Thomas H. Ridgway

Characterization of surface-confined ionic liquid stationary phases: impact of cation and anion identity on retention

A series of surface-confined ionic liquid (SCIL) stationary phases for high-performance liquid chromatography were synthesized in-house. The synthesized phases were characterized by the linear solvation energy relationship (LSER) method to determine the effect of residual linking ligands and the role of the cation and the anion on retention. Statistical analysis was utilized to determine whether the system coefficients returned from multiple linear regression analysis of chromatographic retention data for a set of 28 neutral aromatic probe solutes were significantly different. Examination of the energetics of retention via κ−κ plots agrees with the results obtained from the LSER analysis. Residual linking ligands were determined to contribute reversed-phase-type retention character to the chromatographic system. Furthermore, retention on the SCIL phases was observed to be more profoundly affected by the identity of the anion than by that of the cation.

Development and experimental evaluation of a simple system for scanning electrochemical microscopy

Abstract The development of a scanning electrochemical microscope (SECM) is described. The device was assembled from a commercially available open-loop piezoelectric micropositioning system and a digitally addressable bipotentiostat system. Software development had to be adapted to the special features of the micropositioning system and, along with minor hardware modifications, could eventually provide a partial closed-loop control. The user interface offers convenient access to the core routines, which are tightly time controlled by the clock of the analog-digital converter in order to achieve highly reproducible positioning and significant enhancement of the signal-to-noise ratio. The capabilities and limitations of the system are discussed using measurements of modified analytical electrodes and ultramicroelectrode arrays. A new approach is introduced for quantifying the spatial resolution, which simplifies its experimental determination. A general expression for the proposed resolution limit as a function of the tip radius and the tip-substrate distance was derived, which is based on experiments with different tip electrodes, substrates and mediator solutions. After investigating a set of microelectrodes with decreasing size, the parameter “smallest imageable object” was found to be less useful for comparing the ability of the instrument and the tip for high spatial resolution.

Evaluation of the electrochemical characteristics of a poly(vinyl alcohol)/poly(acrylic acid) polymer blend

Abstract The polymer blend poly(vinyl alcohol)/poly(acrylic acid) (PVA/PAA) has been evaluated as a coating on graphite electrodes. The uptake of tris(2,2′-bipyridyl)ruthenium(II), Ru(bpy) 3 2+ , by the film was monitored by cyclic voltammetry and was found to be pH dependent. A plot of current response versus pH is analogous to a titration curve of a mixture of acids with p K a values in the range of 4–6. At pH 1 where the PVA/PAA film is neutral, voltammograms of Ru(bpy) 3 2+ were comparable to ones recorded at a bare electrode. At pH 7, where the film is anionic because the carboxyl group is deprotonated, a current enhancement factor of 9 to 16 compared to a bare electrode was obtained. This pH-dependent behavior is also observable for plots of the peak separation versus pH in which E p of cyclic voltammograms increases with pH. The effect of different supporting electrolytes was investigated by measuring the current response with LiNO 3 , NaNO 3 , and KNO 3 over a range of pH. Cyclic voltammograms of Fe(CN) 6 3− showed that negatively charged species are rejected by the polymer film.

A polarographic method for following the rates of cholinesterase-catalyzed hydrolyses of acetylthiocholine

Abstract A rapid and simple polarographic method for following the rates of the cholinesterase-catalyzed hydrolysis of acetylthiocholine and the rates of inhibition of this hydrolysis is described. This method has the advantage that it can follow quite rapid reactions, as meaningful data can be obtained after 10 sec of mixing of the solutions. The method follows the rate of increase of the anodic polarographic wave of thiocholine produced by the hydrolysis. The validity of the method was determined by comparing the rates of production of thiocholine spectrophotometrically and polarographically. Also the values of K m for the hydrolysis, and K m and K 1 for the inhibition reaction of neostigmine were determined and compared to values reported in the literature.

Microprocessor-controlled potentiostat for twin-electrode voltammetry

Abstract A microprocessor controlled potentiostat with independent potential conrol of two working electrode is described. A 6502 microprocessor enables voltammetry to be conducted with any number of potential waveforms and current-sampling methods. A unique circuit for 60-Hz line noise elimination is used. The utility of the potentiostat is demonstrated with differential pulse anodic stripping voltammetry in a twin-electrode thin-layer cell.

Voltammetric/amperometric detection for flow-injection systems

Abstract A dual-electrode voltammetric/amperometric detector for flow-injection systems is described. The detector provides qualitative voltammetric information without the charging currents associated with scanning the potential. Selectivity is enhanced relative to direct voltammetric detection at a single electrode because only chemically reversible redox couples are detected. A preliminary evaluation with hexacyanoferrate(III) and ascorbic acid is presented.

New spectroelectrochemical sensor

A new type of spectroelectrochemical sensor that embodies tow modes of instrumental selectivity in addition to selective partitioning through an applied film barrier is described. The sensor consists of a planar optical substrate/electrode coated with a chemically-selective film. SEnsing is based on the change in the attenuation of light passing through the guided wave substrate which accompanies a chemical reaction of an analyte induced by electromodulation. Threefold selectivity for a chosen analyte relative to other environmental components is obtained by the choice of coating material, the electrolysis potential, and the wavelength for optical monitoring. The sensor concept is demonstrated with an indium tin oxide coated glass guided wave device that has been over-coated with a sol-gel derive charge-selective thin film. One such selective coating used was a charge-selective sol-gel processed Nafion-SiO2 composite film. Prototype analytes have been used to demonstrate that the change in the transmittance of the waveguide resulting from electrochemical oxidation/reduction can be used to quantify an analyte.

Design and Characterization of a Four Point Probe for Conducting Polymer Electrode Studies.

ABSTRACT A simple to use, auto-ranging four point probe is described. The full scale ranges for the instrument progresses in decade steps from 2 Giga Ohms to 2 Milli Ohms. The operator intervention is limited to sample insertion, lowering the probe head and recording the results.

Structural Characterization of Organostannane — Kraft Lignin

Lignin is the second most abundant natural, renewable material. It is produced at an annual rate of about 4 × 101 3 lbs. Organostannane moieties were successfully incorporated into Kraft lignin (Indulin AT and C) through the classical interfacial condensation of organotin chlorides with lignin. IR, MS and other physical data are consistent with the presence of organotin-modified lignin formed through condensation with the lignin-hydroxyl groups.

Software and hardware integration of a microprogrammable state machine for NMR imaging

We have integrated a commercially available microprogrammable state machine (Tecmag PULSkit) for use as a magnetic resonance pulse programmer. Providing the capability for active research environment imaging protocols, it features timing resolution of 100 nsec, ten 16-bit loop counters, and individually addressable look-up tables. This integration involved hardware and software integration with a VAX 11/750 at several levels. Hardware: Each of the three gradient channels employs three digital-to-analog converters (DACs). An 8-bit, 4-quadrant, multiplying DAC generates the gradient waveform shape. A 12-bit DAC generates the multiplying DAC scaling voltage, controlling gradient amplitude and sign. A third 12-bit DAC produces a gradient offset (shim) voltage. An eddy current compensation network is present for each gradient channel. Software: The software design philosophy was to create a flexible interface (interactive window environment), while not constraining complex manipulation of the hardware (direct use of the pulse-sequence compiler primitives and microprogramming). The software levels include (a) pulse-sequence microprogramming, (b) pulse-sequence compiler, (c) interactive parameter specification, and (d) canned pulse-sequence microcode library.