Further remarks on <Emphasis Type="Italic">δ</Emphasis>- and <Emphasis Type="Italic">θ</Emphasis>-modifications

In generalizing constructions of N.V. Veličko, the paper starts from two generalized topologies μ and μ′ on a set X and introduces two more generalized topologies gd(μ, μ′) and δ(μ,μ′) with the examination of their properties. Research (partially) supported by Hungarian Foundation for Scientific Research, grant Nos. T 49786, T 046846, K 68398.     

Determination of parabens in house dust by pressurised hot water extraction followed by stir bar sorptive extraction and thermal desorption-gas chromatography-mass spectrometry

This study describes the development of a new method for determining p-hydroxybenzoic esters (parabens) in house dust. This optimised method was based on the pressurised hot water extraction (PHWE) of house dust, followed by the acetylation of the extracted parabens, stir bar sorptive extraction (SBSE) with a polydimethylsiloxane stir bar, and finally analysis using thermal desorption-gas chromatography-mass spectrometry (TD-GC-MS). The combination of SBSE and PHWE allows the analytes to be preconcentrated and extracted from the aqueous extract in a single step with minimal manipulation of the sample. Furthermore the in situ acetylation of parabens prior to SBSE improved their extraction efficiency and their GC-MS signal. The method showed recoveries of between 40 and 80%, good linearity, repeatability and reproducibility (<10% RSD, at 100 ng g(-1), n=5), low limits of detection (from 1.0 ng g(-1) for propyl paraben to 2.1 ng g(-1) for methyl paraben) and quantification (from 3.3 ng g(-1) for propyl paraben to 8.5 ng g(-1) for methyl paraben). The proposed method was applied to the analysis of house dust samples. All the target parabens were found in the samples. Methyl and propyl parabens were the most abundant, with concentrations up to 2440 ng g(-1) and 910 ng g(-1), respectively. The high levels of parabens found in the samples confirm the importance of determining organic contaminants in indoor environments.     

Interactionist perspective on negotiation processes of students’ different understandings during small group work on linear algebra

Student group work represents a central learning setting within mathematics programs at the university level. In this study, a theoretical perspective on collaboration is adopted in which the differences between students’ interpretations of a mathematical concept are seen as an opportunity for individual restructuring processes. This so-called interactionist perspective is applied to student group work on linear algebra. The concepts of linear algebra at the university level are characterized by a versatility of different modes of expression and interpretation. For students of linear algebra, the flexible transitions between the different interpretations of linear algebra concepts usually pose a challenge. This study focuses on how students negotiate their different interpretations during group work on linear algebra and how transitions between interpretations might be stimulated or hindered. Video recordings of eight student groups working on a task that required flexible transition between interpretations of homomorphisms were sampled. The recordings were analyzed from an interactionist perspective, focusing on interaction situations in which the participating students expressed and negotiated different interpretations of homomorphisms. The analyses of students’ interactions highlight a phenomenon whereby differences in students’ interpretations remain implicit in group discussions, which constitutes an obstacle to the negotiation process.     

An Electropolymerized Pyrrole‐based Coating for Stir Bar Sorptive Extraction of Btex from Water Followed by Gas Chromatography‐Mass Spectrometry

A stir bar sorptive extraction (SBSE) technique was developed by the use of polypyrrole (PPy) sorbent, electropolymerized on the surface of a rod, as a possible alternative to solid‐phase microextraction (SPME). Liquid desorption was subsequently employed to transfer the extracted analytes into the injection port of a gas chromatography‐mass spectrometry (GC‐MS). The PPy sorbent including polypyrrole‐dodecyl sulfate (PPy‐DS) was deposited on the surface of a stainless steel rod from the corresponding aqueous electrolyte by applying a constant deposition potential. The developed method was applied to the trace level extraction of BTEX (benzene, toluene, ethylbenzene, o,p‐xylene) from aqueous sample. Optimization of influential experimental conditions including the voltage of power supply, the time of PPy electrodeposition, the extraction temperature, the ionic strength and the extraction time were also investigated. The detection limits of the method under optimized conditions were in the range of 0.01–0.1 ng·mL?¹. The relative standard deviations (RSD) at a concentration level of 1 ng·mL?¹ were obtained between 8% and 13% (n=6). The calibration curves of BTEX showed linearity in the range of 0.03 to 600 ng·mL?¹. The proposed method was successfully applied to the extraction of some selected BTEX from river water samples and the relative recoveries were higher than 90% for all the analytes.     

Determination of phthalate esters in polyvinyl chloride infusion bag by stir bar sorptive extraction combined with GC

A novel, efficient, and sensitive stir bar sorptive extraction method coupled with GC for the detection of four kinds of phthalate esters in plasticized polyvinyl chloride infusion bag has been developed and validated. Some experimental parameters including stirring speed, stirring time, pH value, salt concentration, desorption mode, desorption solvents, and desorption time were investigated and optimized. Under optimum condition, the validated assay was found to be linear (r > 0.9945) and LODs were between 1.07 and 2.67 ng for the four analytes. The method exhibited excellent precision with RSD varied from 4.5 to 6.1% (n = 5). The recoveries of the four phthalate esters at two different concentrations ranged from 80.5 to 93.4%. The results showed that the validated method could meet the need of determination of targets and was successfully applied to the analysis of phthalate esters in real samples.     

