Browsing by Author "Mason, John G."

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    Analysis of wood pulp extracts utilizing gas chromatography-mass spectroscopy
    Sequeira, Anna J. (Virginia Tech, 1991)
    Wood pulp mill effluents continue to attract much attention due to environmental consequences. However, in comparison, very little work has been published on wood pulp extracts themselves. In this investigation, chemithermomechanical (CTMP) pulps as well as Kraft (BKP) pulps were Soxhlet extracted with solvents of different polarity. These two types of pulp extracts were then compared qualitatively using GC-FID and GC-MSD as well as quantitatively based on the percent of extractives obtained. For all the pulps studied, the percent extractives of water > ethyl acetate > cyclohexane. The CTMP extracts exhibited many more components as compared to BKP extracts for all the extractions solvents. The presence of trace chlorinated phenolics in the above wood pulp extracts was also addressed utilizing GC-ECD, GC-EIMS and GC-NCIMS. 4-MCG, 4,5-DCG, 4,5,6-TCG, 3,4,5-TCG, 2,4,6- TCP, 2,3,4,6-TeCP, PCP and 6-MCVN were discovered. Due to a lack of knowledge of the complete history of the wood pulps studied, the exact causes for their discoveries are unknown. Attempts were also made to study the feasibility of Supercritical Fluid Extraction of the above mentioned wood pulps due to the difficulties faced with Soxhlet extractions. The percent extractives obtained using SF-CO₂ and cyclohexane were found to be comparable.
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    The application of planar optical waveguides to absorption spectrometry in flow injection analysis
    Choquette, Steven Joseph (Virginia Polytechnic Institute and State University, 1988)
    Attenuated total reflection techniques have been used extensively as analytical tools for the analysis of thin films and analytes imbedded in complex scattering matrices. However they have not been commonly utilized as detectors in common analytical techniques such as Flow Injection Analysis because of their relatively low sensitivity. The feasibility of using a thin film planar waveguide as an absorption sensor in the Flow Injection Analysis of Urea was investigated. Urea was hydrolyzed to ammonia and carbon dioxide with the enzyme Urease. The ammonia produced was quantitated colorimetrically using Berthelot’s reaction. The reaction product, indophenol blue, was detected using the combination planar waveguide 9.2 microliter flow cell sensor. The planar waveguides used had 2 to 3 orders of magnitude greater sensitivity than typical internal reflection elements. The analytical working range obtained for urea determinations was from 0 to 20 mM urea at a rate of 30 samples per hour. A description of the investigation and the various factors involved in designing and optimizing a planar waveguide for absorption spectrometry is included.
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    The application of thin film planar waveguides as a refractive index detector for microscale high performance liquid chromatography
    Kang, Lee (Virginia Tech, 1991-01-15)
    Thin film planar waveguides were originally introduced in microwave engineering. The spectroscopist began to use such waveguides as tools to solve chemical characterization problems since Harrick and Fahrenfort introduced the Attenuated Total'Reflection (ATR) in the early 1960's. Today, planar waveguides are playing an important and ever-increasing role in modem chemistry. In this thesis, a novel design for a refractometer involving the application of a thin film planar waveguide and coupling prism was demonstrated. This device shows the feasibility of refractive index measurements in a flowing stream. Therefore, an online detector for High Performance Liquid Chromatography (HPLC) was chosen as the vehicle to test out the concepts. The research works were devoted to studies of waveguide properties and flow dynamics in a chromatographic situation. It was found that microscale detection is possible. The sensitivity can be enhanced by using the highest propagation mode as the probe, and by selecting a proper refractive index liquid as the solvent carrier. A description of the investigation and the various factors involved in designing and optimizing a planar waveguide for refractive index detection is included. The results provide guidelines for the device as a realistic analytical detector.
