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Accurate and precise determination of uranium concentration and isotopic composition by multi-collector inductively coupled plasma mass spectrometry
No full text本研究利用多偵測器感應耦合電漿質譜儀發展了一套精準的鈾同位素比值及含量測量技術，精確度可達萬分比等級。我們使用四個法拉第杯同時測量鈾同位素, 233U, 234U, 235U, 及236U。利用233U-236U 雙示蹤劑有效校正質量分化效應以及計算鈾濃度。複雜之背景干擾，主要因不同238U 訊號、真空狀況及基質而改變外，亦235U 拖尾所影響，故同位素之背景值必須個別校正。若以簡單的指數行為描述背景頻譜，對233U, 234U 及236U的訊號強度，將造成萬分比至千分比的錯估。以進樣速率為每分鐘50 ml，測量鈾濃度2 ppm 約2-6 分鐘，僅需200-600 ng U 的標本量，δ234U 和鈾含量的測量精準度可達± 0.1-0.5‰ (2σ)。分析國際及實驗室之標準品，長時間之δ234U 及鈾含量測定重現性分別可達0.12‰及0.47‰，且與其他質譜術量測數值相吻合。重複分析國際標準品NBL-112A 和HU-1 的δ234U 平均結果分別為-37.10 ± 0.03‰ (2σm, n = 17) 和-0.17 ± 0.05‰ (2σm, n = 5)。自然界具有不同基質之各種珊瑚及洞穴石筍碳酸鹽樣品的δ234U 及鈾濃度測量值與其他千分比準度之質譜術所得數值一致。此分析技術將可應用至鈾系定年法，古氣候學，及岩石地球化學之相關研究。Techniques used for precise and accurate determination of natural uranium isotopic ratios and concentration with sub-permil precision have been developed on a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS), Thermo Electron NEPTUNE. Uranium isotopes, 233U, 234U, 235U, and 236U, were statically measured in 4 Faraday collectors. A 233U-236U double spike method was employed to correct mass bias and determine uranium concentration. The behavior of complicated spectral background is attributed to tailing characteristics of 235U and 238U ion beams as well as vacuum condition and matrix effect. Individual background correction should be applied to the measured isotopes, 233U, 234U, and 236U, or a bias of sub-permil to permil level could be caused by using a simple exponential function correction. By introducing 2 ppm uranium with a uptake rate of 50 ml/min for 2-6 minutes, a sample size of 200-600 ng U is needed to offer a within-run precision of 0.1-0.5‰ for d234U and uranium concentration. Measurements made on reference materials and in-house standards demonstrate that an external reproducibility of 0.12‰ can be achieved for d234U and 0.47‰ for uranium concentration. Repeated measurements of NBL-112A and HU-1 give averaged d234U values of -37.10 ± 0.03‰ (2σm, n = 17) and -0.17 ± 0.05‰ (2σm, n =5), respectively, within error of accepted values. MC-ICP-MS data of d234U and uranium concentration for different carbonates, including corals and speleothems with various matrices, are in agreement with those measured by other mass spectrometric methods with a permil-level precision. This technique can be applied to the exploration of the frontiers in diverse fields of Earth Sciences, such as U series chronology, paleoclimatology, and geochemistry.List of Figures ...III
List of Tables ...V
摘要(Abstract in Chinese) ...1
Abstract ...2
Chapter 1 Introduction ...3
Chapter 2 Experimental Methods ...6
2.1 Materials and chemistry ...6
2.1.1 Standards and samples ...6
2.1.2 Chemistry ...6
2.2 Instrumentation ...7
2.2.1 MC-ICP-MS ...7
2.2.2 Introduction system ...8
2.3 Detector calibration ...9
2.3.1 Faraday cup: gain and baseline ...9
2.3.2 SEM: yield and dead time ...9
2.4 Spectral interference ...10
2.4.1 Instrumental memory blank ...10
2.4.2 Scattering of reflected 238U ions ...11
2.4.3 238U and 235U tailing ...13
2.5 Analytical procedure ...14
2.5.1 Analysis protocols ...14
2.5.2 Measurement sequence ...15
2.5.3 Data acquisition ...16
2.5.4 Off-line data reduction ...17
Chapter 3 Results and Discussion ...18
3.1 Background correction for U tailing ...18
3.1.1 Behaviors of 235U and 238U tailing ...18
3.1.2 Correction of tailing background ...19
3.1.3 Mixtures of standard and 233U-236U tracer ...23
3.2 Precision and accuracy ...24
3.2.1 Protocol-1 with 6S-jumping mode ...24
3.2.2 Protocol-1 with 2S-jumping mode ...24
3.2.3 Comparison between the 6-step and the 2-step jumping modes ...26
3.2.4 Protocol-2 ...28
3.3 Natural carbonate standards and samples ...29
3.4 Comparison with other mass spectrometric techniques ...34
Chapter 4 Conclusions ...37
References ...38
Acknowledgments ...4
Simultaneous lead isotope ratio and gold-lead-bismuth concentration analysis of silver by laser ablation MC-ICP-MS
Full text linkA new approach is presented for the simultaneous analysis of lead isotope ratios and gold, lead, and bismuth concentrations in metallic silver using nanosecond laser ablation multi-collector inductively-coupled plasma mass spectrometry (LA-MC-ICP-MS). Corrections for both isotope and concentration analyses are performed using an in-house matrix matched silver reference material RM3834. Accuracy and external reproducibility are demonstrated by repeat analyses of a further seven silver reference materials all characterised by solution (MC)-ICP-MS approaches. Typical internal precisions, expressed as two relative standard errors (S.E.) of the mean of the cycles comprising one analysis, are <0.5% for
206Pb/
204Pb,
207Pb/
204Pb and
208Pb/
204Pb, and <0.03% for
207Pb/
206Pb and
208Pb/
206Pb. External reproducibilities, based on repeat analyses over a 10 month period and expressed as two relative standard deviations (S.D.) of the mean, are <0.4% for
206Pb/
204Pb,
207Pb/
204Pb and
208Pb/
204Pb and <0.2% for
207Pb/
206Pb and
208Pb/
206Pb. Internal precisions and external reproducibilities of gold, lead and bismuth concentration analyses are typically <25%. Data are consistent with solution-based approaches. Methods are further demonstrated through analyses of a set of Islamic silver dirhams from the mint of al-Muhammadiyya, highlighting their applicability to geochemical studies of archaeological artefacts.
