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0.90, corresponding to an anomalous increase in measured analyte intensities in the sample, to 1.10, indicating an anomalous decrease in measured analyte intensities. Experience has shown that analytical accuracy is often compromised when IS correction factors deviate from unity by more than ~15% (i.e., 1.15), and it may be necessary to re-analyze the samples using a different dilution factor to minimize matrix and/or drift effects. Following internal standard corrections, interference corrections for polyatomic species are applied to the sample data. These interference corrections are applied only to elements with atomic masses between 151 (Eu) and 183 (W). A detailed treatment of the determination and application of these interference corrections is beyond the scope of this discussion, and the reader is referred to Dulski (1994). The last correction applied to the sample data before calculating elemental concentrations involves subtracting the internal standard and interference corrected blank contribution, which would be due to the reagents used in preparing the samples, and/or from any laboratory procedures. 4.5. Analytical blanks and quantification limits Two blank contributions may be defined for the described ICPMS methods. The first is the acid blank value, which is used for calculating the instrument detection and quantification limits (IDLs and IQLs). As all ICPMS solutions are analyzed in a 0.5 M acid matrix (either HCl or HNO 3 ), the acid blank is simply the purest 0.5 M acid solution that is capable of being produced within the JUB Geochemistry Lab. Analyses of the acid blank allows the calculation of IDLs and IQLs for any given element, which represent the best achievable (ideal) ICPMS sensitivity using the available laboratory reagents. The IDL and IQL equal 3x and 10x the standard deviation of the acid blank, respectively (MacDougall and Crummett, 1980), and are best characterized through long term, repeated analyses of the acid blank, as daily fluctuations in instrument sensitivity are common. It is important to note that IDLs and IQLs calculated from the acid blank are not equivalent to detection and quantification limits in a solid sample, that is, in a rock or a mineral. As all solid samples are highly diluted before ICPMS analyses, IDLs and IQLs determined for solid samples equal the acid blank multiplied by the appropriate dilution factor. Table 2 lists long term average IQLs in solid samples for 0.5 M HCl and 0.5 M HNO 3 acid matrices. 15
The second blank contribution to ICPMS analyses is the method blank. The method blank is a solution that has been processed in exactly the same manner as a sample during the decomposition method, and represents the contamination added to a sample as a result of the decomposition method and subsequent dilution in acid. While significant amounts of concentrated acid are used in the decomposition methods, these acids (in combination with contamination from labware) contribute insignificant amounts of trace metals to rocks typically analyzed, even for trace metal-poor samples like iron-formations and dolomites (Fig. 4). Method blank values are determined for every ICPMS analysis to monitor possible contamination sources, but as a result of the above observations, ICPMS data are blank corrected by subtracting the acid blank intensities. Only in instances where method blank values are significantly higher than those observed in Fig. 4 are blank corrections performed using the method blank intensities. The reasoning for this approach is as follows; since the method blanks are typically below the IQLs determined from the acid blanks, the method blank is not quantifiable. Only when method blanks exceed the acid blank IQL are they used for the blank correction. It should be noted that with regard to the carbonate decomposition method, many refractory elements (e.g., Ti, Nb, Ta) are not suitable for quantification, as they are expected to be primarily hosted in silicate-bearing phases that are resistant to dissolution with nitric acid, and in particular the poor IQLs for Sc mean that it is generally not quantifiable in trace metal-poor samples regardless of the decomposition method used (Fig. 4). 5. Analytical precision For the analyses conducted within the Geochemistry Lab at JUB, analytical precision may be defined in different ways. The highest degree of precision is expected when considering only the 60 mass scans performed during ICPMS analysis of a single sample solution. If this sample solution is periodically re-measured with time during an ICPMS run, i.e., by occasionally returning the autosampler probe to the vial containing the solution, then analytical precision would be expected to decrease. Further decreases in analytical precision should occur if one considers repeated analyses of the same sample solution on different days, or repeated acid decompositions of the same sample powder, with the decreasing precision reflecting 16
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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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100 10 MORB 1 0.1 raw data (mg/kg)
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Page 42 and 43: Haley B.A., Klinkhammer G.P. (2003)
Page 44 and 45: Lee J.H. and Byrne R.H. (1993) Comp
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Page 52 and 53: Abstract The benefits of inductivel
Page 54 and 55: List of Figures Figure 1. Sample de
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Page 112 and 113: Govindaraju K. (1995) 1995 working
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Page 118 and 119: 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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