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    Applications of supercritical fluids to the extraction and analysis of oligomers and polymer additives
    Via, James C. (Virginia Tech, 1993)
    Supercritical fluids (SF)s have several physicochemical properties that can often make them superior to conventional liquid solvents. These characteristics include relatively high densities, low viscosities, zero surface tension and high diffusivities. This unique combination of properties allows them to have solvating strengths that can approach those of pure liquids while maintaining many of the advantageous transport qualities of gases. In the past decade SFs have seen increased use as solvents for both extraction (SFE) and chromatography (SFC). A particularly exciting area of applications has been in the field of polymeric materials. Since polymers are not discrete molecules, but, rather broad distributions of very similar compounds (oligomers), they can pose interesting challenges for the separation chemist. SFs are uniquely suited to meet these challenges. The goal of the work done in this laboratory over the past few years has been to use SFs to extract and characterize both oligomers and additives from polymeric materials. A method for the post-polymerization fractionation of a low molecular weight, high density polyethylene wax using analytical scale SFE equipment was developed. Supercritical CO₂ was used to separate very narrow molecular weight distributions (MWD)s from the polyethylene feedstock. The resulting MWDs were characterized by SFC and high temperature gel permeation chromatography (GPC) and found to have polydispersities and molecular weights much lower than the parent wax. Supercritical propane was used to fractionate higher MWDs from the feedstock, however its greater solvating strength for the polyethylene oligomers resulted in larger polydispersities. A dual pump SFE system was used to deliver a dynamic mixture of propane in CO₂. The resulting fractions were very similar to those achieved by pure CO₂, but the recoveries were much higher. Increasing the temperature appeared to have both a kinetic and thermodynamic contribution to oligomer extraction. Supercritical CO₂ was also used to extract additives from an insoluble polymeric nitrocellulose (NC). The primary stabilizer additive (diphenylamine) and its nitrated derivatives were extracted from the propellant. The SF extracted stabilizer profile was characterized using liquid chromatography (LC), gas chromatography (GC) and SFC. SFC was shown to provide separations of propellant additives that were superior to the existing LC method while maintaining lower temperatures than GC. Extracts from propellants stored at elevated surveillance temperatures contained more highly nitrated stabilizer derivatives. However, some question was raised as to the actual validity of elevated temperature programs for propellant screening due to potential differences in reaction mechanisms. A LC-thermospray mass spectrometry (MS) interface was modified for use wtih packed column SFC. The system was used as a chemical ionization (CI) source for the high CO₂ flow rates emanating from packed columns. Methane was used as a reagent gas (RG) for positive chemical ionization (PCI). The resulting CO₂ + CH₄ mixed RG was studied at CO₂ pump pressures typical of SFC pressure programming. The background ions varied widely with CO₂ partial pressure and source temperature, however, spectra of a propellant test mixture were relatively unaffected by changing RG. The system was also used to perform negative CI (NCI) using the mobile phase CO₂ as a RG. This method was found to be very useful for nitrated derivatives of diphenylamine. SFC-CH₄-PCI-MS confirmed the intact elution of thermally labile N-nitroso compounds thought to exist in propellants. SFC-CI-MS, both NCI and PCI, was used to characterize the SF extracts of polymeric nitrocellulose and was demonstrated to have potential for the analysis of a wide range of compounds found in the propellant industry. SFC-CH₄-PCI was also determined to be compatible with methanol modified mobile phases. The mobile phases in this case were delivered from premixed cylinders. However, severe limitations regarding the reliability of premixed mobile phases in SFC were shown to exist.
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    Assessment of coal liquefaction behavior through product characterization with hyphenated chromatographic/spectroscopic methods
    Hellgeth, John William (Virginia Polytechnic Institute and State University, 1986)
    The understanding of liquefaction behaviors, related to a coal's properties and a recycle solvent's composition, is essential for the development of an efficient direct liquefaction process. In this dissertation, a study of the liquefaction behaviors of an Eastern us bituminous and four Western US subbituminous coals is presented. The experimental approach has been to examine their behaviors under various reaction conditions with in-house microautoclave reactor and Kerr McGee pilot plant liquefaction runs. In-house runs involved surveys of coal types and process solvent compositions with variations in reaction times, temperatures and atmospheres. Runs performed at Kerr McGee examined the use of tetrahydroquinoline (TBQ) as a process solvent with a Wyoming coal. Liquefaction activities were assessed through determinations of coal conversion to both solvent-soluble products and distillate yields. Per the in-house liquefaction studies, a novel microautoclave reactor design and product recovery methods were developed, evaluated and employed. The reaction chemistries of !n !!S!! metal species and basic nitrogen heterocycles were investigated specifically. Changes in trace element concentrations were ascertained by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) and Size Exclusion Chromatography/ICP-AES (SEC/ICP-AES). Pates of basic nitrogen components in distillate and solvent-soluble residuum products were examined by nitrogen mass balance determinations, Gas Chromatography/Pourier Transform Infrared Spectrometry (GC/PTIR) and Gas Chromatography/Mass Spectrometry (GC/MS). Conversions to soluble products demonstrated the expected dependencies of liquefaction on coal rank, elemental composition and petrography. The western subbituminous coals showed extreme sensitivity to drying and solvent-soaking pretreatments. Metal content analyses revealed that metals exist as complexed species in the liquefaction process. Higher conversions to toluene-soluble materials were obtained with THQ in contrast to other H-donor solvents. Adduction of THQ was significant in the non-distillate product stream, however. The direct coupling of Reversed Phase HPLC separations with PTIR (RP-HPLC/PTIR) detection through on-line, post-column extraction was developed. Though intended for application to coal-liquefied product (CLP) analysis, this system was evaluated rigorously for both chromatographic and spectral performance. Throughout this investigation, the overall utility of these hyphenated methods for CLP analysis was explored. These methods demonstrated exceptional performance in providing a wealth of qualitative and quantitative information in a rapid manner.
