Analysis of volatiles in polymers,part II. Supercritical fluid extraction/open tubular GC/MS

Summary Supercritical fluid extraction (SFE) connected to open tubular gas chromatography has been used for the analysis of non-polar to medium polar volatiles entrained in polymers. This polymer sheets, prepared with the aid of a microtome, have been extracted with supercritical carbon dioxide. The influence of sample area/weight ratio and extraction cell volume on the SFE performance has been investigated. Further, quantitative analysis was executed by stepwise extractions.A set of polybutylene terephthalate polymers, originating from different manufactures, has been analysed regarding entrained volatiles.     

The structure of the grignard reagent : IX. The crystal structure of the dimeric ethylmagnesium bromide/diisopropyl ether complex

The structure of the dimeric ethylmagnesium bromide/diisopropyl ether complex [C₂H₅MgBr - O(i-C₃H₇)₂]₂ was studied with s ingle crystal X-ray diffraction techniques. Crystals are monoclinic with spacegroup P2₁/n and two dimers per unit cell of dimensions a 7.85(1), b 14.43(2), c 11,31(2) °A, β 100.3(1)^o The structure was refined to a final R-value of 0.073. The magnesium is four coordinate; dimers are formed through bridging bromine atoms. p]This result is briefly discussed in the context of factors governing complex formation of Grignard reagents.     

Chemometric modeling of neurotransmitter amino acid separation in normal and reversed migration micellar electrokinetic chromatography

A chemometric experimental design has been applied for the optimization of neurotransmitter amino acid separation in capillary electrophoresis. The optimizations were carried out for normal micellar electrokinetic chromatography (N-MEKC) and reversed migration micellar electrokinetic chromatography (RM-MEKC). In order to optimize three separation factors and study the interaction between factors, a response function was optimized via searching its optimum (minimum/maximum). For this purpose a central composite design with multivariate linear regression (MLR) analysis was utilized. Modeling with good regression coefficients from the MLR adequately described the interaction of factors such as background electrolyte and sodium dodecylsulfate concentrations which had a large impact on selectivity and migration behaviors. Similar optimal conditions regarding resolution and number of theoretical plates but different retention behaviors as a function of background electrolyte and micellar concentrations were observed for N-MEKC and RM-MEKC. Improved overall performance from the RM-MEKC separation of five neurotransmitter acids, superior to N-MEKC, is demonstrated in terms of repeatability, peak symmetry, sensitivity, and in particular, impurity determination in an overloaded separation system.     

Dramatic selectivity changes on carrier gas flow change in a series-coupled GC capillary tandem

We describe the polarity of selectivity of a GC separation system in terms of Retention Index data. In a series-coupled capillary system having stationary phases of differing polarity even slight (independent!) carrier gas flow changes in one part of the capillary series result in a dramatic change of selectivity. “Dramatic” is a relative term! Using a simple electronically controlled flow changing device we found retention index changes of several hundred units for polar test compounds such as phenols on a SE30/Carbowax tandem. This means: The classical theoretical model for understanding retention (and selectivity) in chromatography must be corrected. We propose a very simple approach involving addition of the expression RESIDENCE TIME to the chromatographic vocabulary. Instead of using flow resistors, one can just add a pressure regulator to the coupling point. A powerful analytical concept is now in easy reach.     

Beryllium dimer,a critical test case of MBPT and CI methods

The ground-state potential curve for the beryllium dimer is calculated as a critical test case for methods based on many-body perturbation theory (MBPT ) and configuration interaction (CI ). In particular, the recently proposed double excitation (DE ) MBPT method is compared to the standard SCF-CI method including single and double excitations from a single reference determinant. The SCF-CI method is shown to give surprisingly accurate results compared to more complete CI calculations including a larger configuration space, whereas the DE-MBPT method breaks down more or less completely, particularly for larger basis sets. The results thus demonstrate the importance of including the renormalization terms in this case. Finally, Davidson's correction and related methods lead to an even more severe breakdown than the DE-MBPT method.     

A quartz crystal microbalance study of the adsorption of asphaltenes and resins onto a hydrophilic surface

The adsorption of extracted and purified samples of asphaltenes and resins onto gold surfaces has been studied as a function of bulk concentration using a quartz crystal microbalance with dissipation measurements (QCM-D). With this device, which works equally well in transparent, opaque, and nontransparent samples, the adsorbed amount is measured through a change in resonant frequency of the quartz oscillator. The measured change in dissipation reports on changes in layer viscoelasticity and slip of the solvent at the surface. The results show that the adsorbed amount for resins from heptane corresponds to a rigidly attached monolayer. The adsorbed amount decreases with increasing amount of toluene in the solvent and is virtually zero in pure toluene. Asphaltenes, on the other hand, adsorb in large quantities and the mass and dissipation data demonstrate the presence of aggregates on the surface. The aggregates are firmly attached and cannot be removed by addition of resins. On the other hand, resins and asphaltenes associate in bulk liquid and the adsorption from mixtures containing both resins and asphaltenes is markedly different from that obtained from the pure components. Hence, we conclude that preformed resin aggregates adsorb to the surface. These results are compared and discussed in relation to adsorption from crude oil diluted in heptane/toluene mixtures.     

Refinement of the K0.96Ti0.96Nb0.04OPO4 structure

The structure of a K_0.96Ti_0.96Nb_0.04OPO₄ single crystal was established by the methods of the X-ray diffraction analysis. Additional positions of K atoms were established. In the superionic state, these atoms provide ionic transport in the crystals.     

Characterization of anacardic acids by micellar electrokinetic chromatography and mass spectrometry

A possibility of using capillary electrophoresis for separation of anacardic acids (6-alkylsalicylic acids) has been studied. Conventional micellar electrokinetic chromatography (MEKC) in non-coated fused silica capillaries and reversed-flow micellar electrokinetic chromatography (RF-MEKC) in capillaries coated with polydimethylacrylamide was applied for separation of anacardic acids extracted from cashew nuts. Influence of the composition of background electrolyte on the resolution of anacardic acid isomers was evaluated. Separations were performed using sodium dodecyl sulphate (SDS) micelles and mixed micelles of SDS and polyoxyethylene lauryl ether as a pseudostationary phase. To further improve the separation in RF-MEKC, beta-cyclodextrin and a dual cyclodextrin system of beta-cyclodextrin with heptakis-6-sulphato-beta-cyclodextrin was added to the working electrolyte. Best separation of anacardic acids were achieved in the polydimethylacrylamide-coated capillary using 10 mM phosphate background electrolyte pH 6.5 with addition of 1 M urea, 20% acetonitrile, 10 mM of beta-cyclodextrin and 1 mM of heptakis-6-sulfo-beta-cyclodextrin. Mass spectrometry was used for the identification of anacardic acids in the extract from cashew nuts in single and tandem mode using Q-TOF instrument. Nine anacardic acids were identified in the extract form the cashew nuts.     

Multifingerprint based similarity searches for targeted class compound selection

Molecular fingerprints are widely used for similarity-based virtual screening in drug discovery projects. In this paper we discuss the performance and the complementarity of nine two-dimensional fingerprints (Daylight, Unity, AlFi, Hologram, CATS, TRUST, Molprint 2D, ChemGPS, and ALOGP) in retrieving active molecules by similarity searching against a set of query compounds. For this purpose, we used biological data from HTS screening campaigns of four protein families (GPCRs, kinases, ion channels, and proteases). We have established threshold values for the similarity index (Tanimoto index) to be used as starting points for similarity searches. Based on the complementarities between the selections made by using different fingerprints we propose a multifingerprint approach as an efficient tool to balance the strengths and weaknesses of various fingerprints.