1,720,958 research outputs found
Determination of calibration function in thermal field flow fractionation under thermal field programming
No full textA new procedure for determining the calibration function able to relate retention and operative parameters to molecular weight of the species in Thermal Field Flow (ThFFF) under Thermal Field Programming (TFP) conditions is presented. The procedure involves determining the average values of retention parameters under TFP and determining a numerical function related to the temperature variations that occur during TFP. The calibration parameters are obtained by a procedure fitting the retention and operative parameters that hold true at the beginning of the TFP. The procedure is closely related to the one previously developed to calibrate the retention time axis under TFP ThFFF and, together, they constitute a full calibration procedure. Experimental validation was performed with reference to Polystyrene (PS) -decalin and PS-tetrahydrofurane (THF) systems. The calibration functions here obtained were compared to those derived by the classical procedure at constant thermal field ThFFF to obtain the calibration function at variable cold wall temperatures. Excellent agreement was found in all cases proving “universality” of the ThFFF calibration concept, i.e. it is independent of the particular system on which it was determined and can thus be extended to ThFFF operating under TFP. The new procedure is simpler than the classical one since it requires less precision in setting the instrumentation and can be obtained with a fewer experiments. The potential applications for the method are discussed
Programmed field decay thermal field flow fractionation of polymers: A calibration method
No full textThe universal calibration procedure typical of thermal field
flow fractionation (ThFFF) under constant thermal field
operation was extended to thermal field programming
(TFP) operation. The method requires knowledge of the
following: (a) the programming function, which only
depends on the thermal field decay function, (b) the
physicochemical properties of the solvent, and (c) the
calibration plot under varying channel cold wall temperatures
(Tc). Two field flow fractionation field programming
conditions, with either a constant or a variable in time
carrier flow velocity, are exploited. The method is based
on determination, for each retention time position, of the
average ì retention value typical of TFP ThFFF. This
parameter is then used to obtain the calibration plot (i.e.,
the molecular weight of the species as a function of the
retention time position) by using the programming function
and the calibration plot under varying Tc values. The
procedure approximation errors are also derived as a
function of the programming type and solute-solvent
system. To properly test the procedure, the calibration
plot for the system constituted by polystyrene (PS) in cistrans
Decalin was determined, under varying conditions
Tc and thermal gradients, by using a set of monodisperse
PS standards of different molecular weights (M). The
procedure was first validated by simulation under two
typical cases of TFP ThFFF operation. The approximation
errors were found acceptable (in the worse cases, the
accuracy in M prediction was 3%) and are in agreement
with the theory. The procedure was then experimentally
validated under varying programming decay function
conditions. The reproducibility and accuracy of the M
determination are both better than 2%
Calibration in Thermal Field Flow Fractionation with Polydisperse Standards: Application to Polyolefin characterization
No full textA new procedure for calibrating ThFFF systems by using broad molecular weight standards based on a non-linear least squares fitting of the fractogram to the Molecular Weight Distribution to derive the system calibration parameters is presented and
explored by simulation. This procedure proved robust with respect to noise and it is able to account for detector nonlinearity. The effect of band broadening was evaluated
and it was found that significant bias is observed only for polydispersities lower than 1.3. The method was experimentally validated by using polystyrene in decalin and was then applied to polypropylene under conditions of high temperature ThFFF (Tc = 394 K). It appears that the presented method can be useful in characterising polyolefins polymers for which monodisperse standards are not available
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
Trade-off between sexual activities and parental care: an experimental test using handicapped mates
No full textElecting to invest in parental care is an adaptive decision thought to involve a trade-off between remating and continuing parental effort. The rock sparrow, Petronia petronia, is an unusual species in which parental investment is highly variable and both sexes may desert the brood. Males contemporaneously engage in parental care, mate guarding, and courting their current or new females. In this study we experimentally handicapped male rock sparrows during the nestling period by increasing their body mass in order to study the effects on male behaviour and the female response. Handicapped males exhibited lower sexual activity than control males but handicapped males did not reduce their offspring feeding rates. Females with a handicapped partner significantly increased the number of sexual soliciting postures towards their mates compared to females paired with control males. The females' behaviour is probably a response to the sexual behaviour change of their partners. Our results suggest that with choices involving a trade-off between mating investment and parental investment, handicapped males chose the parental investment option
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