- and β-cyclodextrin complexes with n-alkyl carboxylic acids and n-alkyl p-hydroxy benzoates. A molecular mechanics study of 1:1 and 1:2 associations

Molecular mechanics (MM) methods were employed to evaluate stabilization upon formation of inclusion compounds between two different guest molecules and - and β-cyclodextrins (CDs) for two different stoichiometries 1:1 and 1:2. The two guest molecules studied were n-alkyl carboxylic acids and n-alkyl p-hydroxy benzoates with variety of chain lengths. The computed stability for the inclusion compounds between -CDs and n-alkyl carboxylic acids reproduced experimental data reported in the literature. The transition between 1:1/1:2 complexes occurred at an alkyl chain length of n_C=9. It was previously demonstrated by diffusion coefficients measures that a stable 1:2 stoichiometry inclusion compound could be formed between n-alkyl p-hydroxy benzoates and -CD for the chain length n_C>4. The computed results reproduced the experimental ones. The combination between OPLS and GB/SA resulted in better agreements with experiments than those obtained with MM2 and MM3.     

Recent developments in electrochemical detection for microchip capillary electrophoresis

Significant progress in the development of miniaturized microfluidic systems has occurred since their inception over a decade ago. This is primarily due to the numerous advantages of microchip analysis, including the ability to analyze minute samples, speed of analysis, reduced cost and waste, and portability. This review focuses on recent developments in integrating electrochemical (EC) detection with microchip capillary electrophoresis (CE). These detection modes include amperometry, conductimetry, and potentiometry. EC detection is ideal for use with microchip CE systems because it can be easily miniaturized with no diminution in analytical performance. Advances in microchip format, electrode material and design, decoupling of the detector from the separation field, and integration of sample preparation, separation, and detection on-chip are discussed. Microchip CEEC applications for enzyme/immunoassays, clinical and environmental assays, as well as the detection of neurotransmitters are also described.     

Stereoselective aldol condensations via alkenyloxy dialkoxyboranes : mechanistic and stereochemical details

A detailed investigation of the enolization of ketones with ethylenechloroboronate ( ECB ) in the presence of a tertiary amine and the subsequent aldol condensations of these boron enolates was conducted. The enolization with ECB- DPEA system was found to be regioselective except for the case of butanone. The stereochemistry of the enolates derived from ethyl ketones was defined as Z on the basis of ¹H-NMR comparison to the Z enolates obtained by a stereodefined route. A mechanistic model for the enolization is proposed to explain the enolization selectivity. E enolates were found to be more reactive than the Z enolates. The product aldol stereochemistry ( syn ) was correlated to the enolate geometry via a chairlike transition state ( Z enolates ) or via a boatlike transition state ( E enolates ).     

Evaluation of the sampling procedure adopted by the EU pesticide residue control programme to assess consumer exposure to high acute toxicity pesticides – methamidophos in sweet peppers

Food chemical control, particularly control involving the analysis of pesticide residues in fruits and vegetables, is routinely performed on composite samples. This sample design is adequate for the control of the majority of pesticides which have only long-term effects. However, from the viewpoint of consumer risk assessment, this sample design is inadequate for pesticides with relevant acute toxicity, when their residues occur in large food items the consumption of which may represent an important fraction of a single meal or an important fraction of the consumption in 24 h. This work presents the results of the Portuguese official pesticide residue control authority concerning the variability of methamidophos concentration in sweet pepper units, collected from the market, in the framework of the EU 1999 co-ordinated programme for the official control of pesticides in products of vegetable origin. The results were expressed with uncertainty and the conclusion was that the observed dispersion cannot be allocated to the method precision. The studied samples presented a variability factor (ratio between the maximum and the mean concentration of the units) ranging from 1.8 to 3.5 supporting the need to consider a variability factor representing the potential unit-to-unit variation in residues concentration for the purpose of consumer risk assessment. Received: 8 December 2001 Accepted: 25 February 2001     

Low-energy electron scattering by methane,arsine and phosphine

We present cross sections for the elastic scattering of low-energy electrons by phosphine (PH₃) and arsine (AsH₃), and for electron-impact excitation of the (1t ₂ 3sa ₁)³ T ₂ and¹ T ₂ states in methane (CH₄). These results were calculated using the Schwinger multichannel method as implemented on distributed-memory parallel computers. The PH₃ and AsH₃ cross sections show a pronounced low-energy shape resonance which may provide a pathway to dissociative attachment. The^1,3 T ₂ cross sections for CH₄ correlate fairly well with recent measurements of CH₂ production via electron-impact dissociation of methane.Contribution No. 8587     

Nicotine-related impurities in e-cigarette cartridges and refill e-liquids

The nicotine used in e-cigarettes and refill e-liquids is extracted from tobacco, and its purity can vary depending upon manufacturer and grade. The US and European Pharmacopoeias make recommendations for the purity of nicotine intended for pharmaceuticals; however, there is no official purity recommendation for nicotine used in e-cigarettes. To date, there are few published reports on nicotine-related impurities in e-cigarettes and refill e-liquids. The objective of this work was to develop a sensitive, selective, and robust analytical method for the quantitation of nicotine-related impurities in e-vapor products and to evaluate the nicotine-related impurities in a variety of commercial e-cigarette cartridges (n?=?10) and refill e-liquids (n?=?10). Nicotine-N-oxide, nornicotine, mysomine, and cotinine were observed to increase with time during stability studies. This method was also applied to estimate the transfer efficiency of nicotine-related impurities to the aerosol. Most of the impurities were observed to transfer efficiently. However, nicotine-N-oxides showed low transfer efficiency and demonstrated thermal degradation. This selective and sensitive method is suitable to provide quantitative data for risk assessments and for use in e-cigarette product and refill e-liquid stability studies as one of the stability-indicating measures.     

Metric Properties of Projections in Semi-Hilbertian Spaces

Several results on norms of projections on a Hilbert space are extended for the operator seminorm defined by a positive semidefinite operator . The authors were supported in part by UBACYT I030, ANPCYT PICT 03-09521, PIP 2188/00.     

Proton and carbon-13 chemical shifts: Comparison between theory and experiment

A series of ab initio ¹H and ¹³C NMR chemical shifts are presented for all molecules for which gas-phase experimental measurements exist. Quantitative agreement with this large set of data is achieved by the use of gauge-invariant atomic orbitals in an SCF perturbation theory approach. The effect of basis set completeness on these ¹H and ¹³C chemical shifts is also examined. The 4-31G basis set is found to provide internally consistent results and give satisfactory agreement with gas-phase experimental data. Errors within 6% for ¹H shifts and 3% for ¹³C shifts result. Increasing the basis set to the 6-31G^* level does not significantly improve the agreement. For ¹H shifts only, the 3-21G basis set is adequate. The validity of the particular computational approach employed here is further substantiated by comparison to another ab initio magnetic shielding method.