Counterflow isotachophoresis in a monolithic column

This study describes stationary counterflow isotachophoresis (ITP) in a poly(acrylamide‐co‐N,N′‐methylenebisacrylamide) monolithic column as a means for improving ITP processing capacity and reducing dispersion. The flow profile in the monolith was predicted using COMSOL's Brinkman Equation application mode, which revealed that the flow profile was mainly determined by monolith permeability. As monolith permeability decreases, the flow profile changes from a parabolic shape to a plug shape. An experimental monolithic column was prepared in a fused‐silica capillary using an ultraviolet‐initiated polymerization method. A monolithic column made from 8% (wt.) monomer was chosen for the stationary counterflow ITP experiments. Counterflow ITP in the monolithic column showed undistorted analyte zones with significantly reduced dispersion compared to the severe dispersion observed in an open capillary. Particularly, for r‐phycoerythrin focused by counterflow ITP, its zone width in the monolithic column was only one‐third that observed in an open capillary. These experiments demonstrate that stationary counterflow ITP in monoliths can be a robust and practical electrofocusing method.     

Synthesis,Crystal Structure and Spectroscopy of 3,4-Perylene-dicarboxylic Monoanhydride

The preparation, via a chemical vapor deposition process, of metallic-green whiskers of 3,4-perylenedicarboxylic monoanhydride (PDCMA), is described. The crystal structure and the electronic, infrared and resonance-enhanced Raman spectra of the PDCMA crystals are reported and compared with the corresponding data for the dianhydride, PTCDA. Interesting differences in the spectroscopy of PDCMA compared with that of PTCDA have been found. This has been correlated with the somewhat increased charge delocalization in PDCMA caused by the packing of the molecules in the crystal. The application of some of these results in characterizing the partly solid state, heat-induced polymerization of PTCDA to the intrinsically conducting polymer, polyperinaphthalene, is also pointed out.     

Monitoring FET flow control and wall adsorption of charged fluorescent dye molecules in nanochannels integrated into a multiple internal reflection infrared waveguide

Using Si as the substrate, we have fabricated multiple internal reflection infrared waveguides embedded with a parallel array of nanofluidic channels. The channel width is maintained substantially below the mid-infrared wavelength to minimize infrared scattering from the channel structure and to ensure total internal reflection at the channel bottom. A Pyrex slide is anodically bonded to the top of the waveguide to seal the nanochannels, while simultaneously enabling optical access in the visible range from the top. The Si channel bottom and sidewalls are thermally oxidized to provide an electrically insulating barrier, and the Si substrate surrounding the insulating SiO(2) layer is selectively doped to function as a gate. For fluidic field effect transistor (FET) control, a DC potential is applied to the gate to manipulate the surface charge on SiO(2) channel bottom and sidewalls and therefore their zeta-potential. Depending on the polarity and magnitude, the gate potential can accelerate, decelerate, or reverse the flow. Here, we demonstrate that this nanofluidic infrared waveguide can be used to monitor the FET flow control of charged, fluorescent dye molecules during electroosmosis by multiple internal reflection Fourier transform infrared spectroscopy. Laser scanning confocal fluorescence microscopy is simultaneously used to provide a comparison and verification of the IR analysis. Using the infrared technique, we probe the vibrational modes of dye molecules, as well as those of the solvent. The observed infrared absorbance accounts for the amount of dye molecules advancing or retracting in the nanochannels, as well as adsorbing to and desorbing from the channel bottom and sidewalls.     

Electrophoretic field gradient focusing: an investigation of the experimental parameters

Electrophoretic field gradient focusing has been used to separate the two oxidation states of myoglobin (Mb), and to separate Mb from bromophenol blue (BPB). Polyacrylamide and Sephadex were shown to be suitable packing materials whilst silica led to band broadening with Mb. BPB and Mb could be simultaneously focused apart using either a fixed 21-electrode setup or a dynamic 6-electrode setup. Using a dynamic three-electrode setup either analyte could be focused but not both simultaneously. It was shown that a higher ionic strength buffer in the separation channel compared to the coolant channel enhanced focusing between electrodes due to a conductivity gradient. Different running buffers were investigated and it was found that using a pH 8.6 buffer containing N,N,N-tris(hydroxymethyl)aminomethane (Tris) and phosphate ions the oxidation states of Mb could be separated but the separation of Mb from BPB was not as good as would be hoped for. Using a pH 8.6 buffer containing Tris, N-2-hydroxyethylpiperazine-N'-3-propanesulphonate and chloride ions as running buffer, BPB and Mb could be well separated but the two oxidation states of Mb merged.     

