1,721,006 research outputs found
Pb isotopic analysis of standards and samples using a 207Pb-204Pb double spike and thallium to correct for mass bias with a double focusing MC-ICP-MS
No full textA Pb isotope analytical method using a Pb-207-Pb-204 double spike (DS) and a double-focusing MC-ICP-MS routinely yields standard and sample data with an external reproducibility <100 ppm for all Pb isotope ratios. With modest increases in analysis times (<10 min), external reproducibility of :! 50 ppm is attainable, e.g., SRM 981-Pb-206/Pb-204=16.9418 +/- 6; Pb-207/Pb-204=15.5000 +/- 6; Pb-208/Pb-204=36.7265 +/- 19 (n=26; 2 S.D.). This is achievable even when unspiked and spiked runs are analysed sequentially, demonstrating that memory due to use of the DS can be easily overcome by short and careful washout routines coupled with on-peak zeroes measurements. Replicate analyses of three mixtures of SRM 981 and SRM 982, in variable proportions, lie within 42 ppm of a mixing line between SRM 981 and SRM 982 Pb isotope ratios. Coupled use of thallium to correct for mass bias during the same analyses of standards and samples does not produce Pb isotopic data of comparable quality to the DS-corrected analyses. While the approach of Woodhead (A simple method for obtaining highly accurate Pb isotope data by MC-ICP-MS, J. Anal. At. Spectrom., 17 (2002) 1381-1385) allows us to produce accurate thallium-corrected Ph isotopic data, the DS data are at least five times more precise and accurate than those obtained when using thallium to correct for mass bias. Relatively rapid analytical throughput (20-25 samples/day), elimination of differential loading blanks between unspiked and spiked runs, easy optimal spiking and the lack of anomalous 207 Ph behaviour as observed sometimes during TIMS analysis make DS analysis by MC-ICP-MS an attractive procedure.
New Ph DS data on lavas from the Torfajokull volcanic centre (Iceland) define near-perfect binary mixing within our errors, even over a very small range in Pb-207/Pb-204 (<300 ppm), comparable to or smaller than the reproducibility of thallium-corrected Pb isotopic data. We also show that recently published Pb isotopic data for Theistareykir picrites (Stracke, et al., Theistareykir revisited, Geochem. Geophys. Geosyst., (2003) Paper no. 2001GC000201), corrected for mass bias using thallium, are up to 3300 ppm inaccurate compared to our DS data and also that of Thirlwall (Interlaboratory and other errors in Pb isotope analyses investigated using a Pb-207-Pb-204 double spike, Chem. Geol., 163 (2000) 299-322) and Thirlwall et al. (Mantle components in Iceland and adjacent ridges investigated using double-spike Pb isotope ratios, Geochim. Cosmochim. Acta., 68, (2004) 361386). These Icelandic data highlight both the potential and the dangers of Pb isotopic analysis by MC-ICP-MS and demonstrate the need for all MC-ICP-MS Pb isotopic studies to include analyses of well-characterized and isotopically homogeneous rock standards in addition to SRM 981. To this end, we present ca. 100 analyses of international standard reference materials, including the NIST 610-612-614 glasses that are used for in situ Pb isotope analysis by laser ablation ICP-MS and SIMS. While the Pb isotope compositions of some standards show poor reproducibility due to variable contamination during initial preparation, four of the reference materials (JB-2, NOD-A-1, AGV-1, 13CR-1) and the NIST 610 and 612 glasses reproduce to <200 ppm.
We also note that Pb isotopes are fractionated during normal anion exchange chemical separation of Pb, as collection of Pb in HCl significantly fractionates Pb isotope ratios, with the lighter isotopes preferentially eluted first. Provided a sufficiently large volume of greater than or equal to6 M HCl is collected (>10x the resin volume), there is no measurable (<50 ppm) fractionation of the Pb isotopes as the light-isotope-depleted tail is fully stripped from the resin
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
Sea-level position at ~80 ka based on phreatic overgrowths on speleothems from Mallorca.
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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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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