Browsing by Author "Long, Gary L."
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- Analysis of Alcohol and Alkylphenol Polyethers via Packed Column Supercritical Fluid ChromatographyHoffman, Brian Jeffrey (Virginia Tech, 2004-05-03)Alkylphenol ethoxylates (APEOs), alcohol ethoxylates (AEOs), and alcohol propoxylates (APOs) are non-ionic surfactants used in daily care products and detergents. They are formed as an oligomeric series with a varying distribution, which determines their commercial application. The goal of the research performed was the development of sample characterization methods for non-ionic surfactants utilizing supercritical fluid chromatography (SFC) under mild instrument operating conditions. The aryl group present in APEOs allowed ultraviolet (UV) detection, with an equal molar response for oligomers, allowing average molar oligomer values to be calculated. APEOs were separated by ethoxylate unit via SFC-UV as well as normal phase HPLC-UV employing packed columns. Stationary phase and column length were varied in the SFC setup to produce the most favorable separation conditions. Fractions from SFC runs of APEOs were collected and analyzed by flow injection analysis electrospray ionization mass spectrometry (FIA-ESI-MS) to identify fraction composition. SFC provided shorter retention times with similar resolution as HPLC for separation of APEOs and consumed a smaller amount of organic solvent. AEOs and APOs lack functionality capable of absorbing UV light outside the UV cut-off of normal organic solvents. SFC was able to separate AEOs and APOs derivatized as trimethylsilyl ethers (TMS) with pure CO2 with detection at 195 nm. The instrumental conditions, however, needed for separation necessitated high temperature and high CO2 pressure. Derivatization of alcohol polyether samples with an UV absorbing agent was achieved with phenylated disilazane-chlorosilane mixtures forming phenylsilylethers detected at 215 nm. Use of an organic solvent-modified CO2 mobile phase afforded lower pressure and temperature conditions for oligomer separation. The use of polar embedded alkyl phases combined with use of organic modified CO2 produced good resolution between oligomers. Better peak shape and shorter retention times were realized with methanol-modified CO2 than acetonitrile-modified CO2. Peak assignments were made via SFC coupled with ESI-MS detection in the positive ion mode. SFC-UV and SFC-ESI-MS data were jointly used for calculation of average molar oligomer values. Proton nuclear magnetic resonance (1H-NMR) analysis of non-derivatized samples was performed to determine average molar oligomer values and was used for comparison with values calculated from SFC-UV data.
- Analysis of polar compounds by supercritical fluid chromatographyShah, Swati H. (Virginia Polytechnic Institute and State University, 1989)The analysis of polar compounds has been studied by SFC using 100% CO₂ or methanol modified CO₂ as the mobile phase. Both microbore packed and capillary columns are employed to separate the mixtures of steroids and agricultural compounds of various chemical classes such as amides, sulfonamides and ureas. The highly deactivated and crosslinked stationary phases used for both packed and capillary columns afford the elution of polar analytes with 100% CO₂ which in turn makes the on-line FT-IR detection of these analytes feasible. The flow cell interface is employed which provides very low detection limits. Spectra with high signal to noise ratio are obtained for the analytes with real time data acquisition. Some polar and structurally similar triazine herbicides are separated using a gradient mobile phase and a rapid separation of all the components with complete resolution is achieved. The effect of flow rate, column outlet pressure and the temperature on resolution is also studied for these analytes. The performance of microbore packed and capillary columns is compared using polar and nonvolatile solutes and the retention and resolution offered by both the columns are also compared. Several van Deemter plots are generated at various constant operating densities and temperatures. Also, the effect of density and temperature on efficiency and resolution is studied for capillary column with split and splitless injection techniques. Several conclusions regarding the favorable operating conditions in each case can be drawn based on the results obtained here.
