At present, the reactivity of cyclic alkanes is estimated by comparison with acyclic hydrocarbons. Due to the difference in the structure of cycloalkanes and acycloalkanes, the thermodynamic data obtained by analogy are not applicable. In this study, a molecular beam sampling vacuum ultraviolet photoionization time-of-flight mass spectrometer (MB-VUV-PI-TOFMS) was applied to study the low-temperature oxidation of cyclopentane (CPT) at a total pressure range from 1–3 atm and low-temperature range between 500 and 800 K. Low-temperature reaction products including cyclic olefins, cyclic ethers, and highly oxygenated intermediates (e. g., ketohydroperoxide KHP, keto-dihydroperoxide KDHP, olefinic hydroperoxides OHP and ketone structure products) were observed. Further investigation of the oxidation of CPT – electronic structure calculations – were carried out at the UCCSD(T)-F12a/aug-cc-pVDZ//B3LYP/6-31+ G(d,p) level to explore the reactivity of O2 molecules adding sequentially to cyclopentyl radicals. Experimental and theoretical observations showed that the dominant product channel in the reaction of CPT radicals with O2 is HO2 elimination yielding cyclopentene. The pathways of second and third O2 addition – the dissociation of hydroperoxide – were further confirmed. The results of this study will develop the low-temperature oxidation mechanism of CPT, which can be used for future research on accurately simulating the combustion process of CPT. 相似文献
Derivatization at the injection port following hollow‐fiber‐based liquid–liquid–liquid microextraction with tetramethylammonium acetate as a dual‐function reagent, i.e. an acceptor and derivatization reagent, for the determination of benzoic acid (BA) and sorbic acid (SA) in real samples by GC was developed. BA and SA were extracted from aqueous samples to an organic phase impregnated into the pores of the hollow fiber wall, and then back‐extracted to the acceptor solution located inside the lumen of the hollow fiber. Upon injection, the extracted analytes were quantitatively derivatized to their methyl esters with tetramethylammonium acetate in the GC injection port. Several parameters related to the derivatization and extraction efficiency were optimized. The linearity was satisfactory over a concentration range of 0.1–50 mg/L with r > 0.993 for both analytes. The LODs were 2.0 μg/L for SA and 20 μg/L for BA. The recoveries (83–116%) and precisions (RSDs of 1.2–11.4% (n = 3)) were examined by analyzing real spiked samples. The enrichment factors of BA and SA were 300 and 425. The results demonstrated that this is a simple, rapid, accurate, and sensitive method for the determination of BA and SA in various samples. 相似文献
A novel method of on-line pyrolytic methylation–gas chromatography was developed for the rapid analysis of fatty acid composition in Polysorbate 80 without any tedious pre-treatment steps. Fatty acids in Polysorbate 80 were converted into their corresponding fatty acid methyl esters in the presence of trimethylsulfonium hydroxide with a vertical microfurnace pyrolyzer at 300 °C. The premixing procedure of sample and organic alkali reagent was necessary before the on-line pyrolytic methylation to improve the repeatability. The relative standard deviations for peak areas of fatty acids in Polysorbate 80 were over the range of 0.3–9.1% (n = 5). Six Polysorbate 80 samples, consisting of three samples of pharmaceutical grade and three samples of non-pharmaceutical grade, were analyzed to evaluate the feasibility of the proposed method. The relative percentages (%) of fatty acids for samples of pharmaceutical grade meet the Chinese Pharmacopoeia requirements with the amount of oleic acid varying from 78.4 to 89.3%. On the other hand, the relative percentages (%) for palmitic acid and stearic acid in samples of non-pharmaceutical grade were out of the specification limits, with the amount of oleic acid varying from 62.0 to 63.5%. The quantitative results determined by on-line pyrolytic methylation were in agreement with those obtained by off-line methylation. The result proved that gas chromatography with on-line pyrolytic methylation technique is of great value for rapid screening analysis of Polysorbate 80 samples in bulks. 相似文献
