Signal enhancement by trapping in microscale liquid chromatography: numerical modelling

We report on a series of computer simulations that have been made to explore the operation and performance limits of a periodically heated and cooled trapping segment (liquid analog of the modulator device used in GC×GC) interfacing the separation column and the detector. The initial peak width and the retention of the molecules on the trapping segment have the most important influence on the trapping efficiency. Higher retention of the trapping segment and smaller peaks will lead to higher signal enhancements. However when the resulting concentration gradients become too large, as is the case for very small peaks and/or for very high retention factors on the trapping segment, the dispersion will strongly decrease the focussing efficiency. The molar diffusion coefficient and the linear velocity mainly have an impact on the dispersive behaviour, which can be directly calculated from the associated plate height values. General design rules for the trapping segment, validated with the computer simulations, give a good estimate of the required trapping time and the length of the trapping segment. Equations for the estimation of the trapped peak width and signal enhancement are also given.     

Preparation and Rheological Characterization of Polymer Nanocomposites Based on Expanded Graphite

A new type of conductive filler, namely expanded graphite (EG), was used to prepare novel nanocomposites. The EG was incorporated into several rather different polymers, specifically polycarbonate (PC), low‐density polyethylene (LDPE), isotactic polypropylene (PP), and polystyrene (PS), using melt mixing in a small‐scale DACA‐Microcompounder. The EG content was varied between 1 and 20 wt%. The rheological properties and morphologies of the nanocomposites were characterized by melt rheology and scanning electron microscopy (SEM), respectively. The melt‐state linear viscoelastic properties were investigated using an ARES rheometer, with the measurements performed in the dynamic mode at various temperatures over a wide range of frequencies. Addition of the EG increased the linear dynamic moduli and melt viscosity of the materials. Up to a certain critical concentration of EG, the materials exhibited a simple liquid‐like behavior. Above this concentration, however, significant changes in the frequency dependences of the moduli and viscosity were observed. In addition, the moduli showed a liquid‐solid transition resulting in a second plateau in the low frequency‐regime, and the complex viscosity revealed shear‐thinning behavior. Specific values of this percolation concentration were found to be at around 4 wt% in the case of PC/EG, 9 wt% for PP/EG and PS/EG, and 12 wt% for PE/EG. This critical concentration was correlated to a network‐like structure formed through interactions between the EG platelets and the polymers. The extent of these complications was found to vary from polymer to polymer, presumably due to different degrees of EG exfoliation and dispersion arising from different EG‐polymer interactions and from variable shearing forces dependent on the polymer viscosities. The formation of network‐like structures is very sensitively displayed using van Gurp‐Palmen plots, which are most suitable for identifying “rheological percolation” in our investigated systems.     

High‐resolution peptide separations using nano‐LC at ultra‐high pressure

We report on the optimization of nano‐LC gradient separations of proteomic samples that vary in complexity. The gradient performance limits were visualized by kinetic plots depicting the gradient time needed to achieve a certain peak capacity, while using the maximum system pressure of 80 MPa. The selection of the optimal particle size/column length combination and corresponding gradient steepness was based on scouting the performance of 75 μm id capillary columns packed with 2, 3, and 5 μm fully porous silica C18 particles. At optimal gradient conditions, peak capacities up to 500 can be obtained within a 120 min gradient using 2 μm particle‐packed capillary columns. Separations of proteomic samples including a cytochrome c tryptic digest, a bovine serum albumin tryptic digest, a six protein mix digest, and an Escherichia coli digest are demonstrated while operating at the kinetic‐performance limit, i.e. using 2‐μm packed columns, adjusting the column length and scaling the gradient steepness according to sample complexity. Finally, good run‐to‐run retention time stability (RSD values below 0.18%) was demonstrated applying ultra‐high pressure conditions.     