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    Birefringent single-arm fiber optic enthalpimeter for catalytic reaction monitoring
    Richmond, Eric William (Virginia Tech, 1990)
    Changes in heat content are almost universally associated with chemical reactions. Thermometry as an analytical tool has been extensively researched and developed. Finding solutions to problems involving thermal isolation, specificity, sensitivity, and cross-sensitivity remain as active areas of interest. Fiberoptic interferometers, which use phase phenomena associated with propagating light, are extremely sensitive to heat. This research has focused on a special "birefringent" optical fiber. Two channels of information are generated in this single-fiber interferometer which correspond to the heat evolved from a catalytic reaction isolated on the fiber surface. Because of the unique transduction mechanism associated with the "birefringent" optical fiber, this device is sensitive to heat and remarkably insensitive to pressure. Details of the characterization and development of the birefringent optical fiber into a useful analytical probe are presented.
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    A calorimetric study of the immersion of bituminous coal in liquids
    Hollenhead, James B. (Virginia Tech, 1988-02-05)
    The surface properties of coal greatly affect its use in industrial processes such as gasification, liquefaction and the use of coal slurries. In addition, such processes also involve reagent penetration into coal pores and the oxidative state of the coal surface. Reagent penetration into coal was examined by determining the heat and kinetics of immersion of Pocahontas No. 3 coal in water, methanol, a series of n-alkanes, and several cyclic and heterocyclic hydrocarbons. Results indicate that the heat of immersion is sensitive to the carbon chain length of the wetting liquid and its ability to hydrogen bond to coal. The time of immersion is insensitive to carbon chain length but is increased by the presence of nitrogen in a cyclic hydrocarbon. The oxidation of both unextracted and extracted coal at 3200C was followed by immersional measurements in water. The heat of immersion increased rapidly with oxidation time, leveling off after 3 hours of oxidation. Extraction with methanol or pyridine prior to oxidation and immersion lowered the time of immersion in water compared to the unextracted case. The oxidation of coal was also followed by x-ray photoelectron spectroscopy (XPS). The XPS oxygen/carbon ratios correlated linearly with the heats of immersion for the extracted, oxidized coals. Thus, XPS was shown to be a useful ancillary technique to heat of immersion for the study of coal surfaces.
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    Capillary supercritical fluid chromatography
    Hensley, Jack Lee (Virginia Polytechnic Institute and State University, 1986)
    The modification of a gas chromatograph for capillary supercritical fluid was demonstrated. The resulting instrument was used to characterize the performance of capillary columns manufactured specifically for SFC applications. The columns were tested for inertness, efficiency, and immobilization before and after extraction with the supercritical CO₂ solvent. Stable deactivation layers were obtained with polyethylene glycol pyrolysis of Carbowax 20M and Superox 20M. Crosslinking was evaluated with dicumyl peroxide (DCP) and azo-t-butane (ATB) as free radical initiators with OV-1701 as a stationary phase. ATB was found to yield more efficient crosslinking and had less effect on column polarity. Quantitative reproducibility of SFC was evaluated for alkanes, alcohols and acids on columns which were deactivated by polyethylene glycol pyrolysis or by mixed cyclic siloxanes. Both gave better reproducibility than an untreated column but the overall reproducibility of the polyglycol deactivation was better than the cyclic deactivation. Liquid CO₂ extraction was performed on a variety of spices and food products. This sample preparation proved useful in isolating CO₂ soluble materials from complex matrices which were insoluble in CO₂.