Headspace solid-phase microextraction and gas chromatography/mass spectrometry (HS-SPME–GC/MS) method combined with XCMS Online was tentatively applied to characterize the dysregulated volatile organic metabolites (VOMs) in benign and malignant pleural effusions. A total of 9 dysregulated feature groups were isolated from metabolic features in 35 pleural effusion samples (20 benign effusions and 15 malignant ones from lung cancer patients). Principle component analysis, partial least squares discriminant analysis (PLS-DA) and orthogonal PLS-DA were built to separate benign from malignant pleural effusion groups and to find dysregulated metabolites in significantly different amounts between the two groups. Four dysregulated VOMs such as 2-ethyl-1-hexanol, cyclohexanone, 1,2,4,5-tetramethylbenzene and naphthalene were selected according to the variable influence on the projection value. The concentration of the four dysregulated VOMs in benign and malignant effusions were further determined by external standard method. The median concentrations of 4 VOMs in malignant effusion samples were from 4.7 to 91,121.9 nM, whereas their median levels were only 1.9–318.3 nM in benign ones. The results show that the proposed SPME–GC/MS-based metabolomic approach combined with XCMS Online data processing is a simple, rapid and available method for the characterization of dysregulated VOMs in malignant and benign pleural effusions. 相似文献
A new application of pyrolytic methylation was developed to determine benzoic acid in soft drinks by gas chromatography (GC) without using any pretreatment procedures and special pyrolyzer. With the on-line pyrolytic methylation by tetramethylammonium hydroxide (TMAH), benzoic acid was converted into its corresponding methyl ester in the injector at 280 °C. Thus, samples containing benzoic acid could be well determined by direct-injection in GC on the medium polar stationary phase column. To obtain optimum methylation conditions, important factors were investigated and then applied to the following experiments. The results were obtained as following: 280 °C as reaction temperature, 2:1 as the proportion of TMAH to benzoic acid. The storage time of mixed solution had no obvious effect on the area of benzoic acid methyl ester peak. With the p-xylene as an inter-standard, GC behaviors were investigated under these optimum conditions. The linear range achieved for benzoic acid was 1-10,000 mg/l with the correlation coefficient of 0.9985. The precision was quite high with the R.S.D. of 2.8% and the limit of detection reached 0.1 mg/l. The potential of the proposed method was assessed by applying it to the determination of benzoic acid in soft drinks. The results obtained coincided with the statement on the labels and all of the detected data were below the maximum permitted concentration of the European Union Legislation. This on-line pyrolytic methylation technique was proved to be simple to implement, sensitive and selective. 相似文献
Flash evaporation-gas chromatography/mass spectrometry (FE-GC/MS) with 0.3 mg sample powder in a vertical microfurnace pyrolyzer at 300 °C was applied to analyze Schisandra fruits without any tedious pretreatment. In total, 80 compounds, 74 compounds of which were identified, were observed, including low-molecular-weight compounds, essential oils (especially terpenoids), fatty acids and esters, and lignans, with the relative standard deviations (RSDs) of the relative percent of peak areas less than 7.79 % (n = 5). 32 compounds of terpenoids and lignans were selected as fingerprint components, since they are the main bioactive constituents in Schisandra fruits. The standard characteristic fingerprints of S. chinensis fruits and S. sphenantherae fruits were established, based on the 32 fingerprint components of 11 genuine S. chinensis fruit samples and 9 genuine S. sphenantherae fruit samples. The discrimination of samples from different growing places was achieved by principle component analysis and hierarchical cluster analysis. Furthermore, a similarity evaluation method was developed to evaluate the quality of each Schisandra fruit sample on the basis of the 32 fingerprint components. The results proved that the FE-GC fingerprint combined with a chemometric approach is a simple, rapid, and effective method for the origin discrimination and quality control of Schisandra fruits.
A method to determine alkylphenols in Ginkgo biloba sarcotesta, especially focusing on polyunsaturated ginkgolic acid components, was developed on the basis of thermochemolysis–gas chromatography/mass spectrometry with a DB-23 column in the presence of trimethylsulfonium hydroxide [(CH3)3SOH]. By using this technique, powder sample of Ginkgo biloba sarcotesta can be directly analyzed without any tedious and time-consuming pretreatment. On the resulting chromatograms, seven kinds of ginkgolic acids including polyunsaturated ginkgolic acid GA17:2 and two kinds of ginkgols were clearly observed as their methyl derivative. Based on the peak areas, the ginkgolic acid compositions are in good agreement with these obtained by a high-performance liquid chromatography method dedicated to analysis of ginkgolic acid. 相似文献