Dynamical structure of peptide molecules: Fourier transform microwave spectroscopy and ab initio calculations of N-methylformamide

Rotational spectra of both trans and cis forms of the N-methylformamide normal as well as deuterated (HCONDCH₃, referred to as N-D) species were observed by Fourier transform microwave spectroscopy in the frequency region from 5 to 118 GHz. Samples were prepared in the form of a beam by a pulsed jet valve maintained at 50 °C and were introduced in a high-vacuum cavity cell, with either Ne or Ar as a carrier gas at a backing pressure of 100 kPa. The observed spectra were analyzed to yield molecular parameters including rotational constants and barrier, V₃, to CH₃ internal-rotation: 53.9 (6) and 301 (4) cm⁻¹ for the trans and cis forms of the normal species, respectively, and 41.9 (6) and 309 (4) cm⁻¹ for the trans and cis forms of the N-D species, respectively. Spectra of four trans isotopologues with ¹³C, ¹⁵N, or ¹⁸O singly-substituted in the internal-rotation A state were observed and analyzed to derive the r_s structure of the trans form. For comparison with the experimental data, ab initio calculations were carried out at MP2/6-31G∗∗ level to derive molecular structure, potential barrier to CH₃ internal rotation, and the energy difference between the cis and trans forms. An extensive coupling was found between the CH₃ internal rotation and N-H out-of-plane bending, suggesting that the potential function for the CH₃ internal-rotation deviates considerably from a simple cos(3α) form. The effects of the V₆ term is briefly discussed.     

Analytical characterization of alcohol-ethoxylate substances by instrumental separation techniques

The characterization of alcohol-ethoxylate substances is a significant challenge in analytical science. Their importance in industry and society makes it necessary to have methods for their rapid, reliable characterization and quantification. This overview highlights the instrumental separation techniques for their analysis - chromatographic (e.g., gas, liquid and supercritical fluid chromatography) and electrophoretic (e.g., capillary, gel, capillary zone, and non-aqueous capillary electrophoresis and micellar electrokinetic chromatography). We summarize and explain analytical parameters for their characterization. We include selected references and examples to show the appropriateness of instrumental separation techniques for the analytical characterization of alcohol-ethoxylate substances.     

毛细管气相色谱法分析液化石油气中二甲醚

建立了液化石油气中二甲醚的毛细管气相色谱分析方法.利用丙酮作为溶剂吸收液化石油气中二甲醚,采用HP-1毛细管色谱柱进行分离,以氢火焰离子化检测器检测,外标法进行定量分析.二甲醚的体积分数在0.5%～30.0%范围内与色谱峰面积呈良好线性关系,二甲醚的定量下限为0.2%,测定结果的相对标准偏差为2.3%～7.3%(n=6...     

中药普乐安片质量标准研究

建立中药普乐安片的质量标准.采用TLC法对普乐安片中主要有效成分山奈酚-3-O-β-D-(2-O-β-D-葡萄糖基)吡喃葡萄糖苷(KMP)进行定性鉴别.采用HPLC法对普乐安片中主要有效成分山奈酚-3,4′-双-O-β-D-葡萄糖苷(KMG)和山奈酚-3-O-β-D-(2-O-β-D-葡萄糖基)吡喃葡萄糖苷进行含量测定...     

仪器定量分析中几个问题的探讨

详细介绍了分析方法的评价指标,包括检出限、测定限、灵敏度、精密度、准确度、动态范围和线性范围、抗干扰能力等,单因素优化方法与加标回收实验评定测定结果准确度的可靠性,并就分析工作中遇到的一些实际问题进行了讨论.     

气相色谱-质谱法测定富马酸二甲酯时内标物的选择

探讨了当采用气相色谱-质谱法测定富马酸二甲酯(DMFU)时内标物的选择问题。采用HP-5MS毛细管色谱柱,在50℃～300℃范围内程序升温分离及采用在50～400amu范围内全扫描方式和选择离子监测模式检测。试验结果表明:以丙二酸二甲酯(DMM)、丙二酸二乙酯(DEM)和戊二酸二甲酯(DMG)作为内标物,既能保持DMFU与3种内标物有良好的分离度,又能保持长久的稳定性,且3种内标物与DMFU的保留时间差均小于2.8min。DMFU的线性范围均为0.005～5.0mg·L~(-1)。方法用于实际样品中DMFU的测定,以上3种物质作为内标,所得结果无显著差别,因此DMM、DEM、DMG均可作为比较理想的内标物用于日常用品中DMFU的检测。     

Molecular dynamics simulation of structure H clathrate-hydrates of binary guest molecules

