177,040 research outputs found
Multivariate Charts for Quality Control of Multiresidue Analytical Methods
No full textModern analytical chemistry involves more and more the use of statistical, mathematical and computer tools, both during the methods validation and, successively, during the internal quality control (IQC). Although these activities are far-back mandatory for the official laboratories which are accredited to ISO/IEC 17025 international standard, in practice analysts are not always sufficiently familiar with the statistical process control (SPC) theory. Furthermore, especially for multiresidue methods, the implementation of an ICQ using single univariate Shewhart charts for each quality characteristic (analyte) of the process can be very misleading [1]. This happens because in practice these quality characteristics are correlated. For this reason and to improve the effectiveness of IQC procedures, multi-analyte methods should also be controlled by multivariate methods that consider the joint distribution of variables. From this perspective, the Hotelling T2 chart represents an extension of the usual Shewhart charts to the multivariate case, and it is the most popular tool used for monitoring multivariate processes.
So far there have been few known applications of multivariate statistical control in analytical chemistry, probably due to the lack of adequate skills among chemists. The aim of this work is, therefore, to present a user friendly add-on package, called “izsqcc”, for the R statistical software
Application of the “inverted chirality columns approach” for the monitoring of asymmetric synthesis protocols
No full textIn the present study, the “Inverted Chirality Columns Approach (ICCA)” was applied to follow an asymmetric synthetic reaction, namely, the addition of butan-2-one to trans-β-nitrostyrene, catalysed by (S)-proline, leading to the formation of 3-methyl-4-phenyl-5-nitro-2-pentan-2-one. The ICCA method was applied to overcome the lack of pure enantiomeric standards.
The two widely employed (R,R)- and (S,S)-Whelk-O1 chiral stationary phases (CSPs), incorporating fully synthesized enantiomeric chiral selectors, were profitably used for this purpose. The enantioselective analysis with the two CSPs was performed under optimized reversed-phase conditions with a water/acetonitrile (60/40, v/v) eluent.
In the probe reaction under investigation, a diastereomeric excess > 90% was found according to a well- established reaction mechanism, thus affording the enantiomer couple (3S,4R)-3-methyl-4-phenyl-5-ni- tropentan-2-one and (3R,4S)-3-methyl-4-phenyl-5-nitropentan-2-one as the main product. Therefore, the at- tention was exclusively focused on this enantiomers pair.
Rather similar retention and separation factor [1.12 with (R,R)-Whelk-O1 and 1.13 with (S,S)-Whelk-O1] values as well as resolutions [2.06 with (R,R)-Whelk-O1 and 2.30 with (S,S)-Whelk-O1] were produced by the two enantiomeric CSPs.
Applying the ICCA concept allowed to identify the two enantiomers-related peaks in the chromatograms, ultimately indicating a 65-to-35 enantiomeric per cent ratio. Electronic circular dichroism (ECD) and high- resolution mass spectrometry analyses of the two peaks collected during the enantioselective analyses further confirmed the enantiomeric nature of the identified compounds. The (3S,4R) < (3R,4S) enantiomer elution order with the (R,R)-Whelk-O1 was fully disclosed thanks to ECD studies coupled with in silico quantum me- chanical simulations. As expected, reversed elution order turned out with (S,S)-Whelk-O1
Reaction of allyl acetates and ketene silyl acetals catalyzed by Palladium (0) complexes part II. Cyclopropanation vs. allylic alkylation
No full textPlatinacyclobutane complexes from nucleophilic attack at a coordinated allyl group and catalytic formation of cyclopropanes in the presence of platinum complexes
No full textMonthly variability of heavy metal accumulation and detoxifying response of freshwater Copepoda, Cyclops abyssorum
No full textDeterminazione rapida di residui di fitofarmaci nel miele mediante SPME-HRGC con rivelazione NPD ed ECD
No full textAppropriate Similarity Measures for Author Cocitation Analysis
No full textWe 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
Simultaneous determination of aminoglycosides and colistins in food
No full textA novel method for the simultaneous identification and quantification of twelve aminoglycosides (AGs) and two
colistins in meat and bovine milk has been developed. The analysis was carried out using liquid chromatography
coupled to quadrupole-Orbitrap mass spectrometry (LC-Q-Orbitrap). Among the HILIC (Hydrophilic Interaction
Liquid Chromatography) stationary phases tested, the bare silica Poroshell 120 provided the best results. The
samples were extracted with an aqueous solution followed by an SPE clean up based on the weak cation exchange
mechanism. The validation study was performed carrying out 72 experiments per matrix at six different
concentrations in a range encompassing the Maximum Residue Limits. The recoveries were from 72 to 87% in
meat (except colistins) and from 82 to 96% in milk. Repeatabilities and intra-lab reproducibilities were lower
than 10 and 15%, respectively. Limits of detection were lower than or equal to 33 μg kg−1. Finally, test materials
containing AGs prepared for interlaboratory studies were successfully analysed
Expeditious synthesis of substituted 2-thiazolin-4-ones by one-pot reaction of some conjugated azoalkenes with thioamides
No full text"Closing the R&D Gap, Evaluating the Sources of R&D Spending"
No full textBoth spending and tax policies have been implemented in the United States with the goal of stimulating private sector research and development (R&D). Karier questions whether current R&D policy, especially the research and experimentation tax credit, can contribute to closing the gap between nondefense expenditures on R&D in the United States and such expenditures in other countries, such as Japan and Germany. He also explores possible changes to our current R&D policy to make it more effective.
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