Direct optical emission spectroscopy of liquid analytes using an electrolyte as a cathode discharge source (ELCAD) integrated on a micro-fluidic chip

Atomic emission detection of metallic species in aqueous solutions has been performed using a miniaturised plasma created within a planar, glass micro-fluidic chip. Detection was achieved using an Electrolyte as a Cathode Discharge source (ELCAD) in which the sample solution itself is used as the cathode for the discharge. To realise the ELCAD technique within a micro-fluidic device, a parallel liquid-gas flow was set up in a micro-channel and a glow discharge ignited between the flowing liquid sample surface and a metal wire anode. The detection of copper and sodium was achieved, using atmospheric pressure air as a carrier gas, by observation of atomic emission lines of copper at 324 nm, 327 nm, 511 nm, 515 nm and 522 nm and an atomic emission line of sodium at 589 nm using a commercially available miniaturised spectrometer. A total electrical power of less than 70 mW was required to sustain the discharge. A semi-quantitative, absolute detection limit of 17 nmol s(-1) was obtained for sodium with a sample flow rate of 100 microL min(-1) and an integration time of 100 ms in air at atmospheric pressure. The volume required for such detection is approximately 170 nL. Further analysis was performed with an Echelle spectrometer using both argon and air as a carrier gas. The geometry and flow rates used demonstrate the feasibility of integrating such micro-plasmas into other micro-fluidic devices, such as miniaturised CE devices, as a method of detection. The potential for using such micro-plasmas within highly portable miniaturised systems and mu-TAS devices is presented and discussed.     

Investigation of gas chromatographic interaction mechanism on permethylated cyclodextrins by molecular modelling

Summary The capillary gas chromatographic retention behavior of -pinene and tricyclene has been investigated on stationary phases of different polarities. On all but one of the columns employed, tricyclene eluted before -pinene; only permethylated -cyclodextrins dissolved in moderately polar polisiloxanes gave a reversed elution order. The intermolecular interactions which caused the unexpected retention behavior were investigated in detail, applying methods of computer simulation. To achieve this, we have developed a calculation algorithm on the basis of molecular mechanical optimizations and programmed it in a macro. This makes it possible to systematically investigate a given configuration space in which all the possible interactions can take place. It was shown that permethylated -cyclodextrin as host molecule for both guest molecules offers an optimum cavity size. As a result the number of energetically favorable contacts between host and guest molecules as well as the strength of the interactions in this stationary phase were larger. As a consequence the elution order, normally only influenced by the vapor pressure of the compounds at a given temperature, was changed. Nonspecific interactions played an especially important role for these kinds of substances.     

Determination of 1-vinyl-2-pyrrolidone in human serum by high-performance liquid chromatography

Summary A simple method is described which allows the quantification of 1-vinyl-2-pyrrolidone (NVP) in human serum. NVP is extracted from serum with diethylether and determined with HPLC/UV-detection. 1-Cyclohexyl-2-pyrrolidone serves as an internal standard. The detection limit is 0.1 mg/l. The method has shown that NVP can enter the organisms of workers occupationally exposed to this substance.     

Numerical analysis of electroosmotic flow in dense regular and random arrays of impermeable, nonconducting spheres

We present a numerical scheme for analyzing steady-state isothermal electroosmotic flow (EOF) in three-dimensional random porous media, involving solution of the coupled Poisson, Nernst-Planck, and Navier-Stokes equations. While traditional finite-difference methods were used to resolve the Poisson-Nernst-Planck problem, the (electro)hydrodynamics has been addressed with high efficiency using the lattice-Boltzmann method. The developed model allows simulation of electrokinetic transport under most general conditions, including arbitrary value and distribution of electrokinetic potential at the solid-liquid interface, electrolyte composition, and pore space morphology. The approach provides quantitative information on a spatial distribution of simulated velocities. This feature was utilized to characterize EOF fields in regular and random, confined and bulk packings of hard (i.e., impermeable, nonconducting) spheres. Important aspects of pore space morphology (sphere size distribution), surface heterogeneity (mismatch in electrokinetic potentials at confining wall and sphere surface), and fluid phase properties (electrical double layer thickness) were investigated with respect to their influence on the EOF dynamics over microscopic and macroscopic spatial domains. Most important is the observation of a generally nonuniform pore-level EOF velocity profile in the sphere packings (even in the thin double layer limit) which is caused by pore space morphology and which is in contrast to the pluglike velocity distribution in a single, straight capillary under the same conditions.     

Controlled vesicle self-assembly in microfluidic channels with hydrodynamic focusing

Traditional liposome preparation methods are based on mixing of bulk phases, leading to inhomogeneous chemical and/or mechanical conditions during formation; hence liposomes are often polydisperse in size and lamellarity. Here we show the formation of liposomes that encapsulate reagents in a continuous two-phase flow microfluidic network with precision control of size from 100 to 300 nm by manipulation of liquid flow rates. We demonstrate that by creating a solvent-aqueous interfacial region in a microfluidic format that is homogeneous and controllable on the length scale of a liposome, we can facilitate the fine control of liposome size and polydispersity.     

13C NMR pattern of Sc3N@C68. Structural assignment of the first fullerene with adjacent pentagons

Sc3N@C68 is assigned to isomer Sc3N@C68:6140 on the grounds of relative energies, geometrical data, and its 13C NMR pattern. Sc3N@C68:6140 is an endohedral fullerene where each Sc atom is coordinated to the center of an equatorial pentalene unit. Static and dynamic computer simulations explain the different point groups observed in NMR and X-ray experiments. Computed and experimental 13C NMR pattern are in close agreement except for one low-intensity signal. The competing isomer Sc3N@C68:6275 is found to be 409 kJ/mol less stable and shows a different 13C NMR pattern.     

Reaction Engineering Aspects of α-l,4-D-Glucan Phosphorylase Catalysis

Some important process properties of α-l,4-D-ghican phosphorylases isolated from the bacteriumCorynebacterium callunae and potato tubers (Solatium tuberosum) were compared. Apart from minor differences in their stability and specificity (represented by the maximum degree of maltodextrin conversion) and a 10-fold higher affinity of the plant phosphorylase for maltodextrin (K _M of 1.3 g/L at 300 mM of orthophosphate), the performances of both enzymes in a continuous ultrafiltration membrane reactor were almost identical. Product synthesis was carried out over a time course of 300–400 h in the presence or absence of auxiliary pullulanase (increasing the accessibility of the glucan substrate for phosphorolytic attack up to 15–20%). The effect of varied dilution rate and reaction temperature on the resulting productivities was quantitated, and a maximum operational temperature of 40°C was identified.     

A study on the utility of bismuth-film electrodes for the determination of In(III) in the presence of Pb(II) and Cd(II) by square wave anodic stripping voltammetry

This work is an investigation into the utility of bismuth-film electrodes (BiFEs) for the determination of indium in the presence of cadmium and lead by square wave anodic stripping voltammetry (SWASV). The purpose of this study was to demonstrate that, after judicious choice of the associated chemical and instrumental conditions, it is possible to achieve satisfactory separation of the relevant stripping peak and, thus, to determine simultaneously trace concentrations of cadmium, lead and indium. It was conclusively demonstrated that a BiFE performed better than the equivalent mercury-film electrode (MFE) in this particular application.