- Applications of modifiers in supercritical fluid extraction and chromatographMulcahey, Leah J. (Virginia Tech, 1991)The use of modifiers in supercritical fluid chromatography and extraction has become quite common due to the inability of pure carbon dioxide alone to solvate many of the compounds of interest. The effects of modifiers in supercritical fluid chromatography have been more thoroughly studied than the effects of modifier in supercritical fluid extraction. The effects of modifier on trapping efficiencies for off-line supercritical fluid extraction have been evaluated in this work. Sorbent and solid phase traps were investigated with pure carbon dioxide in order to determine the effect of stationary phase identity, pretreatment, and rinse solvent on the recoveries of a test mixture of compounds of varying vapor pressure and molecular weight. The solid phase traps, which were polyethylene frits, performed as well as the sorbent traps in most cases, and significantly better than the sorbent traps in many cases. The ability to cool these traps to -20°C allowed for efficient trapping of volatile compounds without the benefit of sorptive interactions. Sorbent and solid phase traps were then studied with the addition of 1%, 2%, 4%, and 8% methanol to the mobile phase. The sorbent trap explored consisted of 40 µm ODS packing material, while the solid phase trap consisted of 100 yum stainless steel beads. In this work trap temperatures ranged from 5-80°C. It was found that trap temperature, modifier concentration, and trap type influenced recoveries of the test mixture components. Applications of these solid phase and sorbent traps explored were the extraction of polychlorinated biphenyls from river sediment and the extraction of the active components from a drug formulation. The separation of some compounds of pharmaceutical interest was also explored, where the addition of modifier, and in some cases an additive, was required to elute compounds from the chromatographic column.
- Applications of supercritical fluids to the extraction and analysis of oligomers and polymer additivesVia, 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.
- Aspects of the microwave induced plasma as an element selective detector for supercritical fluid chromatography and supercritical fluid extractionDucatte, Gerald Roger (Virginia Tech, 1993-12-05)The introduction of supercritical CO₂ at packed column supercritical fluid chromatography (SFC) flow rates is shown to significantly affect the excitation characteristics of a helium microwave induced plasma (He MIP). In this work, the influence of CO₂ on specific atomic and ionic transitions of Cl, Br, I, P, and S is described. Also presented is the determined relationship between transition energy and degree of signal depression resulting from the introduction of CO₂ to the plasma. Attempts to enhance the emission signals of non-metals by introducing H₂ into a He MIP are discussed. The inadequacy of excitation temperature, ionization temperature, rotational temperature, and electron number density measurements to determine the effect of CO₂ on the excitation characteristics of a He MIP is also described. However, application of kinetic theory and a recently developed theory on charge transfer allows a reasonable series of mechanisms to be developed that describe the excitation processes of a He MIP to which supercritical CO₂ is added. The remainder of this work describes a direct interface between a supercritical fluid extraction (SFE) vessel and an Ar MIP for the purpose of element selective detection. The response of the plasma to the introduction of sample via SFE at a variety of extraction temperatures, pressures, and sample concentrations is presented.
- Birefringent single-arm fiber optic enthalpimeter for catalytic reaction monitoringRichmond, 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.
- Characterization and optimization of supercritical fluid chromatography with on-line Fourier transform infrared detectionJordan, Jeffrey W. (Virginia Tech, 1985-06-05)Supercritical fluid chromatography (SFC) has been gaining much exposure over recent years. The characteristics inherent to SFC give it capabilities in separating nonvolatile, thermally labile compounds inseparable by GC, and with greater efficiency per unit time than HPLC. As in the past with the more conventional chromatographic methods, the need for higher information detectors, such as IR, exists for SFC. The interface of a Fourier Transform Infrared (FTIR) spectrometer to a packed column (analytical scale) SFC is demonstrated. Characterization of mobile phases for SFC-FTIR is presented. Also, parameters for the optimization of spectral sensitivity, such as mobile phase transparency and flow cell design, are discussed. Demonstrations are made via model mixture separations. Finally, the application of this technique to the analysis of free fatty acids is presented. Identification of carboxylic acids of carbon number 6-18 is made on several standard mixtures and on free fatty acids extracted from various complex samples. The spectral identification of the compounds as free fatty acids is presented, and spectral evidence for dimerization of the acids in CO 2 and the coelution of saturated and unsaturated carboxylic acids is shown.
- Characterization of cellulose esters via GPC/FT-IRSaunders, 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.
