David W. Paul

An electronic oscillator with automatic gain control: EQCM applications

Abstract An electronic oscillator for use with thickness-shear-mode acoustic wave sensors is described. The oscillator monitors not only the resonance frequency of the sensor, but also provides an automatic gain control signal that monitors the energy dissipated by the sensor. Electrochemical quartz-crystal microbalance (EQCM) experiments have been performed in which one of the sensors electrodes serves as the working electrode in an electrochemical cell. Electrochemical deposition of Tl + on the sensors surface results in a frequency shift that is attributed to mass loading, while a negligible amount of energy loss from the sensor is observed. Results from the cyclic voltammetry of the K 3 Fe(CN) 6 / K 4 Fe(CN) 6 couple, which does not alter the mass on the sensor, indicate that the energy loss from the sensor can be directly monitored by the automatic gain control voltage and is related to the microviscosity of the solution adjacent to the sensor. The addition of automatic gain control to an oscillator circuit provides energy-loss information that is usually only given by impedance analyzers. The circuit is applicable to situations where remote sensing or cost are important factors.

Real‐time data acquisition for gas electron diffraction

An instrument for gas electron diffraction (GED) studies is described which eliminates the photographic intermediary and sector device which are currently used almost universally in molecular structure determinations using GED. Specifically, the scattered electrons are detected by a fluorescent screen which is optically coupled to a custom multichannel analyzer (EG&G model 1412 photodiode array detector and model 1218 controller, combined with a MINC‐DEC LSI 11/23 computer). Rapid operation not only eliminates photographic film development and densitometry, but also allows for repeated measurements to be easily made. Dynamic data acquisition provides for virtually steady‐state conditions in the experimental environment, minimizing systematic errors and providing a means for instrument control. The design has been completed to the extent that small angle scattering data are now readily recorded in this novel way. The data range accessible in the current configuration (maximum s values of approximately 20–2...

Instrumentation for gas electron diffraction employing a pulsed electron beam synchronous with photoexcitation

A novel instrument is described capable of recording gas electron diffraction (GED) patterns of excited molecular states or transient species with pulsed electron beams. The system incorporates (1) a pulsed optical beam for electronic excitation of materials under study, (2) a synchronously pulsed source of 30–50 keV electrons in a space‐charge‐limited beam, (3) necessary vacuum environment and sample‐handling capabilities, and (4) detection and signal processing equipment using an on‐line procedure developed at the University of Arkansas. Data obtained for several test gases demonstrate successful operation of the instrument. The 193 nm laser photofragmentation of carbon disulfide, CS2, is described in detail. In agreement with a recent time‐of‐flight mass spectrometric study of the same process, carbon monosulfide was observed as the reaction product. This study is the first quantitatively successful joint exercise of on‐line multichannel GED data recording and a stroboscopic electron source. The method...

Immunoelectrochemical Assay in Combination with Homogeneous Enzyme-Labeled Antibody Conjugation for Rapid Detection of Salmonella

An immunoelectrochemical assay in combination with homogeneous enzyme-labeled antibody conjugation was developed for rapid detection of Salmonella. The assay was performed by mixing alkaline phosphatase linked anti-Salmonella (APLAS) with Salmonella in a solution. The Salmonella-APLAS conjugate separated by polycarbonate membrane filtration was incubated with a phenyl phosphate substrate to produce phenol. The concentration of Salmonella cells was determined by measuring phenol oxidation peak current using differential pulse voltammetry at a renewable carbon paste electrode. This assay could be completed within two hours with a detection limit of 5 × 103 cells/mL. A linear response was found for Salmonella between 5 × 103 and 1 × 106 cells/mL.

Detection of galactosyltransferase using chemically modified piezoelectric quartz

Abstract In this report a piezoelectric crystal (quartz crystal microbalance) was used to detect the enzyme galactosyltransferase. A circular hole was etched in the piezoelectrics gold electrode and glucosamine covalently attached to the surface of the exposed quartz. Separate colorimetric testing showed that the glucosamine surface densities were in agreement with those found in the affinity chromatography literature and were estimated at 1.0 μmol/m2. The modified surface was exposed to a drop of test solution containing excess galactosyltransferase and cofactors sufficient to trap analyte in an enzyme—substrate complex on the surface of the crystal. The piezoelectric was removed from solution, dried and the frequency changes caused by the attraction of the enzyme recorded. Shifts in the resonant frequency of the crystal indicated that a complex between surface-bound glucosamine, Mn(II), UDP (uridine 5-diphosphate) and enzyme formed when the crystal was exposed to the liquid phase, but a complicating background signal existed from non-specific adsorption of enzyme, buffer and cofactors. It was also discovered that etching the piezoelectric electrode caused a degradation in crystal Q. In addition, an environmental chamber was constructed for the determination of crystal frequencies. An average drift rate of 0.05 Hz/h was achieved with a reproducibility of frequency measurement for an individual crystal at ±5 Hz.