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    Characterization of cellulose esters via GPC/FT-IR
    Saunders, Charles William (Virginia Tech, 1990)
    The object of this research was the development of on-line methods for the determination of the degree of substitution of cellulose esters. The focus of the effort was on the application of a Fourier transform infrared spectrometer (FT-IR) as an in-line detector for quantitative gel permeation chromatography (GPC). GPC/FT-IR has been used in the analysis of polymeric materials in the past, but not for quantifying the substituent content of cellulosics. This work has identified the infrared asymmetric nitrate absorptions of cellulose nitrates observed in THF and acetonitrile solutions. Independent absorptions for the primary nitrate at C₆ and secondary nitrate at C₃ were observed at 1651 cm⁻¹ and 1639 cm⁻¹ respectively. In addition, a third absorption at 1667 cm⁻¹ was observed to be dependent upon the degree of nitration of the cellulose nitrate. This absorption was found to be due to steric hindrance between secondary nitrate groups on the C₂ and C₃ sites. The infrared absorptions of cellulose acetate butyrates in THF, acetonitrile, and methylene chloride were also characterized. Separate absorptions for the acetyl and butyryl C-O-C asymmetric stretch were observed at 1235 cm⁻¹ and 1176 cm⁻¹ respectively. In acetonitrile and methylene chloride the carbonyl asymmetric stretching mode frequency observed at ≈1753 cm⁻¹ was found to be dependent upon the extent of acetyl substitution. This work developed a method for the quantitative determination of the degree of nitration of cellulose nitrates as a function of the polymer molecular weight using the asymmetric nitrate stretch. Methods for the determination of the acetyl and butyryl substitution of cellulose acetate butyrates were developed as well. Examples of each method applied to real world samples were carried out.
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    Characterization of resins in alternative fuel mixtures
    Karam, Hani Shukri (Virginia Polytechnic Institute and State University, 1986)
    "Resins" is a class of compounds believed to play an important role in the conversion processes of coal and coal-related materials into oils. Methods currently used to isolate this fraction, generally lack reproducibility and yield impure and strongly overlapping fractions which do not reflect the actual group-type distribution in the liquid fuel. A separation method based on liquid column chromatography was developed, which divides liquid fuels into eight distinct and minimally overlapping chemical classes: five non-polar (saturated, mono-, di-, tri-, and polynuclear aromatics), one intermediate polar (resins) and two polar (asphaltenes and asphaltols) fractions. Chemical characterization of "resins fractions," derived from two alternative fuels (coal-derived liquid and sugarcane bagasse), was achieved by first subjecting them to acid-base-neutral separation, followed by analysis of each subfraction by GC/MS. Identification of the eluted components was carried out utilizing a library search system, by comparing retention times (indices) of 150 model compounds believed to exist in liquid fuels, on two fused silica capillary columns (Carbowax 20 M and SE-54), and by mass spectral interpretation. GC/MS results indicate that "resins" are mainly composed of weakly acidic (phenols, indanols, naphthols), mildly basic (benzoquinolines, chloroanilines, etc.), neutral-nitrogen (indoles and carbazoles), and oxygen (carbonyl) compounds, and are free of hydrocarbons.
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    A chemical sensor based on surface plasmon resonance on surface modified optical fibers
    Bender, William John Havercamp (Virginia Tech, 1993-02-12)
    A sensor is described which utilizes the phenomenon of surface plasmon resonance to detect changes in refractive index of chemical or biochemical samples applied to a surface modified optical fiber. The sensor is constructed by polishing a short section of the lateral surface of an optical fiber to its evanescent field surrounding the fiber core. One or more thin films are applied to the polished section of the fiber to produce the sensing element. One of the films is the metal silver, which acts as the support for the surface plasmon. Under the proper conditions, TM polarized energy propagating in the fiber can be coupled to a surface plasmon electromagnetic mode on the metal film. This coupling depends on the wavelength, the nature of the fiber, the refractive index and thickness of the thin films applied to the fiber, and the refractive index of a chemical sample in contact with the modified surface. The fiber to plasmon coupling is seen as a large attenuation of the light reaching the distal terminus of the fiber.
