David S. Ballantine

Trace Chemical Vapor Detection Using SAW Delay Line Oscillators

The resonant frequencies of SAW delay line oscillators are shown to be very sensitive to the presence of thin organic films that are deposited onto the delay line surface. Theory suggests that the sensi- tivity of the SAW device to mass loading depends on the square of the resonant frequency. This dependence has been studied experimentally using devices resonating at 31, 52, and 112 MHz that were coated with organic films of precisely controlled composition and thickness. The mass sensitivity of SAW oscillators can be exploited to make very sen- sitive chemical sensors if an appropriate sorptive coating is applied to the device. Results obtained from a delay line oscillator operating at 290 MHz (the highest frequency SAW chemical sensor reported to date) suggest that the rapid detection of organic vapors at concentra- tions substantially below 100 parts per billion (by volume) is readily achievable.

Revised linear solvation energy relationship coefficients for the 77-phase McReynolds data set based on an updated set of solute descriptors

Linear solvation energy relationship (LSER) coefficients for the 77-phase McReynolds data set have been recalculated using updated solute descriptors in the revised solvation equation: [equation: see text] These revised LSER coefficients are presented and classified by cluster analysis into groupings of stationary phases which have comparable solubility properties. It was found that the groupings were similar to those proposed by Abraham using the original solvation equation and that any dissimilarities were readily explainable by the grouping methods that were applied. Comparison of the original coefficients with the revised set also shows that several stationary phases which had a statistically insignificant b1 value with the original equation now have significant b1 values when utilizing the revised solvation equation.

Use of SAW devices to monitor viscoelastic properties of materials

Preliminary data are presented which demonstrate the potential sensitivity of SAW (surface acoustic wave) to the elastic properties of polymer coatings. 158-MHz dual SAW devices were used to demonstrate the sensitivity of the SAW to changes in coating elastic properties, and to identify the T/sub g/ and/or T/sub m/ of several polymer coatings. Data for PCAP (poly(caprolactone)) and FPOL (fluoropolyol) indicate that the SAW frequency is sensitive to changes in the shear modulus of the coatings. The steady decrease in frequency with increased temperature observed for FPOL-coated devices may be the result of increased surface coverage due to expansion of the film. Increased surface coverage would result in an increase in surface wave interaction with the film and a corresponding steady decrease in frequency. This indicates that these polymer films may not be true lossless films. It is suggested that in light of this, previous assumptions made in the development of predictive equations using SAW data must be reevaluated. It is concluded that the sensitivity of SAWs to elastic properties is indicative of their potential as monitors for cure processes, or other applications where changes in the elasticity of materials are of interest.<<ETX>>

Detection of Cyclic Volatile Organic Compounds Using Single-Step Anodized Nanoporous Alumina Sensors

Conventional methods of fabricating anodic aluminum oxide (AAO) materials have utilized a multistep anodization process to manufacture sensor substrates and templates for nanostructured materials. The multistep anodization produces structured, highly ordered hexagonal nanopores. In this paper, it is demonstrated that a single-step anodization process employed in the manufacturing of moisture sensors produces a nanoporous AAO material suitable for the detection and discrimination of low molecular weight volatile organic compounds. Electrical impedance methods have been employed to analyze the electrical response of the single-step anodized AAO materials in the presence of cyclic organic vapors. The sensor exhibits an impedance response that discriminates cyclohexane, cyclohexene, benzene, toluene, and the three isomers of xylene. The impedance measurements have a direct correlation with the relative permittivity, polarizability, and dipole moments of the organic analytes.

Calculation of Abraham solute descriptors from McReynolds gas chromatographic retention data.

Quantitative descriptors of solubility properties are useful in the investigation of a wide variety of chemical and biological phenomena. Several solutes which may be useful in such studies are not suitable because these values have not been previously determined experimentally. Several solute descriptors used in the linear solvation energy relationship developed by Abraham and co-workers have been calculated either algebraically or by multiple linear regression analysis. Values for those descriptors which have been calculated are reported and the methods of calculation of these descriptors are also discussed. It is shown that both methods of determination of missing solute descriptor values agree statistically with each other and that the values reported for the calculated descriptors correlate well with data previously reported for similar homologs.