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Controls on nickel isotope compositions of modern seawaters and marine sediments
No full textIn this thesis, Ni isotope fractionation during marine phytoplankton uptake and amorphous Ni-sulfide formation are characterized, to improve the understanding of biogeochemistry of Ni and its isotopes in the ocean, from the surface to the bottom of the ocean. The first direct Ni isotope measurement in phytoplankton cells suggests that marine phytoplankton preferentially take up heavy Ni isotopes from the culturing medium with species-dependent isotope fractionation factors. This finding contrasts to previous hypothesis in the community that biological activity consumes isotopically light Ni in the surface ocean where the enrichment of isotopically heavy Ni is observed. The contradiction inspires a new framework of considering relative binding strength between biological metal acquisition mechanism and ambient ligands, which reconciles findings from culturing and the modern ocean. Coccolithophores show an enhancement of Fe, depletions of Co and Cu, and a decreasing isotopic pattern; with increasing Ni availability in the media, which are not seen in cultures of other species. The distinct Ni/Co efflux proteins for regulating excess cellular Ni shown in coccolithophores may cause their contrasting isotopic pattern. The Ni isotope fractionation during amorphous sulfide formation and the mineral transformation from amorphous to crystalline sulfides; suggest a potential benthic flux of isotopically heavy Ni from sulfide-containing sediments in euxinic ocean settings. This diagenetic remobilization of Ni is supported by the slightly lighter Ni isotope compositions found in nearshore euxinic sediments off Namibian margin; compared to the offshore sediments, which are anoxic but not euxinic according to the porewater chemistry. This thesis addresses important processes in the biogeochemical cycling of Ni and its isotopes in the modern ocean, demonstrating the potential and limitation of its application as a novel geochemical tracer
Controls on distributions of aluminium, manganese and cobalt in the South Atlantic Ocean along GEOTRACES transect GA10
Full text linkTrace metals (TMs) manganese (Mn), cobalt (Co), and aluminium (Al) have important geochemical and biological roles in the ocean. Here, we present full depth profiles of dissolved (d) and particulate Al, Mn, and Co along the latitude of 40 °S in the South Atlantic Ocean from the GEOTRACES GA10 cruises that operated in austral spring 2010 and summer 2011. The region is characterized by enhanced primary productivity and forms a key transition zone between the Southern Ocean and South Atlantic Subtropical Gyre. The mean concentrations of dAl, dCo, and dMn (±standard deviation) were 3.36 ± 2.65 nmol kg−1, 35.3 ± 17.6 pmol kg−1, and 0.624 ± 1.08 nmol kg−1, respectively. Their distributions in surface waters were determined by external sources and complex internal biogeochemical processes. Specifically, surface ocean dCo was controlled by the interplay between phytoplankton uptake, remineralization and external inputs; dMn was likely determined by the formation and photoreduction of Mn-oxides; and dAl was supplied by atmospheric deposition and removed by scavenging onto particles. Fluvial and sedimentary inputs near the Rio de La Plata estuary and benthic sources from the Agulhas Bank resulted in elevated dTM concentrations in near-shore surface waters. These externally sourced dTMs were effectively delivered to the open ocean by offshore diffusion and/or advection, and potentially facilitated enhanced primary productivity along the transect.
The distributions of dTMs at depth were predominantly controlled by the mixing of North Atlantic Deep Water (NADW) and waters of Antarctic origin (e.g., Upper Circumpolar Water (UCDW) and Antarctic Bottom Water (AABW)). The calculated endmember concentrations of dAl and dCo in NADW showed minor decreases in the SASTG following north–south transport, suggesting removal rates of 0.064 nM/year and 0.035–0.075 pM/year, respectively. The endmember concentration of dCo in AABW was maintained at ∼30 pmol kg−1 without evidence for scavenging removal in the Southern Ocean and SASTG (time frame >400 years). The concentrations of dMn in NADW and AABW were between 0.1 and 0.16 nmol kg−1, and any elevated dMn concentrations were ascribed to local external inputs (e.g., from sediments in the Argentine Basin and hydrothermal activity near the Mid-Atlantic Ridge). Hence, four controlling factors (sources, internal cycling, water mass mixing and time) need to be considered when assessing TM distributions in the global ocean, even for TMs that are vulnerable to scavenging removal processes. Because the deep waters formed in high latitude oceans are crucial components of the global thermohaline overturning system, any processes (e.g., glacier melting, upwelling and sinking, and biological activity) that impact the preformed dTM concentrations in high latitude oceans will determine the downstream dTM distributions. Therefore, the sources and sinks of TMs and associated biological activity in high latitude oceans could engender basin to global scale impacts on seawater distributions of Al, Co, and Mn and their stoichiometric relationships with macronutrients, and the global biogeochemical cycles of these scavenged-type TMs
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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