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    cis-Arenediols as versatile chiral synthons in the synthesis of prostaglandins, cyclitols, carbohydrates, and alkaloids
    Contla, Hector Luna (Virginia Tech, 1992-09-15)
    The oxidation of simple benzene derivatives by a mutant of Pseudomonas putida, called 39-D, produces cis-arenediols (1). The diols are enantiomerically pure and can be used as synthons for the preparation of a variety of interesting compounds because of their stereochemistry and the special array of functional groups. See: Figure 1 cis -Toluenedio] (2) served as a chiral intermediate in an efficient synthesis of enone (3). which has been used to attain prostaglandin Fra and Neplanocin A. The same diol (2) was transformed into both enantiomers of a terpene synthon (4). See: Figures 2, 3, 4 Oxidative functionalization of cis-chlorobenzenediol (5) afforded intermediates suitable for transformation into L-erythrose (6), conduritol C (7), dihydroconduritol C (8) and aminoconduntol F-4 (9). See: Figures 5, 6, 7, 8 The application of this versatile synthetic protocol culminated in an approach to kifunensine (10), an important glycosidase inhibitor, which was approached according to the following retrosynthetic analysis: See: Figures 10, 11, 12, 5, 14, 13 A detailed study of the nucleophilic opening of epoxide 13 was carried out in order to better understand the parameters of the diastereoselective functionalization of arenediols. Details are provided for the oxidative functionalization of chlorobenzenediol (5), the key compound in all of the projects discussed.
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    Deactivation and preparation of fused silica open tubular columns for gas and supercritical fluid chromatography
    Ogden, Michael Wayne (Virginia Polytechnic Institute and State University, 1985)
    The activity and wettability of raw fused silica capillary tubing was found to be widely variable which places severe limitations on the reproducibility of column deactivation and inertness. Hydrothermal treatment of the raw fused silica with nitric acid was proven to be very effective for cleaning and maximizing the degree of silanol coverage of the surface. The capillary rise method was used to obtain contact angle data for untreated fused silica and fused silica treated with a variety of deactivating reagents. This contact angle data was used in the construction of Zisman plots which allowed quantitative comparison of the wettability and degree of surface coverage obtained with the different deactivants. The thermal stability of the final column was related to the success of the deactivation procedure. The choice of cross-linking initiator was also found to have an affect on column inertness. In the synthesis of intermediate polarity polysiloxane stationary phases, mixtures of commercially available cyclic siloxanes were shown to be a viable alternative to the use of dichlorosilanes as starting material. The main advantages were the simplification of the synthesis procedure, simpler and better molecular weight control of the polymer, and the elimination of HC1 as a by-product of both the polymerization and endcapping steps. A new stationary phase, 7% cyanoethyl, 7% phenyl, 1% vinyl, methyl polysiloxane was synthesized and found to be more polar than OV-1701 with higher temperature stability, easily cross-linked, and suitable for use in supercritical fluid chromatography.
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    The design and application of an automated luminometer for chemiluminescence
    Chapple, Ian (Virginia Polytechnic Institute and State University, 1984)
    An automated luminometer was designed and constructed to facilitate the investigation of reaction conditions and quantitation of the chemiluminescence observed during the oxidation of NADH by horseradish peroxidase in the presence of eosin. The luminometer design incorporates a computer controlled reagent induction system, pulse counting detector and real time display of luminescence profiles. Each of the major reaction conditions was studied individually to determine the most favorable conditions for the analytical application of this reaction. In addition, this chemiluminescent reaction was studied to determine if the luminescing species is recycled. If this were the case it would provide the first essential step necessary for the future design of a chemiluminescent probe that could be reusable or be used for continuous monitoring. During the course of this investigation it was observed that the luminescing species, eosin, was being bleached. A number of experiments were undertaken to determine the nature of the bleaching process. Those steps which suppressed the bleaching process, unfortunately, also suppressed the emission process. Despite the fact that this reaction does not appear to recycle the luminescing species, it does provide a chemiluminescent method for the determination of NADH over the range 5x10⁻⁶ M to 5x10⁻⁴ M. This chemiluminescent reaction has also been coupled to several enzyme systems, which reduce NAD⁺ to NADH, in order to obtain a chemiluminescent signal proportional to the substrate concentration.