Characterization of olefinic gas chromatographic stationary phases by linear solvation energy relationships

Abstract Three olefinic gas chromatographic stationary phases were characterized using a linear solvation energy relationship (LSER) of the type log Vg=c+r1R2+s1πH2+a1ΣαH2+b1ΣβH2+l1log L16, and the results were compared with the LSER coefficients for other, previously characterized, olefinic stationary phases. Based on the coefficients obtained, the presence of olefinic functional groups tends to contribute to the hydrogen bond acceptor basicity, molar refractivity, and dipolarity–polarizability interactions (a1, r1, and s1 values, respectively, in the above equation). It is also shown that the presence of the electron withdrawing chlorine atom in polychloroprene contributes to its ability to act as a hydrogen bond donor acid. The results are viewed from the perspective of using quantitative structure–solubility relationships in the determination of LSER coefficients.

2D bitmapping approach for identification and quantitation of common base flavor adulterants using surface acoustic wave arrays and artificial neural network data analysis.

Arrays of polymer-coated surface acoustic wave microsensors are used in conjunction with a variety of signal-processing algorithms known as artificial neural networks (ANN). This format of data analysis has the capability to characterize complex mixtures of volatile and semi-volatile organic compounds commonly found in base flavors. The approach described, which minimizes the number of training sets while retaining the robustness of an ANN, utilizes a 2D bitmap matrix. The matrix is obtained by converting the time domain kinetics of sensor response into a bitmap. The high data throughput of this approach enables quantitation on the order of ppm of common base flavor adulterants.

Characterization of phosphorus-containing gas chromatographic stationary phases by linear solvation energy relationships

Linear solvation energy relationships allow the prediction of a variety of solubility interactions based on a set of descriptors found in the following equation: [equation: see text]. SP refers to an intrinsic thermodynamic property that can be found experimentally for a series of solutes. Phases containing phosphate, phosphite and phosphine functional groups were studied in this work. Coefficients obtained during this work, as well as those available for previously characterized phases, were correlated with molecular structural descriptors. When effects of non-phosphorus functional groups are estimated and subtracted out, hydrogen bond acceptor capability, a1, shows a positive trend when correlated with percent functional group. Correlation of the dipolarity/polarizability coefficient, s, with calculated atomic polarizability shows stationary phases group according to like functional groups. A similar correlation with dipole moment gives a trend of increasing dipole as s1 increases. Further quantitative structure-solubility relationship work is planned to better describe the contributions of inner shell and valence electrons to the chemical and physical properties of these compounds.

Enhanced hydrogen sensing employing electrodeposited palladium nanowires enclosed in anodized aluminum oxide nanopores

Palladium nanowire sensors have been limited to detecting small concentrations of molecular hydrogen (H2). Upon excessive hydrogen uptake, tunneling currents fuse the nanowires creating a short circuit causing permanent failure. Here we demonstrate that electrodeposited palladium nanowires enclosed within single-step anodized aluminum oxide nanopores reliably detect hydrogen concentrations greater than four percent and do not suffer the mechanical and electrical failures of conventional self-supporting nanowires. Multiple cycling of molecular hydrogen at 100% concentration and long-term exposure to high concentration of hydrogen does not contribute to permanent short-circuit failure of the sensor.

Detection and discrimination of alcohol vapours using single-step anodised nanoporous alumina sensors

The electrical response of single-step anodised alumina sensors to alcohol vapours containing one to four carbon atoms has been studied. In this investigation, the real and imaginary impedance components have been measured in response to the equilibrium saturation vapour pressure of methanol, ethanol, butanol and two structural isomers of propanol. The response to each type of alcohol produces characteristic impedance spectra that allow discrimination and possible recognition of each alcohol.