correction factors typically range from 14
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Iron sedimentation and the neodymiu
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The process(es) by which IFs were d
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This thesis represents original and
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viii
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goal of the research is to discern
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Figure 1. Map of southern Africa sh
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appropriate to briefly discuss: 1)
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Page 52 and 53: Abstract The benefits of inductivel
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Appendix 1 continued. Data in mg/kg
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the concentration of the major elem
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Potential interferences on monoisot
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1500 0.5 M HCl 4.0 interference on
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4000 0.40 0.20 3500 0.17 mg/kg 0.18
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interference on 95 Mo in sample sol
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Nd isotopes in 2.9 Ga Archean surfa
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Table 2 Trace element concentration
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Author's personal copy B.W. Alexand
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and preservation of oxide-facies IF
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TOP relative stratigraphic position
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The mechanism by which the alkali m
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6. Widdel, F. et al. Ferrous iron o
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82 PRESERVATION OF PRIMARY REE PATT
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ulk, open ocean seawater tended to
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