- Characterization of Electrochemical Interfaces by INfrared SpectroscopyHuang, Jimin (Virginia Tech, 1996-08-30)The properties of electrochemical interfaces are studied using Fourier transform infrared spectroscopy. Potential difference infrared spectroscopy (PDIRS) was used in the investigation of carbon monoxide adsorbed on polycrystalline platinum electrodes. It is found that the infrared peak position of adsorbed carbon monoxide is linearly dependent on the applied electrode potential, and that the Stark tuning rate is a function of system temperature. The change in Stark tuning rate is the result of the variation of the interfacial dielectric constant with temperature. Self-assembled alkoxyalkanethiol monolayers were formed on gold substrates as surface modifiers of low dielectric constant designed to influence the interfacial capacitance. Polarization modulation infrared spectroscopy (PMIRS), ellipsometry, interfacial wetting, and cyclic voltammetry were conducted to characterize the modified interfaces. The interfacial capacitance is greatly reduced due to the adsorption of w-mercapto ethers on substrates. It was found that the solvation of the monolayer by solution is capable of improving the mass transport to maintain the Faradaic current while lowering the interfacial capacitance. The oxygen group in w-mercapto ethers at the monolayer-water interface interacts with water molecules to improve the monolayer solubility in water. The w-mercapto ether monolayers were found to be fluid-like in structure, giving better freedom to undergo structural change. The repulsion from the oxygen atoms in adjacent w-mercapto ether molecules adsorbed on the substrate introduces structural disorder to the alkyl chains in the monolayer, allowing better solvent permeation. This relieves some of the current blocking character of long chain alkanethiol monolayers. The interfacial contact angle to water for the w-mercapto ether monolayers is dependent on the oxygen position in the monolayer. 12-Methoxydodecanethiol has the lowest contact angle among all the w-mercapto ethers studied while 12-butoxydodecanethiol through 12-hexoxydodecanethiol have similar contact angles due to the ether oxygen being buried beneath several layers of methylene groups. The film thickness is roughly proportional to the total number of methylene groups in the two alkyl chains on w- mercapto ethers. w-Mercapto ethers that have a longer alkyl chain between the oxygen and thiol tend to form thicker monolayers on the substrates. In situ PMIRS measurements show that w-mercapto ether monolayers do not undergo structural change in the alkyl chains when in contact with either water or acetonitrile. The terminal methyl group, however, suffers from a shift in infrared peak position to lower frequency, and a decrease in peak height as the result of solvent load.
- Chelate Assisted, Pressurized, Liquid Extraction for the Removal of Adsorbed Metal Contaminants From SoilsMarshall, Karen L. (Virginia Tech, 2000-03-03)Chelate Assisted, Pressurized, Liquid Extraction (CAPLE) has been developed in our laboratory as an efficient, separation-based, extraction methodology for heavy metals in soils. Unlike current extraction methods used in environmental determination of contaminated soils, CAPLE is able to selectively remove adsorbed metals from the soil matrix without requiring the total destruction of the sample. By not fracturing the soil matrix particles, as with hot acid digestion methods, geologically bound metals are not liberated in the CAPLE process. This unique feature of CAPLE allows us to quantify levels of contaminant metals and correlate them to anthropological activity in the area. CAPLE requires the use of a modified supercritical fluid extractor for operation with water at sub-critical levels. The extraction of the sorbed metals is facilitated by the use of a chelating agent. Metal determinations are performed by atomic absorption (FAAS or GFAAS) or ICP emission spectrometry. CAPLE has been subjected to a variety of experimental conditions in order to elucidate the strengths and possible weaknesses of the extraction technique. The uses of the chelating agents (type and concentration) have been optimized. Possible release of metals from the resulting ionic strength of the chelating solutions have been shown not to be a factor. Both pressure and temperature effects have been studied and adjusted for optimal conditions. The majority of the research lies in the application of CAPLE to a variety of soil conditions. The effect on particle size of the soil and soil coating (humic acid and iron oxides) has been studied. In all soil systems and coatings studied, CAPLE could be optimized to completely remove chemisorbed metals. Tests of CAPLE on Cu-sludge amended soils provided excellent agreement with traditional methods of soil analysis. Not only was good agreement obtained between the recoveries of the methods, but CAPLE was also found to be much faster, more environmentally friendly, and much less prone to sample loss or sample contamination compared to traditional soil extraction methods. A final portion of this work involves a rigorous statistical analysis of CAPLE to a sequential extraction method. Since a Standard Reference Material (SRM) has not been provided for chemisorbed metals onto soils, a comparative analysis was chosen to validate the technique. Using the Cu-amended soils, CAPLE was found to effectively liberate all chemisorbed metals as compared to the sequential extraction technique. There was no statistical difference in recovery between the two extraction methods. CAPLE is shown in this work to be a viable extraction method for analyzing contaminant metals in soils. It is a rapid and efficient technique. Unlike traditional digestion methods, it is able to differentiate anthropological metals from geologically occurring metals. Its ease of use, coupled with simplicity of instrumental design and analytical reagents make it an attractive extraction technique for environmental analysis.