Improvements in real‐time data acquisition for gas electron diffraction

Several improvements are described of a recently reported prototype real‐time data‐acquisition system for gas electron diffraction (GED), in which the scattered intensity distribution is recorded by a photodiode array. The use of a computer‐designed and photographically produced ‘‘butterfly’’ slit in the optical track of the instrument, which simulates the effects of the rotating sector of conventional GED, as well as a number of other modifications, now makes it possible to record, for the first time by this technique, scattered electron intensities in a region around s=300 nm−1. Modeling of several test molecules shows that structural parameters can be determined accurately by the novel procedure, but systematic inaccuracies still exist in the determination of mean amplitudes of vibration. Data evaluation is based on a novel ratiometric analysis procedure, whose essential features are briefly discussed.

The molecular structures of cis- and trans-1,2-dichloroethene: a real-time gas electron diffraction and ab initio study

Abstract The molecular structures of cis- and trans -1,2-dichloroethene have been studied in the gas phase by electron diffraction, using our new real-time data acquisition tecnique. For cis -dichloroethene the structural parameters are ( r a distances(A) and angles (deg.), with 3σ in parentheses): CCl 1.717(2); CC1.337(4); ClCC 124.0(2); HCC 120(3). For trans -dichloroethene: CCl 1.725(2); CC 1.332(8); ClCC 120.8(6); and HC 124(3). The experimental work was augemented by ab initio gradient geometry refinements of the two compounds and of cis- and trans -dichloroethene. The 4–21 G, 5–31 G ** , and 3-3-21G basis sets were used in various series of calculations. Without any exception, the calculated structural trends for the cis- and trans -configurations are in excellent agreement with the experiments.

On‐line gas electron diffraction identification of gas chromatography effluents (GC‐GED)

Experiments are described which document the utility of gas electron diffraction (GED) as a means of detecting analytes emerging from the column of a gas chromatograph (GC). In these experiments electron diffraction patterns from GC effluents are made visible on a phosphor screen, and the intensities of each GC peak are recorded on line by a 1024‐channel linear photodiode array detector. The technique not only indicates the presence of elutants but also provides auxiliary information on their molecular structures under favorable conditions. Minimum sample sizes of about 1 μl per component are currently required for GC‐GED, but indications are that further detector enhancements will allow the recording of electron intensities from smaller samples.

Real-Time Electron Diffraction. Part III: Image Transfer via Fiber Optics

Improvements are described in photodiode-array real-time data recording for gas electron diffraction (GED). When the conventional glass window and lens optics in a previously reported detector configuration are replaced by fiber optic components, two significant effects arise: (1) detector gain is enhanced to the extent that it is now possible to detect nanoliter samples in combined GED-GC (gas chromatography) experiments, and (2) for the first time since the development of the real-time recording scheme, molecular mean amplitudes of vibration are within error limits of literature values. Thus the method now affords full molecular structure determinations, including bond distances and angles, and their associated mean vibrational properties.

An acoustic wave biosensor for human low-density lipoprotein particles: construction of selective coatings

Cholesterol is found in four major classes of blood particles including chylomicrons, very low-density lipoproteins (VLDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL). The most studied fraction is LDL as it is most closely associated with heart disease. The challenge in current methods of analysis is the determination of the cholesterol in the individual lipoprotein fractions. Accordingly, the critical step in any analysis is the complete separation of the lipoprotein fractions. In this work, enhanced selectivity for the LDL fraction was achieved by the covalent binding of dextran sulfate (DS) to the gold surface of a thickness shear-mode acoustic wave sensor. The thickness and surface concentration of the DS layer was estimated by in situ ellipsometry to be 219 A and 0.8 ng/mm(2), respectively, but it was difficult to construct the sensing layer reproducibly. The DS coated sensor was ten times more responsive to LDL than the other lipoprotein (LP) fractions. The sensor was a main component in a flow injection analysis system that exposed LDL, VLDL and HDL to not only the DS layer, but also to the underlayers used in the construction of the DS layer. A possible regeneration solution was found which would rinse the LDL from the layer, restoring the sensor for repeated use. Frequency shifts from LP absorption into the DS layer were corrected for dissipative losses through the DS layer using an oscillator circuit equipped with an automatic gain control feature.