Polyhydroxylated steroids with insect molting hormone activity were discovered in plants over thirty years ago. The major endogenous molting hormone of insects is believed to be 20-hydroxyecdysone (20E) and interestingly, it is also the most commonly encountered phytoecdysteroid (PE) in plants. Ecdysteroids control developmental programs in both immature and adult insects however, the role of PEs in plants has not been demonstrated. PEs are hypothesized to function as either plant hormones or plant defenses against phytophagous insects. Many toxic secondary metabolites are concentrated in apical meristems where herbivory would result in the greatest reduction in plant fitness. Similarly, the highest concentrations of 20E in spinach were associated with the stems and vasculature while old leaves and roots displayed low levels. In plants, concentrations of toxic or deterrent metabolites are often rapidly induced following attack. In spinach roots, both mechanical damage and insect herbivory resulted in rapid increases in 20E concentrations. The plant wound signal, jasmonic acid was strongly implicated in signaling this response. Known plant hormones and chemical defenses are regulated differently. Pulse chase studies with [2-¹⁴C] mevalonic acid demonstrated that de novo root 20E biosynthesis occurred during the induction and, once synthesized, 20E was stable for over one month. This result is does not support the plant hormone hypothesis, as plant hormones typically undergo rapid conjugation or catabolism. The induction of root 20E concentrations occurred without similar changes in related membrane phytosterols. Simply, pathway specificity was demonstrated as increased 20E accumulation was not part of an overall increase in steroids. To empirically examine the hypothesis that PEs function as plant defenses against insects, a series of experiments were designed with the fungus gnat Bradysia impatiens. Results indicated that root herbivory by larvae induced 20E levels in roots, larval preference for diets containing induced 20E levels was reduced, larval survivorship on 20E containing diets was lower, and plants with induced root 20E levels were better protected from attack. Together, these results support the plant defense hypothesis at both the physiological and ecological level.

The work detailed in the dissertation has resulted in a photoacoustic gas detector chamber that has been proved to be applicable for measuring very low concentration gas traces in ambient air. Calculation tools were developed for photoacoustic cell design, namely (i) a method estimating the acoustic quality factor of a cavity even for open configurations; and (ii) a technique calculating the effectiveness of light-sound energy conversion. An open, windowless resonant photoacoustic chamber was designed, that has high acoustic quality factor and good noise suppression. In such a chamber neither the window material nor the contamination adsorbed on the window surface can contribute to the generation of unwanted coherent background signal. The most important factor limiting the applications of high quality factor resonant photoacoustic cells is the resonant frequency shift due to the possible temperature and gas density variations in the chamber. To compensate this drift, a unique electronic resonance tracking system was constructed. A calibration experiment applying a grating tuned CO₂ laser was performed. The achieved detection limits were 8 ppb for ethylene, 50 ppt for sulphur-hexafluoride, and 11 ppm for carbon-dioxide. The reliability of the system was determined by three repeated measurement campaigns over a month, and it was found outstanding. The photoacoustic detector system was also tested against a conventional analytical technique and good agreement was found with the colorimetric ammonia detection method recommended by the NIOH.

Of the halogens, the gold iodide complexes are the most stable in aqueous solutions. A series of experiments was performed to investigate the kinetics and mechanism of the leaching reaction between gold and iodide. Using a rotation disk technique, the most important kinetic parameters were measured. The reaction rate was found to be first order with respect toI⁻₃ and half order with respect to I⁻. A gold leaching rate of about 2.6 x 10⁻⁹ mol/cm²·sec for 10⁻² M I⁻ and 5 x 10⁻³ M I₂ was obtained. This value is close to that for typical cyanidation. The reaction rate appears to be controlled mainly by diffusion of reactants through the boundary layer of solution to the gold electrode under the conditions studied. The electrochemical study of gold in different halide solutions, with emphasis on iodide, was also carried out. The electrochemical techniques used in this investigation include cyclic voltammetry and linear sweep voltammetry. The results displayed the sequential oxidation for gold dissolution in iodide solution and confirmed that iodide has the strongest oxidation capability of dissolving gold of the halides.