- Chemical demonstrations: a compendium of resources in print and on the InternetEddleton, Jeannine E. (Virginia Tech, 1996-11-21)This masters report pulls together the body of resources available to assist the lecture demonstrator. professor. and teacher of chemistry in incorporating the very powerful teaching tools of chemical demonstrations. The lecture demonstration lies somewhere in the continuum between laboratory exercise and magic trick; but it is not my intention here to debate the pedagogical implications of a chemical demonstration's place in that continuum. I assume simply that the reader is in a position to motivate and excite students about chemistry and will benefit from the material contained herein. The most recent chemical demonstrations materials in print are listed and annotated in this report. The most useful chemical demonstrations-related sites on the internet are also listed and reviewed. The combination of a committed teacher and the following resources cannot but improve both the teaching and learning of chemistry at all levels of education.
- A chemical sensor based on surface plasmon resonance on surface modified optical fibersBender, 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.
- Chiral Separations on HPLC Derivatized Polysaccharide CSPs: Temperature, Mobile Phase and Chiral Recognition Mechanism StudiesCabusas, Maria Elena Ybarbia III (Virginia Tech, 1998-04-03)Direct chiral separations of the non-steroidal drugs of 2-methylarylpropionic acids (profens) on the chiral stationary phases (CSPs) of amylose tris(3,5-dimethylphenyl-carbamate), Chiralpak AD, and cellulose tris(3,5-dimethylphenylcarbamate), Chiralcel OD, were investigated. Chiralpak AD and Chiralcel OD are CSPs coated on silica gel and have the same type of constituents. However, they have different higher order structures arising from their different arrangements of the glucose units, i.e., the former has an a-(1,4)-D-glucose linkage and the latter has a b-(1,4)-D-glucose linkage. The orders of optimum enantioselectivity of racemic acids were reversed on the two CSPs which demonstrated that the enantioseparating abilities of these CSPs are complementary. This phenomenon also confirmed that the chiral recognition abilities of both CSPs were dependent on their higher order structures. Mechanisms for retention and chiral recognition for the separation of racemic 2-methylarylpropionic acids on Chiralpak AD and Chiralcel OD were explored. In depth studies of the dependence of retention and enantioselectivity on temperature and mobile phase compositions were made. The thermodynamic parameters, the differences in free energy, enthalpy, and entropy of association between enantiomers and the CSP were evaluated. The results indicated that the retention of racemic acids on both CSPs is mainly dependent on the hydrogen bonding interaction between the acid proton of the carboxyl moiety of the analyte and the carbonyl oxygen of the carbamate moiety of the CSP. The chiral recognition mechanism for Chiralpak AD involves: (1) the formation of transient diastereomeric analyte-CSP complexes through hydrogen bonding interactions between the carboxyl and the carbamate moieties of the acid and CSP, respectively; (2) stabilization of these complexes by insertion of the aromatic portion of the analytes into the chiral cavities of the CSP, as well as pi-pi, dipole-dipole, and additional hydrogen bonding interactions between analyte and CSP; and (3) chiral discrimination between enantiomer analytes arising from the additional hydrogen bond between analyte and CSP. For Chiralcel OD, the chiral recognition mechanisms involve: (1) the formation of transient diastereomeric analyte-CSP complexes through hydrogen bonding interactions between the carboxyl and the carbamate moieties of the acid and CSP, respectively; (2) stabilization of these complexes by insertion of the aromatic portion of the analytes into the chiral cavities of the CSP, as well as pi-pi and dipole-dipole interactions between analyte and CSP; and (3) chiral discrimination due to: (a) the difference in the steric fit of enantiomers into the chiral cavity of the CSP (entropy controlled); and (b) dipole-dipole or p-p interactions between enantiomer analytes and CSP (enthalpy controlled). Chromatographic and quantitative thermodynamic data showed that there are at least two different chiral recognition mechanisms for Chiralcel OD. One mechanism was characterized by negative values for the enthalpy and entropy differences of the association between enantiomers and CSP that classifies the enantioseparation to be enthalpy controlled. This behavior was exhibited by racemic 2-methylarylpropionic acids with fused rings that were favorably separated at low temperatures. The other mechanism was associated with positive values for the enthalpy and entropy differences of the association between enantiomers and CSP, and the enantioseparation is said to be entropy controlled. The analytes with "free" phenyl moieties favored high temperatures for their enantioseparations. Both studies on the effects of temperature and mobile phase composition also indicated that the higher order structures of CSPs influence their chiral recognition abilities.
