PFG-NMR approach to determining the water transport mechanism in polymer electrolyte membranes conditioned at different temperatures

Two types of polymer electrolyte membranes (PEMs), Nafion (NR-212) and sulfonated poly(4-phenoxybenzoyl-1,4-phenylene) (sPPBP), were conditioned under air flow in independently controlled temperature and relative humidity conditions. The effects of temperature and relative humidity on the diffusion behavior of water in the PEMs were investigated by pulsed field gradient (PFG) NMR. The two PEMs show different tendencies, especially with respect to their temperature effects. In a comparison of diffusion data with proton conductivity, mechanisms for proton conduction in the PEMs were considered. It was suggested that proton conduction by proton hopping occurs to a larger extent in NR-212 than in a hydrocarbon-based membrane. Temperature-affected microstructural changes below the glass transition temperature are also discussed.     

Analysis of tinuvin 1577 polymer additive in edible oils using on-line coupled HPLC-GC

An application of automated on-line HPLC-HRGC is described for direct analysis of edible oils for migrated polymer additives. The sample preparation, separating the additive from the oil triglycerides, is carried out using normal phase HPLC. The fraction of the eluent containing the additive is automatically transferred to a HRGC where a second and final separation of the additive from minor oil components takes place. The method compares well with off-line separation methods. Migration data for Tinuvin 1577 from PET and PC polymers as well as an unspecified experimental polymer is given. The advantages and disadvantages of using different edible oils as food simulants are discussed.     

Effect of a resistance-free tank on the resistance to gas transport in high vacuum tube

The expression for the time lag in a cylindrical tube, into which a gas at very low flow rate enters at one end while the other end is connected to a resistance-free accumulation tank, has been derived assuming that the gas transport in the tube is a diffusive process. Assuming a constant diffusion coefficient of the gas in the tube allowed obtaining an analytical expression for the time lag using the concept of linear asymptotes and Laplace transformation of the governing partial differential equation. The obtained expression indicates that if the pressure response is monitored in the tube, the presence of the tank at the end of the tube would lead to a negative time lag in the tube. The time lag becomes more negative as the distance from the tank increases and the volume of the tank increases while the cross-sectional area of the tube decreases.

The comparison of the model with the experimental data obtained in tests with nitrogen in which the pressure response to a step increase in feed pressure of membrane was monitored in the tube at two different distances from the membrane cell, indicates that the error due to resistance to gas transport in the tube on the experimental time lag of tested medium is even greater than that predicted by the model. This is because of the assumption of constant diffusion coefficient in the tube, which does not allow predicting the experimentally observed increase in the slope of the asymptote with the distance from the membrane cell.     

Separation,pre-concentration and determination of trace amounts of lead(II) ions in environmental samples using two functionalised nanoporous silica gels containing a dipyridyl sub-unit

In this study, two functionalised nanoporous silica gels containing dipyridyl sub-unit (SiL1 and SiL2) as selective solid-phase extraction materials for separation, pre-concentration and determination of trace levels of Pb(II) ions by inductively coupled plasma optical emission spectroscopy (ICP-OES) was investigated. The experimental parameters including pH, amounts and type of sorbent, sample volume, eluent type and interfering ions on the recovery of the target analytes were investigated, and the optimal experimental conditions were established. Under the optimised operating conditions with the SiL2 as sorbent, an enrichment factor of 300 was obtained. The detection limit based on three times standard deviations of the blanks was 150 ng L^–1. The proposed method was applied to the determination of lead in natural and wastewater samples with satisfactory results (recoveries greater than 96.5%, RSDs lower than 5.0%).     

Comparison of two different electrophoretic methods in studying the genetic diversity among plantains (Musa spp.) using ISSR markers

Inter simple sequence repeat markers were employed for the genotyping of 16 plantain ecotypes. Two different electrophoretic systems namely conventional gel electrophoresis (CVGE) and fully automated high‐resolution CGE were used to evaluate the genetic diversity. Comparative analysis indicated that all parameters related to marker informativeness were higher in CGE except polymorphic information content. But genetic diversity parameters like effective number of alleles, Nei's gene diversity (1973) and Shannon's information index showed higher values (1.52 ± 0.12, 0.34 ± 0.05 and 0.52 ± 0.05, respectively) in CVGE as against CGE (1.29 ± 0.04, 0.22 ± 0.02 and 0.38 ± 0.03, respectively) system. The unweighed pair group method with arithmetic averages was used to obtain the dendrogram for both analyses. The results of dendrogram and principal component analysis were found to be consistent in both systems except for some minor disagreements. The clone‐specific bands could be used in the identification and development of SCAR markers. Inter simple sequence repeat markers used in this study provided sufficient polymorphism and reproducible banding pattern for evaluating the genetic diversity of different plantain ecotypes. Lack of accuracy and consistency of the CVGE warrants the employment of high‐throughput CGE for diversity analysis as it provided better separation of bands with higher resolution.     