The beam-foil source can be developed as an absolute intensity secondary standard light source. A spectrometer and optical system are calibrated using a standard tungsten filament (blackbody) lamp whose intensity calibration is traceable to the National Bureau of Standards. Knowing the number of photons emitted by the lamp, we determine a wavelength dependent system efficiency. By careful comparison of the intensity of spectral lines emitted from the beam-foil source to an appropriate portion of the continuous spectrum emitted from the standard blackbody lamp, the beam-foil source is calibrated over the same region of the spectrum as is the standard lamp. Branching ratios are then used to extend this calibration into the vacuum ultraviolet.

Phosphorus magnetic resonance spectroscopy (31P-MRS) permits the measurement of high-energy phosphates in the gastrocnemius muscle at rest and during recovery from exercise. This examination is a powerful non-invasive method to evaluate muscle mitochondrial function in vivo. Data analysis, however, can be both time-consuming and user-dependent. I designed a fully-automated system to quantify both resting and recovery spectra, followed by quantification of metabolite recovery kinetics. This included quantifying the recovery kinetics of adenosine diphosphate (ADP), an index of mitochondrial function which previously had not been properly characterized. I modeled the transition from ischemic-exercise to perfused-recovery as a step function, and fitted the ADP recovery with a second-order step-response function. Furthermore, I used nearest-neighbour methods to account for the effects of physical conditioning and metabolic work on the recovery kinetics. This fully-automated method of analyzing 31P-MRS data provides comprehensive results relevant to the diagnosis and monitoring of patients with metabolic myopathies.

The use of ion-selective field-effect transistors (ISFETs) as potentiometric microsensors was investigated. In the first stage, an instrument was designed and built to operate an array of ISFETs. A microcomputer was used for instrument control and acquisition of data. / The second phase of research focussed on the development of a pH sensitive radiotelemetric device that could eventually be used for the noninvasive monitoring of gastric pH. The first attempt used an ISFET as a variable resistor in a simple telemetry circuit. The drift in the pH dependent signal from this device was significant. The use of a differential sensor was studied as a possible way to minimize the effect of signal drift. This system measured the differential output of a pH ISFET and a pH insensitive ISFET. The pH insensitivity was due to an alkanethiol monolayer at the ISFET$ vert$solution interface. / It was shown that ISFETs are well suited for use as sensors in telemetry devices. The union of these previously independent research areas has been achieved.

The goal of this study was to design, characterize and test a flow-through extraction cell for rapid determination of toxic metals present in soil and sediment samples. / Based on a design elaborated by Jean Bouffard, a Teflon cell was first machined, but it appeared to be leaky and hard to disassemble without breaking the fritted glass discs. To overcome these difficulties, a poly ether ether ketone (PEEK) cell was machined and several parts were modified. Even though the shape and seal of the components seemed to be affected when heated, the cell was working at room temperature and proved promising for future work. / Finally, the EPA (Environmental Protection Agency) method 1311 was applied on some real samples, and the extracts were analyzed in order to get reference results that could eventually be compared to results given by extracts obtained with the flow-through cell.* / *This dissertation is a compound document (contains both a paper copy and a CD as part of the dissertation). The CD requires the following system requirements: Microsoft Office.

