Brian G. Frederick

Detection and quantification of nitric oxide in human breath using a semiconducting oxide based chemiresistive microsensor

Abstract Nitric oxide (NO) is recognized as playing a critical role in an ever-increasing list of diseases. An important requirement for more extensive utilization of the potential diagnostic value of NO concentrations in human breath is the development of low-cost, reliable NO monitoring devices. This paper describes a promising approach to meet this requirement using a semiconducting metal oxide based chemiresistive sensor. We have shown that it is possible to monitor NO levels in human breath samples with a WO 3 based thin film chemiresistive sensor element. The sensor element is highly sensitive to nitrogen dioxide (NO 2 ). Monitoring of NO is achieved via oxidation of the NO component in breath samples by an oxidizing agent such as alumina supported potassium permanganate (KMnO 4 ). Human breath contains a large number of organic compounds that can interfere with the response of the sensor element as well as NO 2 . Molecular sieve filter materials such as silicalite are used to remove these interfering compounds from breath samples without affecting their NO concentrations. Verification of this monitoring scheme is demonstrated with data which correlates sensor response with NO concentrations in human breath samples, as determined by a chemiluminescence NO analyzer.

Defects and morphology of tungsten trioxide thin films

Abstract Tungsten trioxide is a wide band-gap n-type semiconductor that has been used as a sensing material in chemiresistive gas sensors. The microstructure and morphology are important characteristics that have a large influence on the sensitivity, selectivity, and stability of the sensor. We have produced tungsten trioxide thin films 15–600 nm thick by reactive r.f. magnetron sputtering onto r-cut sapphire substrates. The microstructure of the films was characterized by reflection high-energy electron diffraction (RHEED), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). Films with local epitaxy or randomly oriented textures were produced by controlling the substrate deposition temperature and by post-deposition annealing treatments. All films were found to be dense with low porosity. Grain boundaries were identified in the films with randomly oriented texture and these films were composed of either the monoclinic or orthorhombic crystallographic phase. No grain boundaries were found for the locally epitaxial films. These films were discontinuous during early growth, exhibited evidence of crystallographic shear planes, and had a cubic crystallographic phase.

Substrate dependence of adlayer optical response in reflectance anisotropy spectroscopy

Reflectance anisotropy spectroscopy (RAS) offers extreme surface sensitivity and can be used to study gas-solid and solid-liquid interfaces. Although the technique has already made a considerable contribution to in situ monitoring of the epitaxial growth of semiconductors, its impact has thus far been limited by difficulties in interpreting experimental results. In particular, RAS spectra are related to both Δe2′ and Δe2″, the real and imaginary parts of the surface dielectric anisotropy, with weighting factors determined by the complex dielectric function of the substrate. Here RAS calculations are performed for a variety of substrates assuming a model overlayer dielectric anisotropy. Three distinct regimes of behavior are observed: RAS spectra can resemble either Δe2′, Δe2″ or −Δe2″ (or some combination of the three) depending on the substrate and the photon energy. It is therefore crucial to properly account for these substrate effects if RAS is to be used to determine the azimuthal orientation of adso...

Adsorption of O, HCO2H and C6H5CO2H on Cu(110) studied using reflectance anisotropy. Chemical and structural influences on an optically active surface resonance

Real-time reflectance anisotropy (RA) transients have been recorded from a cu(110) surface during exposure to O2, O2–HCO2H and C6H5CO2H, using a simple HeNe laser-based system operating at 1.96 eV polarised parallel to first the [110] and then the [100] directions on the crystal surface. The marked changes in the RA response observed during the formation of the reconstructed overlayers is attributed to the quenching of an allowed optical transition of the surface, as identified using electron spectroscopy, by the process of adsorption and restructuring. However, the influence of new adsorbate-induced states on the RA response cannot be ruled out at present. The magnitude of the observed changes in RA are comparable for the formation of the p(2 × 1)O and (3 × 1)O/HCO2H structures and are tentatively assigned to a combination of the rapid quenching of an allowed surface transition and the possible modification of new adsorbate-induced states. For the formation of the c(8 × 2) benzoate overlayer the RA change is considerably larger, which is believed to be due to a more thorough quenching of the allowed surface transition than that which occurs for the other systems studied. For the formation of the initial stages of the (2 × 1)O overlayer, in particular, the RA response would be consistent with simple Langmuir adsorption kinetics providing that a model in which the RA response was directly proportional to coverage was applicable.