Modeling and simulation of IEF in 2-D microgeometries

A 2-D finite-volume model is developed to simulate nonlinear IEF in complex microgeometries. This mathematical model is formulated based on the mass conservation and ionic dissociation relations of amphoteric macromolecules, charge conservation, and the electroneutrality condition. Based on the 2-D model, three different separation cases are studied: an IPG in a planar channel, an ampholyte-based pH gradient in a planar channel, and an ampholyte-based pH gradient in a contraction-expansion channel. In the IPG case, cacodylic acid (pK(1) = 6.21) and Tris (pK(1) = 8.3) are used as the acid and base, respectively, to validate the 2-D IEF model. In the ampholyte-based pH gradient cases, IEF is performed in the pH range, 6.21-8.3 using 10 ampholytes in the planar channel and 20 ampholytes in the contraction-expansion channel. The numerical results reveal different focusing efficiencies and resolution in the narrow and wide sections of the contraction-expansion channel. To explain this, the expressions for separation resolution and peak concentrations of separands in the contraction-expansion channel are presented in terms of the channel shape factor. In a 2-D planar channel, a focused band remains straight all the time. However, in a contraction-expansion channel, initially straight bands take on a crescent profile as they pass through the trapezoidal sections joining the contraction and expansion sections.     

True moving bed electrophoresis using stepped electric field gradients

True moving bed electrophoresis has been shown to be an effective technique for the bench-scale separation of enantiomers, and it is desired to increase the maximum possible throughput attainable with the process by using electric field gradients. Homatropine enantiomer separations were performed and results using a stepped electric field gradient were compared to those using a traditional non-gradient separation. In order to accomplish this, a newly designed stator was constructed for use with the Vortex-Stabilized Electrophoresis Apparatus that has three sets of electrode housings, one set at both ends and one in the middle of the chamber. There were several problems related to the membranes used at the middle electrode. The dialysis membranes were permeable to the homatropine enantiomers, and while a switch to anion exchange membranes prevented the permeation of the homatropine, this caused a pH shift that interrupted binding to the hydroxypropyl-ss-cyclodextrin chiral selector. These problems prevented any meaningful data from being collected using homatropine enantiomers, and due to this, a proof of concept study was conducted using two bovine proteins. The separations using fluorescein-labeled BSA and bovine hemoglobin showed that a 63% increase in the maximum processing rate was attainable. The maximum throughput using the non-gradient process was 30.6 mg/h and the maximum was 50.0 mg/h using an electric field gradient that was 10% lower than the non-gradient field in section II and 10% higher in section III.     

Continuous voltage gradients and their application to true moving bed electrophoresis

Gradient elution has been practiced in chromatographic separations for many years. The application of discontinuous "step" gradients in simulated moving bed (SMB) chromatography has been very successful in increasing both processing rates and column productivity, resulting in a reduction in the number of SMB columns required. With the advent of the field gradient focusing techniques, electrophoresis has gained the ability to apply a continuous electric field gradient to a true moving bed (TMB) electrophoretic separation. Application of a spatial gradient allows a large degree of control of the product concentrations inside the separation unit as well as a large increase in product throughput. A model of moving bed electrophoretic separations has been developed that demonstrates the potential advantages of applying a continuous gradient to the moving bed process. These advantages include the reduction of detrimental peak tailing and the ability to decrease the concentrations of the compounds being separated in comparison with commonly used step gradient elution.     

Elucidating reactivity differences in palladium-catalyzed coupling processes: the chemistry of palladium hydrides

This communication describes a series of studies directed at obtaining a better understanding of the Heck reaction. For the first time, the postulated palladium-hydride intermediate (L2PdHX) in the catalytic cycle of the Heck arylation has been identified. In addition, this study establishes that the base-mediated Pd(0)-regeneration step (L2PdHX --> PdL2) of the cycle can be kinetically slow and thermodynamically unfavorable and that the process is remarkably sensitive to the structure of L (PCy3 vs P(t-Bu)3). Finally, this investigation demonstrates that, for certain catalyst systems, slow rates of Heck arylation can be correlated with reluctant reductive elimination of L2PdHX, furnishing a possible rationalization for Br?nsted-base (Cs2CO3 vs Cy2NMe) and ligand (PCy3 vs P(t-Bu)3) effects that have been observed.