<p> Commercially available Ga⁺ focused ion beam (FIB) instruments with nanometer size probe allows for in situ materials removal (sputtering) and addition (deposition) on a wide range of material. These spatially precise processes have enabled a wide range of nanofacbrication operations (e.g. specimen preparation for analysis by scanning electron microscope, transmission electron microscope, and secondary ion mass spectrometer). While there exists an established knowledge of FIB methods for sample preparation of hard materials, but FIB methodology remain underdeveloped for soft materials such as biological and polymeric materials. </p><p> As FIB is increasingly utilized for specimen preparation of polymeric materials, it is becoming necessary to formulate an information base that will allow established FIB techniques to be generalized to this spectrum of materials. A thorough understanding of the fundamental ion-solid interactions that govern the milling process can be instrumental. Therefore, in an effort to make the existing procedures more universally applicable, the interrelationships between target material, variable processing parameters, and process efficiency of the milling phenomena are examined. The roles of beam current, distance (i.e. step size) between successive FIB beam dwell and the time it spent at each dwell point (i.e. pixel dwell time) are considered as applied to FIB nanomachining of four different thermoplastic polymers: 1. low density polyethylene (LDPE), 2. high density polyethylene (HDPE), 3. Polystyrene (PS), and 4. nylon 6 (PA6). Careful characterization of such relationships is used to explain observed phenomena and predict expected milling behaviors, thus allowing the FIB to be used more efficiently with reproducible results. Applications involving different types of polymer composite fiber are presented.</p>

<p> The development of better energy conversion and storage devices, such as fuel cells and batteries, is crucial for reduction of our global carbon footprint and improving the quality of the air we breathe. However, both of these technologies face important challenges. The development of lower cost and better electrode materials, which are more durable and allow more control over the electrochemical reactions occurring at the electrode/electrolyte interface, is perhaps most important for meeting these challenges. Hence, full characterization of the electrochemical processes that occur at the electrodes is vital for intelligent design of more energy efficient electrodes. </p><p> X-ray absorption spectroscopy (XAS) is a short-range order, element specific technique that can be utilized to probe the processes occurring at operating electrode surfaces, as well for studying the amorphous materials and nano-particles making up the electrodes. It has been increasingly used in recent years to study fuel cell catalysts through application of the &Delta;&mgr; XANES technique, in combination with the more traditional X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) techniques. The &Delta;&mgr; XANES data analysis technique, previously developed and applied to heterogeneous catalysts and fuel cell electrocatalysts by the GWU group, was extended in this work to provide for the first time space resolved adsorbate coverages on both electrodes of a direct methanol fuel cell. Even more importantly, the &Delta;&mgr; technique was applied for the first time to battery relevant materials, where bulk properties such as the oxidation state and local geometry of a cathode are followed.</p>

This thesis represents the development of the first laser spectroscopy based trace-gas sensors with sensor characteristics which simultaneously satisfy low cost, handheld footprint, low power, and long term autonomous operation while still providing part-per-billion detection sensitivity and negligible interference to enable trace gas sensor networks and wearable sensors. In order to realize these demanding criteria, this work describes the development of a complete laser spectroscopic sensor platform from the ground up to determine all of the tradeoffs inherent to photonic chemical sensing, and presents a sensor platform with a configuration to meet as many application requirements as possible. Specifically, complete photonic sensor integration and design optimization (e.g. digital signal processing, low power analog, digital control technology, high speed digital design, efficient programming, infrared laser technology, mechanical design) provides sensor characteristics which are significantly improved over the current sensor technology. These sensors can permit the portable deployment of trace gas sensors and enable applications previously unattainable with any other gas sensing method. A performance comparison of the various different types of sensors measured according to these new metrics of cost, size, power consumption in addition to standard metrics (such as sensitivity and specificity) will provide a complete description of advantages and disadvantages of each trace gas sensing technique. Performance characteristics of an open-access handheld sensor platform also provide the baseline for comparison in terms of all of these new criteria. This work will also detail the development path of each major sensor component to allow new technologies to update the original modules. This thesis also describes a scalable network of high sensitivity trace gas sensors, something which has not been achieved to-date. Additionally, issues such as variable-power consumption sensor management and gas sensor data harvesting and analysis will be addressed. Several new applications will be described which may be performed with the optimized sensors which were difficult to perform previously. Finally, this thesis will extrapolate future optimal sensor configurations based on current research in MEMS, photonics, networking, integration, and sensing and will conclude with a discussion of the impact of the various advances achieved in this work.