Capillary zone electrophoresis (CZE) devices fabricated in poly(dimethylsiloxane) (PDMS) require continuous voltage control of all intersecting channels in the fluidic network in order to avoid catastrophic leakage at the intersections. This contrasts with the behavior of similar flow channel designs fabricated in glass substrates. When the injection plugs are shaped by voltage control and leakage from side channels is controlled by the application of pushback voltages during separation, fluorescein samples give 64 200 theoretical plates (7000 V separation voltage, E = 1340 V/cm). Native PDMS devices exhibit stable retention times (+/- 8.6% RSD) over a period of five days when filled with water. Contact angles were unchanged (+/- 1.9% RSD) over a period of 16 weeks of dry storage, in contrast to the known behavior of plasma-oxidized PDMS surfaces. Electroosmotic flow (EOF) was observed in the direction of the cathode for the buffer systems studied (phosphate, pH 3-10.5), in the presence or absence of hydrophobic ions such as tetrabutylammonium or dodecyl sulfate. Electroosmotic mobilities of 1.49 x 10(-5) and 5.84 x 10(-4) cm2/Vs were observed on average at pH 3 and 10.5, respectively, the variation strongly suggesting that silica fillers in the polymer dominate the zeta potential of the material. Hydrophobic compounds such as dodecyl sulfate and BODIPY 493/503 adsorbed strongly to the PDMS, indicating the hydrophobicity of the channel walls is clearly problematic for CZE analysis of hydrophobic analytes. A method to stack multiple channel layers in PDMS is also described.

Electrokinetic control of fluid flow in native poly(dimethylsiloxane) capillary electrophoresis devices.

V. SUMMARY To the reader who is only casually familiar with electrochemical measurements based on periodic perturbations, perhaps one of the most surprising aspects of this review is its illustration of the level and scope of influence of the methodology considered. Hopefully, it has been made evident that the literature gives witness to notable growth in the three areas of importance to any experimental method:theory, instrumentation, and application. One finds successful application to chemical systems whose types encompass the entire range of solution chemistry, from the molecule of life (DNA) to the simplest elemental ions. Having never bothered to actually keep a quantitative record of the output, even this writer was rather surprised upon first realizing that a yield in the neighborhood of 100 publications per year has characterized this field in the last 3½ years. Even in the present era of the "information explosion" this must be considered a nontrivial level of activity. Because many scientists, ...

Recent Developments in Alternating Current Polarography

Instrument for the automatic measurement of the electrode admittance

This paper describes a simple method for the effective and rapid separation of hydrophobic molecules on polydimethylsiloxane (PDMS) microfluidic devices using Micellar Electrokinetic Chromatography (MEKC). For these separations the addition of sodium dodecyl sulfate (SDS) served two critical roles – it provided a dynamic coating on the channel wall surfaces and formed a pseudo-stationary chromatographic phase. The SDS coating generated an EOF of 7.1 × 10−4 cm2 V−1 s−1 (1.6% relative standard deviation (RSD), n = 5), and eliminated the absorption of Rhodamine B into the bulk PDMS. High efficiency separations of Rhodamine B, TAMRA (6-carboxytetramethylrhodamine, succinimidyl ester) labeled amino acids (AA), BODIPY® FL CASE (N-(4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-s-indacene-3-propionyl)cysteic acid, succinimidyl ester) labeled AA's, and AlexaFluor® 488 labeled Escherichia coli bacterial homogenates on PDMS chips were performed using this method. Separations of Rhodamine B and TAMRA labeled AA's using 25 mM SDS, 20% acetonitrile, and 10 mM sodium tetraborate generated efficiencies > 100 000 plates (N) or 3.3 × 106 N m−1 in <25 s with run-to-run migration time reproducibilities <1% RSD over 3 h. Microchips with 30 cm long serpentine separation channels were used to separate 17 BODIPY® FL CASE labeled AA's yielding efficiencies of up to 837 000 plates or 3.0 × 106 N m−1. Homogenates of E. coli yielded ∼30 resolved peaks with separation efficiencies of up to 600 000 plates or 2.4 × 106 N m−1 and run-to-run migration time reproducibilities of <1% RSD over 3 h.

High efficiency micellar electrokinetic chromatography of hydrophobic analytes on poly(dimethylsiloxane) microchips.

Alkali metal amides typically aggregate in solution and the solid phase, and even in the gas phase. In addition, even in the few known monomeric structures, the coordination number of the alkali metal is raised by binding of Lewis-basic solvent molecules, with concomitant changes in structure. In contrast, the simplest lithium amide LiNH(2) has never been made in a monomeric form, even though its structure has been theoretically predicted several times. Here, the first experimental structural data for a monomeric, unsolvated lithium amide are determined using a combination of gas-phase synthesis and millimeter/submillimeter-wave spectroscopy. All data point to a planar structure for LiNH(2). The r(o) structure of LiNH(2) has a Li-N distance of 1.736(3) A, an N-H distance of 1.022(3) A, and a H-N-H angle of 106.9(1) degrees. These results are compared with theoretical predictions for LiNH(2), and experimental data for oligomeric, solid-phase samples, which could not resolve the question of whether LiNH(2) is planar or not. In addition, comparisons are made with revised gas-phase and solid-phase data and calculated structures of NaNH(2).

First Synthesis and Structural Determination of a Monomeric, Unsolvated Lithium Amide, LiNH2

/sup 14/N NMR spectra of solutes dissolved in supercritical fluid (SCF) solvents are examined by using a sapphire high-pressure sample cell in an unmodified spectrometer. Examples using carbon dioxide, ethane, and ethylene as solvents are reported. The /sup 14/N line widths are reduced in the SCF solvents compared to those in typical liquid solvents. Solubilities large enough to yield signal-to-noise ratios > 5 in 6-24 h are obtained for a variety of liquid solutes at reduced densities of 1.0-2.0, but solid compounds had only limited solubilities. Sample pressures and temperatures were varied to observe their effects on the spectral line width. The spectral line-width improvement appears to have a functional group dependence. Line-width enhancement factors of 3-25 were obtained experimentally and varied between approximately 10% and 70% of the enhancements predicted from the differences in solution viscosity. The results are interpreted to indicate a substantial viscosity-independent contribution to the rotational correlation time.

Nuclear magnetic resonance studies of nitrogen-14-containing species in supercritical fluids

Alternating current polarograph with simplified phase selective detection

Studies of the electrochemical reduction of the monohalopyridines

Etude d'une nouvelle methode pour l'attribution des details de coordination dans des composes contenant des liaisons zinc-azote: observation, sur des complexes du bis(diethylenetriamine) et du bis(histidine), d'une amplification du temps de relaxation T 2 de 15 N par 67 Zn

Ronald F. Evilia

Papers

Nuclear magnetic resonance studies of nitrogen-14-containing species in supercritical fluids

Alternating current polarograph with simplified phase selective detection

Studies of the electrochemical reduction of the monohalopyridines

Investigation of zinc-67 coordination effects on the nitrogen-15 NMR spectra of coordinated ligands

Minimization of nuclear magnetic resonance spinning sidebands by spinning rate modulation