Femtomolar isothermal desorption using microhotplate sensors

The authors describe a technique that utilizes the fast heating rates (106K∕s) of a microhotplate sensor along with a calibrated thermal desorption system to determine the initial coverage and kinetic parameters using isothermal desorption on a millisecond time scale. Models for isothermal desorption including both pumping and desorption rate effects are presented for zero, first, and second order kinetics. Analysis of the first order model illustrates the domain of the desorption, pumping speed, and heating rate time constants that permit the desorption parameters to be estimated from the mass spectrometer signal. The technique is demonstrated using isothermal temperature programed desorption of benzoic acid from a single SnO2 covered microhotplate at surface temperatures ranging from 296to347K. The data indicate that desorption is best represented by first order kinetics. The first order preexponential factor and the desorption energy in the zero coverage limit are determined to be 1×1017s−1 and 97kJ∕mo...

Quantifying gas sensor and delivery system response time using GC/MS

Semiconducting metal oxide (SMO) chemiresistive sensors have applications for gas detection in chemical, biological, medical and environmental fields. We examine how properties of the sensor test system can introduce errors into measurements of sensitivity, reproducibility and response time of SMO sensors. We describe and characterize the performance of a typical gas delivery system using a gas chromatograph/quadruple mass spectrometer (GC/MS) and custom-designed gas sampling system. The accuracy of parameters extracted from the sensor response is assessed from a model that incorporates both the delivery system characteristics and surface reaction kinetics on SMO sensors. Our analysis shows that effects of the test system on response time are significant, particularly at lower concentrations and for less volatile compounds. Sensor to sensor reproducibility is critically dependent upon sensor RTD temperature calibration, while the variation in sensor response from pulse to pulse is controlled by the gas delivery system.

Complementary vibrational and reflectance anisotropy spectroscopic determination of molecular azimuthal orientation

Abstract Azimuthal orientations of 9-anthracene carboxylate (9-AC) on clean and p(2×1)O/Cu(110) surfaces were determined from a reflectance anisotropy spectroscopy (RAS) signal derived from an intramolecular electronic transition. The magnitude of the molecular signal on the p(2×1)O/Cu(110) surface is 4–5 times larger than on the clean surface. We present a complete vibrational assignment of adsorbed 9-AC based on Fourier transform infrared and on- and off-specular high resolution electron energy loss spectroscopy (HREELS) and ab initio calculations. Correlation of the off-specular HREELS on p(2×1)O/Cu(110) with the RAS results demonstrates that the magnitude of the RAS signal depends on the degree of azimuthal orientation.

Application of the interleaved comb chopper to time-of-flight electron spectrometry

Abstract We examined the use of the interleaved comb chopper for time-of-flight electron spectrometry. Both static and dynamic behaviors are simulated theoretically and measured experimentally, with very good agreement. The finite penetration of the field beyond the plane of the chopper leads to non-ideal chopper response, which is characterized in terms of an “energy corruption” effect and a lead or lag in the time at which the beam responds to the chopper potential.

Identification of dimethyl sulfide in dimethyl sulfoxide and implications for metal-thiolate disulfide exchange reactions

The concentration of dimethyl sulfide (DMS) in seven different samples of research grade dimethyl sulfoxide (DMSO), including one deuterated sample, was measured by GC-MS. The average concentration of DMS is 0.48 ± 0.14 mM (range: 0.44–0.55 mM) and ca. 0.35 mM in DMSO-d6. The presence of DMS in DMSO is potentially problematic for compounds that are susceptible to reaction with DMS and are present at μM–mM concentrations. Standard methods of purification of DMSO were unsuccessful in removing all traces of DMS.

Advantages of maximum likelihood methods for PRBS modulated TOF electron spectrometry

Abstract We consider the recovery of spectra from pseudo-random binary sequence (PRBS) modulated TOF-HREELS data. The effects of the Poisson noise distribution and the non-ideal behavior of an “interleaved comb” chopper are simulated. We show, for the first time, that maximum likelihood methods can be combined with PRBS modulation to achieve resolution enhancement, while properly accounting for the Poisson noise distribution and artifacts introduced by the chopper. Our results indicate that meV resolution, similar to that of modern high resolution electron spectrometers, can be achieved with a dramatic performance advantage over conventional, serial detection analyzers.