Reference materials for measuring the size of nanoparticles

This article discusses the requirements for reference materials (RMs) for measuring the size of nanoparticles (NPs). Such RMs can be used for instrument calibration, statistical quality control or interlaboratory comparisons. They can come in the form of suspensions, powders or matrix-embedded materials [i.e. NPs integrated in a natural matrix (e.g., food, soil, or sludge)].At present, uncertainty about the most suitable form of material, the most relevant measurands and the most useful metrological-traceability statement inhibits the production of NP RMs. In addition, the lack of validated methods and qualified laboratories to produce NP RMs present formidable challenges.Metal, inorganic and organic NPs are available, but most of them are intended to be laboratory chemicals. With the exception of latex materials, certified RMs are not available, although some metrology institutes have started to develop such materials for colloidal gold and silica particles.     

Modelling the thermal behaviour of the low-thermal mass liquid chromatography system

We report upon the experimental investigation of the heat transfer in low thermal mass LC (LTMLC) systems, used under temperature gradient conditions. The influence of the temperature ramp, the capillary dimensions, the material selection and the chromatographic conditions on the radial temperature gradients formed when applying a temperature ramp were investigated by a numerical model and verified with experimental temperature measurements. It was found that the radial temperature gradients scale linearly with the heating rate, quadratically with the radius of the capillary and inversely to the thermal diffusivity. Because of the thermal radial gradients in the liquid zone inside the capillary lead to radial viscosity and velocity gradients, they form an additional source of dispersion for the solutes. For a temperature ramp of 1 K/s and a strong temperature dependence of the retention of small molecules, the model predicts that narrow-bore columns (i.d. 2.1 mm) can be used. For a temperature ramp of 10 K/s, the maximal inner diameter is of the order of 1 mm before a substantial increase in dispersion occurs.     

Scroll wave instabilities in an excitable chemical medium

Two kinds of scroll wave instabilities were studied experimentally in the excitable Belousov-Zhabotinsky reaction: three-dimensional meandering and negative line tension of the scroll wave filament. The filament displays a flat zigzag shape in the initial stages of the experiment. As the chemical medium ages, the filament assumes a wiggly shape while its length increases substantially. Numerical simulations underpin the experimental findings and their interpretation.     

The effect of a cluster on a chemical reaction: a quasiclassical trajectory study

We have calculated product rotational state distributions for the D + H₂ reaction trapped in the center of icosahedral argon cluster at various cluster temperatures. All the degrees of freedom are treated classically in the present calculations. The rotational state distributions exhibit considerable dependence on the cluster environment and its temperature as compared with those obtained in the gas phase reaction.     

New Polymer Syntheses, 108. Controlled Degradation and In‐Situ Functionalization of Polycarbonates by Means of Bu2SnO

It was found that diphenyl carbonate reacts with Bu₂SnO above 140°C yielding CO₂ and Bu₂Sn(OPh)₂. This insertion reaction was also successfully applied to a polycarbonate prepared from polytetrahydrofuran diol‐1000 and Bu₂SnO containing macrocyclic polyTHF dioxides. Bu₂SnO reacted analogously with poly(bisphenol A) carbonates yielding tin‐containing rapidly crystallizing cyclic oligocarbonates. Their treatment with 1,2‐ethanedithiol gave linear oligocarbonates having two OH end groups. Bu₂Sn‐containing macrocyclic oligocarbonates reacted in‐situ with several carboxylic acid chlorides, forming telechelic oligocarbonates with functional ester end groups. Bu₂Sn‐containing macrocycles also reacted in‐situ with terephthaloyl chlorides yielding a poly(ester carbonate).     

A molecular mechanics force field for rhodium(I) carbonyl phosphine complexes and its application on the oxidative addition reactions of these complexes

The main purpose of the development of an Rh(I) Carbonyl Phosphine force field was to predict the molecular structure of Rh(I) complexes as well as to compute possible intermediates or transition states during the oxidative addition of CH₃I to these complexes. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 692–703, 2000