Achievable sets, brambles, and sparse treewidth obstructions

One consequence of the graph minor theorem is that for every k there exists a finite obstruction set Obs(TW?k). However, relatively little is known about these sets, and very few general obstructions are known. The ones that are known are the cliques, and graphs which are formed by removing a few edges from a clique. This paper gives several general constructions of minimal forbidden minors which are sparse in the sense that the ratio of the treewidth to the number of vertices n does not approach 1 as n approaches infinity. We accomplish this by a novel combination of using brambles to provide lower bounds and achievable sets to demonstrate upper bounds. Additionally, we determine the exact treewidth of other basic graph constructions which are not minimal forbidden minors.     

A new technique for the identification of chloride ions by the formation of chromyl chloride

Summary A new technique is described for the identification of the chloride ion through the formation of chromyl chloride, based on the extraction of the latter with carbon tetrachloride. The identification limit is 2g of chloride. It is possible to carry out the test in the presence of many other anions, including fluoride, which cannot be present when the traditional distillation technique is employed.

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Zusammenfassung Vorgeschlagen wird eine neue Technik zum Chloridnachweis durch Bildung von Chromylchlorid. Dieser Nachweis beruht auf der Extraktion des Chromylchlorids mit Tetrachlorkohlenstoff. Die Nachweisgrenze liegt bei 2g Chlorid, wobei die Möglichkeit besteht, den Test auch in Gegenwart von Fluorid durchzuführen, das bei Anwendung der üblichen Destillationsmethoden nicht anwesend sein darf.

     

Multipurpose chamber for the implementation of gas diffusion, dialysis, solid-phase extraction and precipitation/dissolution in continuous flow analyzers

A multipurpose chamber capable of implementing a variety of continuous separation techniques is reported. The proposed module can be used in conjunction with flow manifolds coupled on-line to optical detectors in order to develop simple autoanalyzers for full sample pretreatment. The analytical applicability of such a device is discussed with specific regard to gas diffusion, dialysis, solid-phase extraction (SPE), and precipitation/dissolution techniques. Its versatility and simplicity are among the main advantages. Four different automated analyzers based on the use of the multipurpose chamber have been developed for monitoring of different quality control parameters of liquid food samples, namely, carbon dioxide in beer, ascorbic acid in fruit juice, caffeine in soft drinks and lactose, fat and protein in milk. The results obtained were compared with those provided by the existing alternatives for the same applications in terms of sensitivity and precision.     

Polarographic determination of glycine with a dropping copper amalgam electrode

The anodic oxidation of a dropping copper amalgam electrode in presence of dilute solutions of glycine in 0.50 M NaClO₄ has been studied. An anodic wave at ?0.28V (SCE) is observed, yielded by diffusion of glycinate anion in the solution towards the electrode surface. The wave-heights increase with the glycinate concentration (function of glycine concentration and pH value) until the anodic oxidation is controlled by the metal diffusion into mercury. The effect of pH is interpreted by attributing it to the depolarizer effects at glycinate anion even though the zwitterion is present in much larger concentrations. The applicability of anodic oxidation of a dropping copper amalgam electrode in the determination of glycine in the range of concentrations 10^?4–10^?2M with a rigorous control of pH (8.0<pH<10.5) is shown. The standard deviation of the proposed method is 4.1% and the minimum concentration determinable is in the 1×10^?4M range.     

Magnetically confined hydrophobic nanoparticles for the microextraction of endocrine-disrupting phenols from environmental waters

In this article, a solid-phase extraction approach, which takes advantage of the good extraction capabilities of hydrophobic magnetic nanoparticles (MNPs), is presented. The new approach involves the deposition of a thin layer of MNPs in a dedicated stirring unit based on the dual function of a mini-magnet. The system allows the extraction of the analytes in a simple and efficient way. The approach, which reduces the negative effect of the aggregation tendency of hydrophobic MNPs, is characterized for the resolution of a model analytical problem: the determination of some endocrine-disrupting phenols in water by liquid chromatography–photometric detection. All the variables involved in the extraction process have been clearly identified and optimized. The new extraction mode allows the determination of these compounds with limits of detection in the range from 0.15 μg/L (for 4-tert-octylphenol) to 2.7 μg/L (for 4-tert-butylphenol) with a relative standard deviation lower than 5.3 % (for 4-tert-butylphenol).     

An easy route to pentacyclic terpenylquinones

Cyclisation of terpenylnaphthohydroquinone derivatives or terpenylquinone cycloadducts, obtained from methyl myrcecommunate, was performed in the presence of HI, I₂ or BF₃·OEt₂ and provides efficient synthetic strategies for the preparation of pentacyclic terpenylquinones.