Surface diffusion in reversed-phase liquid chromatography using silica gels bonded with C1 and C18 ligands of different densities

Surface diffusion in reversed-phase liquid chromatography (RPLC) using silica gels bonded with C₁ and C₁₈ alkyl ligands of different densities was studied from the viewpoints of two extrathermodynamic relationships, i.e., enthalpy-entropy compensation (EEC) and linear free energy relationship (LFER). First, according to the four methods proposed by Krug et al., the values of surface diffusion coefficient (D_s) were analyzed to confirm that an actual EEC resulting from substantial physico-chemical effects takes place for surface diffusion. Then, it was also demonstrated that a LFER is observed between surface diffusion and the retention equilibrium. The establishment of EEC and LFER suggests a mechanistic similarity of molecular migration by surface diffusion, irrespective of the alkyl chain length and the densities of C₁ and C₁₈ ligands. Finally, a thermodynamic model for the LFER based on the real EEC was used to estimate D_s values under various RPLC conditions. The D_s values can be estimated with a mean square deviation of about 25–30%. The agreement between the D_s values estimated and those experimentally measured suggests that the total mass flux by surface diffusion consists of the two contributions due to C₁ and C₁₈ ligands and that the contribution of each ligand is proportional to the ligand density.     

恒压-恒流充电容量比值法测定石墨电极中的锂离子扩散系数

通过球形扩散模型推导了恒压-恒流充电容量比(简称RPG)与石墨电极中锂离子固相扩散系数的关系,给出了一种测量锂离子在石墨负极中固相扩散系数的新方法即RPG法.理论结果表明固相扩散系数D与RPG值q,颗粒半径R和恒流充电时间tG有关.当q ≤ 0.5时,其关系式可通过最小方差线性化拟合为D=R2/(15.36qtG) (q ≤ 0.5).实验结果表明,RPG法具有非常好的重现性,不同恒流充电电流和时间下测得1.5 V(vs Li/Li＋)的锂离子固相扩散系数值均在1.060×10－10 cm2•s－1左右.采用RPG法对石墨电极在1.0～2.5 V(vs Li/Li＋)高电压区的锂离子固相扩散系数进行了测试,结果表明在1.0～2.5 V的高电压区,随着电压的升高,D值从4.292×10－10 cm2•s－1减小到1.219×10－11 cm2•s－1.     

Comparison of different methods to estimate the uncertainty in composition measurement by chromatography

Natural gas is a mixture that contains hydrocarbons and other compounds, such as CO₂ and N₂. Natural gas composition is commonly measured by gas chromatography, and this measurement is important for the calculation of some thermodynamic properties that determine its commercial value. The estimation of uncertainty in chromatographic measurement is essential for an adequate presentation of the results and a necessary tool for supporting decision making. Various approaches have been proposed for the uncertainty estimation in chromatographic measurement. The present work is an evaluation of three approaches of uncertainty estimation, where two of them (guide to the expression of uncertainty in measurement method and prediction method) were compared with the Monte Carlo method, which has a wider scope of application. The aforementioned methods for uncertainty estimation were applied to gas chromatography assays of three different samples of natural gas. The results indicated that the prediction method and the guide to the expression of uncertainty in measurement method (in the simple version used) are not adequate to calculate the uncertainty in chromatography measurement, because uncertainty estimations obtained by those approaches are in general lower than those given by the Monte Carlo method.     

Prediction of HPLC retention times in gradient elution from isocratic data application to a set of non-homologous compounds and different stationary phases

Summary Polymethyloctylsiloxane-coated stationary phases have been prepared for liquid chromatography, by thermal reaction. The influence of the reaction conditions on retention and efficiency of test substances with different structures has been discussed. The materials have good stability in both acidic and basic eluents.     

A synopsis of empirical gel fraction analytical methods: Application of the Wanxi equation to polyethylenes irradiated in vacuo and in the presence of acetylene

Gel fraction data from linear low-density polyethylene (LLDPE) films and high-modulus polyethylene (HMPE) fibers, which had previously failed to give reasonable results when treated with Charlesby-Pinner and Saito-Kang-Dole theory, have been fitted to a modified form of the Wanxi equation. The fitting yields parameters that compare very well with literature values derived by the Wanxi equation, confirming that fracture and cross-linking densities are proportional to a power function (β) of the radiation dose. Extrapolated G(S) values have been obtained from LLDPE using the fit parameters and UV-Vis spectroscopy data. A brief synopsis of the empirical analytical methods available for gel fraction analysis is given. © 1994 John Wiley & Sons, Inc.     

Comparison between different data pre-treatment methods in the analysis of forage samples using near-infrared diffuse reflectance spectroscopy and partial least-squares multivariate calibration method

Moisture and protein content of alfalfa samples from Catalonia (Spain) have been analyzed by near-infrared (NIR) diffuse reflectance spectroscopy and multivariate calibration methods. In order to remove systematic variation in experimental data, such as base-line and multiplicative scatter effects, the evaluation of different data pre-processing methods is performed. Different figures of merit are used for quality assessment and comparison of these pre-treatment methods.