The natural-like assessment of essential oils is a demanding task due to the growing trend toward adulterations. Usually chiral chromatography was used for this purpose due to the capability of assessing stereospecificity which is directly related to the enzymatic pathways of each plant species. On the other hand, the quality of an essential oil involves also the evaluation of its oxidative state, mainly connected with the age and storage conditions. In fact, some modifications in the chemical profile of the oil can occur if not properly preserved. Alterations of the components due to oxidative reactions lead to the formation of peroxides, endoperoxides and epoxides, such as ascaridole and 1,2,4-trihydroxymenthane, usually present in very low amount, formed by the oxidation of terpinen-4-ol and α-terpinene, respectively. Therefore, in the present research, the quality of Australian Tea Tree oil (Melaleuca alternifolia (Maiden & Betche) Cheel, Myrtaceae) was investigated by means of a multi heart-cut multidimensional gas chromatographic system coupled to a mass spectrometer detector and by conventional enantio-GC. The MDGC system allowed the complete separation of the compounds of interest transferred from the first column to a second dimension based on a different separation mechanism. The MS detector at the end of the second column provided the identification of the peaks with high similarity values because of their high purities after the multidimensional separation. Method validation was carried out, in order to use this procedure for routine application, monitoring the repeatability of 1D retention times and 2D peak areas, LoD and LoQ. Finally, enantiomeric ratios for chiral compounds were established to support quality data obtained. 相似文献
The identification of gasoline adulteration by organic solvents is not an easy task, because compounds that constitute the solvents are already in gasoline composition. In this work, the combination of Hydrogen Nuclear Magnetic Resonance (1H NMR) spectroscopic fingerprintings with pattern-recognition multivariate Soft Independent Modeling of Class Analogy (SIMCA) chemometric analysis provides an original and alternative approach to screening Brazilian commercial gasoline quality in a Monitoring Program for Quality Control of Automotive Fuels. SIMCA was performed on spectroscopic fingerprints to classify the quality of representative commercial gasoline samples selected by Hierarchical Cluster Analysis (HCA) and collected over a 6-month period from different gas stations in the São Paulo state, Brazil. Following optimized the 1H NMR-SIMCA algorithm, it was possible to correctly classify 92.0% of commercial gasoline samples, which is considered acceptable. The chemometric method is recommended for routine applications in Quality-Control Monitoring Programs, since its measurements are fast and can be easily automated. Also, police laboratories could employ this method for rapid screening analysis to discourage adulteration practices. 相似文献
This article reports the composition of the essential oil from the leaves of Stachys yemenensis. The essential oil was extracted by supercritical CO? (90 bar; 40 °C) and its chemical composition was determined by gas chromatography and gas chromatography-mass spectrometry. The major components of the sample were α-phellandrene (13.9%), β-phellandrene (11.7%), elemol (12.0%), spathulenol (6.7%), β-eudesmol (5.0%), α-eudesmol (4.75%) and squalene (4.8%). On the exhausted matrix, deprived of the volatiles, we carried out a high-pressure (250 bar) treatment for the extraction of squalene (49.7%). The antimicrobial activity of the essential oils has been assayed by using the broth dilution method on two American Type Culture Collection (ATCC) strains, Escherichia coli ATCC 35218 and Staphylococcus aureus ATCC 43300, and two clinical strains, Candida albicans and Candida glabrata. 相似文献
Free‐radical polymerization of vinyl chloride is investigated computationally with special attention to the secondary reactions involving mid‐chain radicals (MCRs). Namely, the rate constants of backbiting, chain scission, chain transfer, and propagation reactions are evaluated using a density functional theory method. The rate coefficients of such reactions are estimated taking into account the position of the radical along the chain as well as its distance from the chain‐end. In particular 1:5, 5:1, and 5:9 backbiting are the most relevant secondary reactions, followed by the slower propagation of MCRs. Finally, a kinetic model of suspension polymerization including the investigated reactions is developed, in order to determine their impact on the quality of the final polymer.
Within the framework of a project aimed to develop protective coatings for antique glass windows, three glass varieties of medieval-like composition were prepared using recipes deduced from archaeometric studies and different amounts of potassium for fluxing (15–20–25 K2O wt.%). Batches were melted in mullite crucibles using an electric furnace at 1350 °C, the glass was fast cooled in air and annealed at temperatures 20 °C higher than the sample glass transition temperatures (726, 702 and 683 °C for V1, V2 and V3, respectively). The chemical composition of the glass was determined by X-Ray Fluorescence Spectroscopy (XRF) and the glass transition temperature was checked by differential thermal analysis (DTA). X-Ray Powder Diffraction (XRPD) data and vibrational spectroscopic experiments (FT-IR and Raman) revealed a direct dependence of glass de-polymerization on potassium content. In particular, the Raman data were interpreted on the basis of SiO4 structural units Qn and the polymerization index Ip. 相似文献
The three-dimensional elasticity energy for a notched rod under axial loads is shown to converge in a variational sense to a class of one-dimensional models with a localized compliance when the beam becomes very slender and when the ratio between the depth and the width of each notch vanishes in a suitable way. Also the case of notched thermal conductors is considered, in a parallel treatment, because it falls in a similar but simpler framework. 相似文献
A novel ‘delay vector variance’ (DVV) method for detecting the presence of determinism and nonlinearity in a time series is introduced. The method is based upon the examination of local predictability of a signal. Additionally, it spans the complete range of local linear models due to the standardisation to the distribution of pairwise distances between delay vectors. This provides consistent and easy-to-interpret diagrams, which convey information about the nature of a time series. In order to gain further insight into the technique, a DVV scatter diagram is introduced, which plots the DVV curve against that for a globally linear model (surrogate data). This way, the deviation from the bisector line represents a qualitative measure of nonlinearity, which can be used additionally for constructing a quantitative measure for statistical testing. The proposed method is compared to existing methods on synthetic, as well as standard real-world signals, and is shown to provide more consistent results overall, compared to other, established nonlinearity analysis methods. 相似文献