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Activated aluminum oxide selectively retaining long chain n-alkanes. Part I, description of the retention properties
No full textAluminum oxide activated by heating to 350–400 ◦C retains n-alkanes with more than about 20 carbon
atoms, whereas iso-alkanes largely pass the column non-retained. Retention of n-alkanes is strong with
n-pentane or n-hexane as mobile phase, but weak or negligible with cyclohexane or iso-octane. It is
strongly reduced with increasing column temperature. Even small amounts of polar components, such
as modifiers or impurities in the mobile phase, cause the retention of n-alkanes to irreversibly collapse.
Since n-alkanes are not more polar than iso-alkanes and long chain n-alkanes not more polar than those
of shorter chains, retention by a mechanism based on steric properties is assumed. The sensitivity to
deactivation by polar components indicates that polar components and n-alkanes are retained by the
same sites. The capacity for retaining n-alkanes is low, with the effect that the retention of n-alkanes
depends on the load with retained paraffins. These retention properties are useful for the pre-separation
of hydrocarbons in the context of the analysis of mineral oil paraffins in foodstuffs and tissue, where plant
n-alkanes, typically ranging from C23 to C33, may severely disturb the analysis (subject of Part II)
On-line solvent evaporation for coupled LC systems: further developments
No full textAn on-line solvent (eluent) evaporator is used for reconcentrating a fraction from a first LC column before introduction into a second LC separation step, possibly involving another mobile phase. For pentane and pentane containing 10% of dichloromethane, evaporation rates of 1 ml/min were achieved, On evaporating a volume of 6 ml, 2-ethylnaphthalene was retained to more than 75%, Separation of aliphatic and polyaromatic hydrocarbons in a linseed oil is shown as an application
Activated aluminum oxide selectively retaining long chain n-alkanes: Part II. Integration into an on-line high performance liquid chromatography-liquid chromatography-gas chromatography-flame ionization detection method to remove plant paraffins for the determination of mineral paraffins in foods and environmental samples
No full textAluminum oxide activated by heating to 300–400 ◦C retains n-alkanes with more than about 20 carbon
atoms, whereas iso-alkanes largely pass non-retained (with characteristics described in more detail in
Part I). This property is useful for the analysis of mineral oil contamination of foods and other matrices: it
enables the removal of plant n-alkanes, typically ranging from C23 to C33, when they disturb the analysis
of mineral paraffins (usually almost exclusively consisting of iso-alkanes). An on-line HPLC–LC–GC–FID
method is proposed in which a first silica gel HPLC column isolates the paraffins from the bulk of edible
oils or extracts and is backflushed with dichloromethane. In a second separation step, a 10 cm×2mmi.d.
column packed with activated aluminum oxide separates the long chain n-alkanes fromthe fraction of the
iso-alkanes which is transferred to GC–FID by the on-column interface and the retention gap technique.
The retained n-alkanes are removed by flushing with iso-octane
On-line HPLC (LC)-Solvent Evaporation-LC-GC-FID for the determination of aromatic hydrocarbons in food contaminated with mineral oil
No full textRelative hopane content confirming the mineral origin of hydrocarbons contaminating foods and human milk
No full textHopanes, triterpenoid hydrocarbons formed under
geological conditions, were analysed to confirm the
mineral origin of the unresolved complex mixtures of
hydrocarbons observed in the gas chromatography with
flame ionization detection chromatograms of human
milk and certain foodstuffs. The ‘relative hopane content’
(RHC) is introduced, i.e. it is the area ratio of
the sum of the hopanes and the paraffins in the same
segment of the chromatogram. The RHC in various
mineral oil products (motor oils, hydraulic oils, lubricating
oils, Vaseline) was 3.4%, with a relative
standard deviation of 19%. The RHC determined in
samples of vegetable oils, mussels and clams as well
as of human milk containing an unresolved complex
mixture of hydrocarbons was in the same range,
confirming that these samples were contaminated by
mineral oil material
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Get PDFWe 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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