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Fractionation between the three isotopes of Pb monitored during ICPMS analyses, 206 Pb, 207 Pb, and 208 Pb, will occur due to radioactive decay of U and Th within the sample. The half-lives of the respective decay series, 238 U → 206 Pb, 235 U → 207 Pb, and 232 Th → 208 Pb, range between ~700 million to ~14 billion years (Faure, 1986). Therefore, changes in the natural modern abundances of the Pb isotopes ( 206 Pb = 24.1%, 207 Pb = 22.1%, 208 Pb = 52.4%), due to the generation of radiogenic Pb within the sample may be significant, particularly in the geologically old (>2.5 Ga) samples commonly analyzed at JUB. Therefore, only in the case of Pb is Eq. 1 not used to monitor agreement between the concentrations determined from individual isotopes for a given element. For the remaining elements with more than one atomic mass available for monitoring, the application of Eq. 1 also provides a method for identifying interferences that may be affecting isotopes of a particular element. The issue of ions with masses similar to the isotopes of interest producing undesirable interferences in ICPMS analyses is common. The elements in geological materials most suitable for routine ICPMS analyses have masses greater than 80 atomic mass units (amu). This primarily results from the fact that lower mass elements (e.g., many transition metals) may suffer severe interferences from polyatomic species generated within the plasma during ionization of the sample. Many of these polyatomic interferences result from the use of argon as the plasma source gas in most ICPMS instruments, and the subsequent formation of Ar-oxides and Ar-hydroxides (ArO + and ArOH + ), though other interfering species may be significant due to the choice of acid used for decomposing and diluting samples (e.g., chloride and nitrogen species). A common example of the difficulties such polyatomic interferences present is evident in determinations of Fe, in which the isotope of choice for analysis is the most abundant one, 56 Fe (91.72%). However, within the plasma large numbers of 40 Ar 16 O + ions are produced which are indistinguishable from 56 Fe in low-resolution ICPMS analyses. High-resolution ICPMS instruments are capable of distinguishing between the 40 Ar 16 O + and 56 Fe peaks, but can only achieve this peak resolution at the expense of ion-beam transmission efficiency and a consequent reduction in instrument sensitivity. As a result high-resolution ICPMS methods are generally not suitable for element determinations in the sub-ppb (parts-per-billion, μg/kg) range necessary for analyses of many trace metals in geological samples, unless intensive 7
sample preparation methods are employed which isolate and concentrate the elements of interest from undesirable matrix elements. Table 2 contains interferences commonly observed in ICPMS analyses performed within the JUB Geochemistry Lab. The inability of the low-resolution DRC-e ICPMS to resolve interferences from the isotopes of interest does not significantly inhibit the accurate determination of the 32 elements routinely analyzed, as correcting for these interferences is possible through careful characterization and quantification of the interfering polyatomic species. The majority of these corrected interferences are for the REE (Table 2), as the REE can form significant amounts of oxide and hydroxide species during ionization. For example, at typical instrument settings as much as 3% of the Ce present in the sample solution will be ionized to 140 Ce 16 O + , and the 140 Ce 16 O + ion will consequently contribute to any Gd determinations that utilize the 156 Gd isotope. The isotopes monitored for quantification of the REE are therefore carefully selected to minimize these effects and maximize analytical accuracy, though as seen in Table 2 a significant number of corrections are necessary. While implementing such corrections is not trivial, uncorrected values may result in reported concentrations for REE elements that are erroneously high by as much as several percent, as illustrated by the example with CeO + . Corrections for polyatomic interferences are performed mathematically, typically offline using commercially available spreadsheet software such as Microsoft Excel (the approach followed at JUB). Interferences are identified and quantified by analyzing solutions that contain relatively high concentrations (50-1000 μg/kg) of a single element, and surveying the range of masses that might be affected by interferences produced by oxide, hydroxide, chloride, or nitrogen species of that particular element. For the REE, the most significant interferences are generated by REE-oxides or -hydroxides, and therefore are found at 16 and 17 amu above the interfering element (i.e., REE 16 O + and REE 16 O 1 H + ). The measured intensities of the interfering element, and the various interferences it produces, allow calculation of ratios expressing the relative amount of interfering species generated as a function of the concentration of the interfering element. For example, if as mentioned above, 3% of 140 Ce in the sample is ionized to 140 Ce 16 O + (i.e., 140 Ce 16 O + / 140 Ce equals 0.03), and if the Ce concentration in the sample corresponds to a measured intensity of 100,000 8
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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
Page 12 and 13: goal of the research is to discern
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Page 52 and 53: Abstract The benefits of inductivel
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Govindaraju K. (1995) 1995 working
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Appendix 1. Analytical data Appendi
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Appendix 1. Literature reference va
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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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