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    Development and characterization of a hydrogen peroxide sensor using catalase immobilized on a pyroelectric poly(vinylidene flouride) film
    Arney, Lawrence Hinkle (Virginia Polytechnic Institute and State University, 1989)
    This dissertation describes the design, development and results of a simple, inexpensive, rugged, pyroelectric heat-of-reaction detector that can be made in many configurations. The measured heat of reaction results from the reaction of a substrate on an enzyme. The enzyme is immobilized in a flow channel with a pyroelectric polymer film, poly(vinylidene fluoride) or PVDF. The sample is introduced into the flow channel using flow injection analysis technology. The heat from the reaction causes the pyroelectric material to produce an electrical potential proportional to the change in temperature which, in turn, is proportional to the substrate concentration. This potential is amplified and recorded. A differential instrument amplifier produces a difference signal from a sample and reference PVDF film. This removes noise caused by stray electromagnetic radiation and piezoelectric pressure responses. A conventional Flow Injection Analysis unit was employed. The FIA flow rate was four ml/min and the time from injection to peak maximum was less than three seconds, with a return to baseline of less than thirty seconds. This gives a quick analysis time and a reasonable number of analyses per unit time. Data interpretation is straight forward, peak height is proportional to the concentration. A 70 μl sample gives a good response. Larger samples do not improve the signal. The system showed minimum detectable number of moles that is comparable to other methods, 7 x 10⁻⁸ moles. The detector showed good response for more than two orders of magnitude. The results show excellent correlation to the modeled system of heat trans+er through the PVDF sensor.
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    Development and characterization of a low power helium microwave induced plasma for spectrometric determinations of metals and nonmetals
    Perkins, Larry D. (Virginia Polytechnic Institute and State University, 1989)
    This dissertation centers on the development of a new helium microwave induced plasma. The analytical utility of this new plasma source is critically evaluated. To sustain the helium plasma a TM ₀₁₀ high efficiency microwave induced plasma, HEMIP, was used. The HEMIP is a modification of the original Beenakker cavity that precludes the use of external matching devices, such as the highly popular double tuning stub. The He-HEMIP was analytically characterized as an atomization source for metals and nonmetals with the use of atomic emission spectrometry (AES) and atomic fluorescence spectrometry (AFS). A torodial plasma was sustained in the cavity solely by the helium gas output of the nebulizer. Aqueous samples from a pneumatic glass nebulizer/Scott spray chamber were aspirated into the cavity without a desolvation apparatus. With AES, detection limits for metals and nonmetals were in the sub-ppm range. with AFS, detection limits for metals were determined to be in the low ppm to sub-ppb range and were found to be not statistically different from those reported for HCL-ICP-AFS. Linear ranges for AES and AFS ranged from four up to five and one-half orders of concentrative magnitude. The effect of sample uptake rate on the emission intensity was investigated. Ionization interferences were determined to be minimal and phosphate interferences were found not to occur. Development and characterization also included studies of the He-HEMIP's physical characteristics. Excitation and ionization temperatures were found to be approximately equal, suggesting that the He—HEMIP approaches local thermodynamic equilibrium. Evaluation of the He—HEMIP as a routine detector for sulfur during coal pyrolysis and coal extracted samples was investigated. Results showed that the He-HEMIP is selective and sensitive. Detection values compared favorably to those of certified coal samples.
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    Development and evaluation of an acylating agent detector using surface acoustic wave devices
    Wollenberg, Glen David (Virginia Tech, 1992)
    The monitoring of harmful ambient vapors is of major concern in the industrial environment. To this end, the development of systems which detect and respond in real time to specific vapors is a highly desirable goal. Surface Acoustic Wave (SAW) devices have been used for chemical analysis since 1978. While sensitive to mass changes occurring on their surfaces, they are not selective to the mass they will detect. Their use as chemical sensors requires the development of specificity for a vapor (or class of vapors) using selective chemical reagents suspended in film media that can have their permeability easily changed. This dissertation presents the development of an automated dosimeter for the detection of phosgene using SAW devices. By changing the film media from a very permeable material to a film exhibiting less permeability, the analytical range of the device using the same suspended selective chemical reagent is expanded to concentrations which the very permeable film is incapable of accurately measuring.