Molecularly imprinted stir bars for selective extraction of thiabendazole in citrus samples

Stir‐bar sorptive extraction is based on the partitioning of target analytes between the sample (mostly aqueous‐based liquid samples) and a stationary phase‐coated magnetic stir bar. Until now, only PDMS‐coated stir bars are commercially available, restricting the range of applications to the non‐selective extraction of hydrophobic compounds due to the apolar character of PDMS. In this work, a novel stir bar coated with molecularly imprinted polymer as selective extraction phase for sorptive extraction of thiabendazole (TBZ) was developed. Two different procedures, based on physical or chemical coating, were assessed for the preparation of molecularly imprinted stir bars. Under optimum conditions, recoveries achieved both in imprinted and non‐imprinted polymer stir bars obtained by physical coating were very low, whereas TBZ was favourably retained by imprinted over non‐imprinted polymer stir bars obtained by chemical coating and thus the latter approach was used in further studies. Different parameters affecting both stir‐bars preparation (i.e. cross‐linker, porogen, polymerization time) and the subsequent selective extraction of TBZ (i.e. washing, loading and elution solvents, extraction time) were properly optimized. The molecularly imprinted coated stir bars were applied to the extraction of TBZ from citrus samples (orange, lemon and citrus juices) allowing its final determination at concentrations levels according to current regulations.     

Coupling stir bar sorptive extraction‐dispersive liquid–liquid microextraction for preconcentration of triazole pesticides from aqueous samples followed by GC‐FID and GC‐MS determinations

Stir bar sorptive extraction (SBSE) combined with dispersive liquid–liquid microextraction (DLLME) has been developed as a new approach for the extraction of six triazole pesticides (penconazole, hexaconazole, diniconazole, tebuconazole, triticonazole and difenconazole) in aqueous samples prior to GC‐flame ionization detection (GC‐FID). A series of parameters that affect the performance of both steps were thoroughly investigated. Under optimized conditions, aqueous sample was stirred using a stir bar coated with octadecylsilane (ODS) and then target compounds on the sorbent (stir bar) were desorbed with methanol. The extract was mixed with 25 μL of 1,1,2,2‐tetrachloroethane and the mixture was rapidly injected into sodium chloride solution 30% w/v. After centrifugation, an aliquot of the settled organic phase was analyzed by GC‐FID. The methodology showed broad linear ranges for the six triazole pesticides studied, with correlation coefficients higher than 0.993, lower LODs and LOQs between 0.53–24.0 and 1.08–80.0 ng/mL, respectively, and suitable precision (RSD < 5.2%). Moreover, the developed methodology was applied for the determination of target analytes in several samples, including tap, river and well waters, wastewater (before and after purification), and grape and apple juices. Also, the presented SBSE‐DLLME procedure followed by GC‐MS determination was performed on purified wastewater. Penconazole, hexaconazole and diniconazole were detected in the purified wastewater that confirmed the obtained results by GC‐FID determination. In short, by coupling SBSE with DLLME, advantages of two methods are combined to enhance the selectivity and sensitivity of the method. This method showed higher enrichment factors (282–1792) when compared with conventional methods of sample preparation to screen pesticides in aqueous samples.     

Synthesis, structure, and sugar dynamics of a 2′-spiroisoxazolidine thymidine analog

Bicyclic nucleoside analogs have shown promise in Antisense and RNA interference strategies. Ribofuranosyl ring conformation is a controlling factor in this regard. We have introduced a spiroisoxazolidine ring at the 2′-position of the sugar to gauge the effect it has on sugar dynamics. Proton relaxation measurements and coupling constant analysis indicate the sugar is locked in the North conformation with substantial sugar rigidity evident.     

Flexibility and sorption selectivity in rigid metal-organic frameworks: the impact of ether-functionalised linkers

The functionalisation of well-known rigid metal-organic frameworks (namely, [Zn(4)O(bdc)(3)](n), MOF-5, IRMOF-1 and [Zn(2)(bdc)(2)(dabco)](n); bdc = 1,4-benzene dicarboxylate, dabco = diazabicyclo[2.2.2]octane) with additional alkyl ether groups of the type -O-(CH(2))(n)-O-CH(3) (n = 2-4) initiates unexpected structural flexibility, as well as high sorption selectivity towards CO(2) over N(2) and CH(4) in the porous materials. These novel materials respond to the presence/absence of guest molecules with structural transformations. We found that the chain length of the alkyl ether groups and the substitution pattern of the bdc-type linker have a major impact on structural flexibility and sorption selectivity. Remarkably, our results show that a high crystalline order of the activated material is not a prerequisite to achieve significant porosity and high sorption selectivity.     

Rectifications of A∞-Algebras

In this article, in the setting of connected DG-modules, we prove that, for any A _∞-algebra (M, {m _i } _i≥1), there is a chain contraction from a DG-algebra A _M onto the DG-module M such that the A _∞-algebra structure induced by perturbation theory on M is precisely the original one. In fact, the mentioned DG-algebra can be considered a rectification of the A _∞-algebra in the sense of (Boardman and Vogt, 1973 Boardman , J. M. , Vogt , R. M. ( 1973 ). Homotopy Invariant Algebraic Structures on Topological Spaces . Lecture Notes in Mathematics . Springer-Verlag , pp. 347 . [Google Scholar]). Appropiate dual results are given for A _∞-coalgebras.