Selection of terms for a CI wavefunction to preserve potential surface features

A cumulative selection procedure for choosing configuration functions for inclusion in CI calculations is described. The objective of the method is to obtain equal energy loss, relative to unselected calculations, for different states and different regions of the potential surface. Results obtained from calculations on the BH molecule indicate an overall advantage in comparison to the threshold selection procedure, particularly with regard to molecular geometry changes.     

Frequent analytical/experimental problems in lipase-mediated synthesis in solvent-free systems and how to avoid them

Compared with chemical catalysis, enzymatic catalysis is a relatively new topic. Experimental work involving lipases deserves careful attention and accurate procedures still need to be implemented. A rapid but careful survey of published data immediately demonstrates that experiments performed under similar conditions with similar reagents have led to very different results. The aim of this work is to point out the importance of accurate and systematic procedures in order to ensure the reproducibility of experimental data. We strongly believe that different results found by different labs are due to problems detected in the procedures used. Quantification of the immobilisation efficiency of lipase on several supports through UV/visible methods and sampling methods used to obtain correct enzymatic activity values are specifically analysed. After a brief review which demonstrates the big discrepancies found in the literature, original data from Candida rugosa lipase adsorption on polypropylene powder and its use in the solvent-free synthesis of ethyl oleate are introduced in order to exemplify the difficulties found in these kinds of systems. Several procedures described in the literature are assayed and the accuracy of the results obtained is carefully analysed. The aim of the whole analysis performed is that it would be useful for any powdered solid to be used as a support for a lipase in a solvent-free system for any synthesis reaction, especially for those involving a volatile reagent. Throughout this contribution, special emphasis is placed on how catalytic reaction results using enzymes (free and immobilised) are reported so as to allow comparison between published data, something which is usually difficult since very different units are used and often complementary data are not included.     

Preparation of apolar glass capillary columns by the barium carbonate procedure

As an extension of previous reports, the barium carbonate procedure has been optimized in detail for the preparation of apolar columns. The aim was to produce optimum overall column characteristics, and to maintain them unchanged under the prolonged influence of the highest possible temperature. The main parameters under optimization were glass variety, leaching of glass surface with aqueous HCl, and amount of barium carbonate deposited, while deactivation and coating were kept constant. The basic column characteristics were adsorption properties and thermostability of deactivation, acid/base behaviour and separation efficiency. They were determined by a new, quantitative testing procedure. Intense leaching was able to eliminate almost totally the differences between glass varieties and to create a well-defined glass surface. While untreated glass, leached glass, and barium carbonate treated glass showed specific weak points in the respective column quality, the combination of leaching and barium carbonate treatment yielded the highest and most stable quality. Some technical modifications of the preparation procedure are described, including deactivation in the gas phase, and use of pentane as a solvent for static coating.     

Capillary gas chromatography of pesticide residues in foods: Application of a split/splitless inlet system

Residue mixtures of pesticides in foods which were difficult to separate by packed-column gas chromatography were determined by capillary gas chromatography and electron capture detection. Manual injections of sample and reference standard were made with a 10 μL syringe on a septum-enclosed, split/ splitless inlet system. Optimal peak height repeatability (± 3%( was achieved with three-μL injections which were made within time periods of four hours or less. The average linearity coefficient (slope( obtained for detector response as a function of solution concentration was 1.1.     

皮革中六价铬含量的IUC-18测定法

Ｃｒ(ＩＩＩ)是制革业最重要的鞣剂 ,制革业所用的Ｃｒ(ＩＩＩ)鞣剂由Ｃｒ(ＶＩ)还原而得到。皮革中除Ｃｒ(ＩＩＩ)外 ,还可能含有微量的Ｃｒ(ＶＩ) ,目前欧盟等国要求Ｃｒ(ＶＩ)的含量小于 5ｍｇ/ｋｇ[1 ] ;国内则还没有分析测定皮革中六价铬的国家标准 ,本文介绍六价铬的ＩＵＣ - 1 8国际标准分析方法 (ＩＵＣ ,Ｉｎｔｅｒ ｎａｔｉｏｎａｌＵｎｉｏｎｏｆＣｈｅｍｉｓｔｒｙ) ,并分析了该方法的不足及改进途径。1　实验部分ｐＨ7 5～ 8 0、惰性气体保护下用磷酸盐缓冲溶液浸取皮革的六价铬 ,六价铬将 1 ,5 二苯卡巴肼氧化为 1 ,5 二苯…     

Direct and simultaneous spectrofluorometric determination of naproxen and salicylate in human serum assisted by chemometric analysis

A direct method for the simultaneous determination of naproxen and salicylate in human serum is reported, based on a combination of spectrofluorometric measurements with two multivariate calibration techniques: partial least-squares (PLS-1) and the novel net analyte preprocessing (NAP). The method is rapid, selective and sensitive, and is based on the measurement of the fluorescence spectra of NH₃ alkalinized whole human sera at the excitation wavelength of 315 nm. It can be applied within the ranges of concentrations 50-200 ng ml⁻¹ for naproxen and 100-300 ng ml⁻¹ for salicylate. The employed chemometric techniques have been compared on the basis of the statistical indicators for calibration and validation. Reproducibility and interference studies in abnormal sera have also been carried out.     

Performance of quantitative analyses by liquid chromatography–electrospray ionisation tandem mass spectrometry: from external calibration to isotopomer-based exact matching

Liquid chromatography–tandem mass spectrometry (LC-MS/MS) is a versatile coupling system which combines both selectivity and sensitivity and certainty. Hence, it is generally considered as the most reliable technique to quantify chemical compounds in complex matrices. In the present paper, we evaluate the performance of LC-MS/MS methods for the quantification of 3-nitrotyrosine in human urine in order to point out its dependence on the design of the quantification method, and emphasize the role of matrix effects in the performance. We compare external and internal calibrations, isotope dilution and isotopomer-based exact matching. The role of both sample preparation and multiple transitions monitoring is particularly addressed.     

The preparation of thick-film glass capillary columns

A detailed method for the routine preparation of glass capillary columns is presented. The method consists of coating a glass tube with quartz powder prior to pulling the tube into a capillary. The inner surface of the capillary consists of an even distribution of quartz particles fused to the walls. This surface has been found readily deactivated by standard procedures and ideal for the preparation of thick-film glass capillary columns. The method has been thoroughly tested in two independent laboratories to ensure that the procedures described are reproducible.     

Classifying Structural Characteristics to Enhance Computation of Enthalpy of Formation of Halohydrocarbons

Density functional theory (DFT) calculations are made and least squares calibration performed for various halohydrocabons, which were 27 straight‐chain alkyl halides, 20 branch‐chain alkyl halides and 19 aromatic halides, to determine their enthalpies of formation (ΔH_f). The mean absolute error (M. |A.E.|) in ΔH_f across 66 molecular computations was only 7.8 kJ/mol (1.9 kcal/mol). Grouping the molecules by their structural characteristics improved M. |A.E.| of ΔH_f by 0.2–2.2 kJ/mol over that obtained using corresponding modified data for the same 66 unclassified molecules.