Molecular dynamics (MD) simulations are performed to study the stability of structure H clathrate-hydrates of methane+large-molecule guest substance (LMGS) at temperatures of 270, 273, 278 and 280 K under canonical (NVT-) ensemble condition in a 3×3×3 structure H unit cell replica with 918 TIP4P water molecules. The studied LMGS are 2-methylbutane (2-MB), 2,3-dimethylbutane (2,3-DMB), neohexane (NH), methylcyclohexane (MCH), adamantane and tert-butyl methyl ether (TBME). In the process of MD simulation, achieving equilibrium of the studied system is recognized by stability in calculated pressure for NVT-ensemble. So, for the accuracy of MD simulations, the obtained pressures are compared with the experimental phase diagrams. Therefore, the obtained equilibrium pressures by MD simulations are presented for studying the structure H clathrate-hydrates. The results show that the calculated temperature and pressure conditions by MD simulations are consistent with the experimental phase diagrams. Also, the radial distribution functions (RDFs) of host-host, host-guest and guest-guest molecules are used to analysis the characteristic configurations of the structure H clathrate-hydrate.     

Dimensional considerations on the mechanical properties of 3D printed polymer parts

Additive manufacturing offers a useful and accessible tool for prototyping and manufacturing small volume functional parts. Polylactic acid (PLA) and thermoplastic polyurethane (TPU) are amongst the most commonly used materials. Characterising 3D printed PLA and TPU is potentially important for both designing and finite element modelling of functional parts. This work explores the mechanical properties of additively manufactured PLA/TPU specimens with consideration to design parameters including size, and infill percentage. PLA/TPU specimens are 3D-printed in selected ISO standard geometries with 20%, 60%, 100% infill percentage. Tensile and compression test results suggest that traditional ISO testing standards might be insufficient in characterising 3D printed materials for finite element modelling or application purposes. Infill percentage in combination to design size, may significantly affect the mechanical performance of 3D printed parts. Dimensional variation may cause inhomogeneity in mechanical properties between large and small cross section areas of the same part. The effect was reduced in small cross section parts where reducing the nominal infill had less effect on the resulting specimens. The results suggest that for 3D printed functional parts with significant dimensional differences between sections, the material properties are not necessarily homogeneous. This consideration may be significant for designers using 3D printing for applications, which include mechanical loading.     

自支撑NH2-MIL-125/聚噁二唑复合正渗透膜的制备及性能

合成了高强度亲水性含羧基聚噁二唑材料(POD-COOH)和含氨基金属有机框架材料(NH₂-MIL-125), 以NH₂-MIL-125为填料, 与POD-COOH基体材料进行溶液共混, 并通过溶液浇铸法制备系列新型自支撑复合正渗透膜, 研究NH₂-MIL-125的引入对复合正渗透膜结构和性能的影响. 研究结果表明, 所制备的系列复合正渗透膜均呈致密结构, 且随着NH₂-MIL-125含量的增加, 复合膜的表面亲水性增加、 电负性增强, 并保持良好的机械性能. 以去离子水为进料液, 1.5 mol/L硫酸钠溶液为汲取液, 对上述自支撑复合膜进行正渗透性能测试, 发现由于消除了传统正渗透膜支撑层的内浓差极化现象, 该新型复合正渗透膜在分离过程中具有优异的正渗透性能.     

Experimental validation of an effective carbon number-based approach for the gas chromatography–mass spectrometry quantification of ‘compounds lacking authentic standards or surrogates’

For the quantitative analysis of ‘compounds lacking authentic standards or surrogates’ (CLASS) in environmental media, we previously introduced an effective carbon number (ECN) approach to develop an empirical equation for the prediction of their response factor (RF). In this research, a series of laboratory experiments were carried out to benchmark the reliability of an ECN approach for sorbent tube/thermal desorption/gas chromatography (GC)/mass spectrometry (MS) applications. First, the ECN values were determined using external calibration data from 25 reference volatile organic compounds (VOCs) using two MS dectectors (quadrupole (Q) and time-of-flight (TOF)). Then, a certified standard mixture of 54 VOCs was analyzed by each system as a simulated unknown sample. The analytical bias, assessed in terms of percentage difference (PD) between the certified and ECN-predicted mass values, averaged 19.2 ± 16.1% (TOF-MS) and 28.2 ± 27.6% (Q-MS). The bias using a more simplified carbon number (CN)-based prediction increased considerably, yielding 53.4 ± 53.3% (TOF-MS) and 61.7 ± 81.3% (Q-MS). However, the bias obtained using the ECN-based prediction decreased significantly to yield average PD values of 9.84 ± 7.28% (TOF-MS) and 16.8 ± 8.35% (Q-MS), if the comparison was limited to 26 (out of 54) VOCs with CN ≥ 4 (i.e., 25 aromatics and hexachlorobutadiene).