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    Development of a low-power molecular microwave plasma and its application as an atom source for atomic spectroscopy
    Lysakowski, Rich (Virginia Polytechnic Institute and State University, 1987)
    The major thrusts of this work have been: 1) To develop a high-efficiency low-power TM-010 microwave cavity for nitrogen support gas at atmospheric pressure, 2) To discover and physically characterize potential laser and emission spectroscopic applications of this atom source, with a particular emphasis on laser-induced fluorescence. The result is the most efficient microwave-induced plasma cavity for nitrogen at one atmosphere that exists to date, giving stable and analytically useful molecular plasmas with only 50 Watts applied power. It is called the “High-Efficiency Molecular Microwave Plasma" (HEMMP) cavity. The HEMMP possesses excellent vaporization and atomization properties. It can handle aqueous sample flows of around 1 mL/min, introduced as an aerosol from a nebulizer. A detection system and sampling system were designed and an analytical instrument was built around the HEMMP cavity. Details of construction, operating conditions and operation of the instrument are described. Applications investigated include laser-induced fluorescence (LIF), atomic emission spectroscopy (AES), and laser-enhanced ionization (LEI) [also known as the opto-galvanic effect (OGE)]. The major emphasis of the application work has been physical characterization of the low-power nitrogen plasma as an atom source for LIF. This is the first time that either laser-induced fluorescence or laser-enhanced ionization have been observed and extensively characterized in any microwave-induced plasma (MIP). This is also the first time that atomic emission has been studied in a low-power N₂-MIP. LIF, AES, and LEI signal intensities were studied as a function of applied microwave power, support gas flow rate, signal observation height, and support gas composition using nitrogen and argon mixtures. Results for LIF yielded detection limits in the very low parts per billion range, and for AES in the low parts per billion range. Limit of detection (LOD) and background noise studies were done for all 3 techniques. Signal intensities were measured as a function of laser light intensity for LIF and LEI. Laser saturation was not observed with 300 mW power from the CW dye laser. The effects of electrode geometry and applied electrode voltage on LEI signals were also studied. Extensive background spectral studies were done for the nitrogen plasma. Analytical feasibility has been demonstrated for AES, LIF, and LEI in the low-power nitrogen MIP. The results presented provide the background physical investigations required for a full-scale development of these techniques for chemical analysis.
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    The effect of denticity on the electrochemistry and oxygenation kinetics of polydentate Schiff base complexes of manganese
    Frederick, Fred Charles (Virginia Tech, 1983-12-15)
    Manganese(II) and (III) complexes of potentially bidentate and tridentate Schiff base ligands have been prepared. The ligands were prepared from substituted salicylaldehyde or pyridine-2-carboxaldehyde and amines with hydrocarbon or alkylpyridyl substituents. The electrochemistry and the oxygenation kinetics of these and similar tetradentate, pentadentate, and hexadentate complexes have been studied. The electrochemistry of the majority of the complexes involves the Mn(III)/Mn(II) couple. However, varying the solvent shows that electron transfer is often accompanied by slow changes in the number of solvent molecules coordinated to the metal or changes in the actual denticity of the ligand. Activation energies and entropies for the reactions with 0₂ show that a large number of parameters influence the rate of reaction. Primary among these is competition between 0₂, solvent molecules, and donor atoms from the ligands for coordination sites on the metal. However, the reactions were all (with one exception) found to be first order in both complex and 0₂, implying that the slow step is formation of a Mn(III)-superoxo complex. The exception was with complexes of the tetradentate Mn(SALCn) type, where a simple rate law could not be fitted. This was explained by either steric hindrance or polymerization of the complex due to the flexibility imparted by the long polymethylene chain in the tetradentate ligand.
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    Effect of heat treatment on dyeability, glass transition temperature, and tensile properties of polyacrylonitrile fibers (orlon 42)
    Sarmadi, Abdolmajid (Virginia Polytechnic Institute and State University, 1986)
    Deniers of treated and untreated fibers were determined and the „ results were used in calculations of tenacity and initial modulus. Tensile properties were measured on a constant—rate—of—extension machine. Shrinkage of treated and untreated fibers were measured after they were boiled in water for 15 min. The glass transition temperatures (Tg) were obtained by differential scanning calorimetry. The ratio of the intensities of the CN/CH stretching bands were found by infrared spectroscopy, using the KBr method