- A comparison of extraction techniques for the determination of metals in soilBurcham, Shannon Tomoe (Virginia Tech, 1995-08-05)A growing issue in groundwater studies concerns the distinction between anthropological and geological metal contaminants in soil. Current extraction methods, such as hot acid extraction, result in the digestion of the sample matrix which may obscure the origin of the contaminant. Since metals from anthropological sources are most often adsorbed to the surface of the soil sediment, the dissolution of the geological matrix is not necessary to release the contaminat metals. The resolution to this problem may lie in the use of chelating agents, where the adsorbed metals can be removed from the sediments without disrupting the geological matrix. This would allow the surface bound metal contaminants to be differentiated from the geological metals which are a natural part of the soil's composition. The basis of this study is a comparison of a hot acid extraction method vs. a chelation extraction method for the determination of metals in soils. Statistical comparisons on the precision and accuracy of the these methods on a reference soil, salt amended soil, and sewage sludge amended soil is discussed.
- Comparison of packed and wide-bore, open-tubular gas chromatographic columnsShagena, Elizabeth C. (Virginia Tech, 1987-05-15)Wide-bore, fused-silica capillary columns can be an excellent replacement for packed columns in many cases. They can provide higher surface inertness and thermal stability, as well as higher efficiency or speed of analysis. Wide-bore, open-tubular (WBOT) columns with thick films have higher sample capacities than conventional narrow-bore capillary columns and can be used in unmodified packed-column gas chromatographs. The majority of gas chromatographic analyses are still performed using packed columns. Many chromatographers associate open-tubular columns exclusively with split injection, often perceived as less repeatable or quantitative. Also, many packed-column methods use application-specific stationary phases which are not readily available on capillary columns. WBOT columns with standard phases can often be substituted due to higher available efficiencies. While several studies have been published on qualitative differences, there is little comparative quantitative data available on O.53â mm I.D. and packed columns.
- Coupled solid phase extraction-supercritical fluid extraction on-line gas chromatography of explosives from waterSlack, Gregory C. (Virginia Tech, 1992)A method has been developed for the quantitative extraction of nitrotoluenes (2,6-dinitrotoluene, 2,4-dinitrotoluene, and trinitrotoluene) from water. Three types of solid sorbents were investigated: two 47 mm Empore disks™ - octadecylsilane (C18) and styrene-divinylbenzene (SDVB); and one Bakerbond spe*™ Phenyl stationary phase. The phenyl sorbent yielded the highest recoveries. The average SPE recoveries for spike standards ranged from 80 to 95 percent for Millipore water and 55 to 95 percent from well and surface water in the low ppb and ppt levels. After the nitrotoluenes were trapped on the solid sorbents they were quantitatively eluted by first doping the bed with toluene and then extracting with supercritical carbon dioxide. Doping with toluene was found to increase the rate of extraction. The extracts were analyzed off-line via GC-ECD using an internal standard. Extraction losses are due to analyte break through, and not from poor SFE recoveries. This demonstrates that supercritical fluid extraction is a suitable elution technique for analytes trapped on solid phase extraction (SPE) cartridges. A method has also been developed and evaluated for the direct on-line coupling of SPE to GC. SPE-SFE-GC-ECD analysis eliminates off-line collection and subsequent handling of hazardous materials. SFE is an ideal means of directly coupling SPE to GC, since carbon dioxide is a gas at ambient temperatures and pressures and thus easily removed. One potential problem for SPE-SFE on-line GC is the presence of residual water trapped on the active sites of the bonded silica sorbent. The presence of water can interfere with the cryogenic trapping of the analytes on the capillary GC column. The water becomes ice at cryogenic temperatures and in large quantities blocks the GC column. This problem has been avoided by using a split injection interface previously described by Hawthorne. The quantitative reproducibility of this interface will be investigated for nanogram quantities of nitroaromatics.
- Design and Construction of a High Vacuum Surface Analysis Instrument to Study Chemistry at Nanoparticulate SurfacesJeffery, Brandon Reed (Virginia Tech, 2011-04-21)Metal oxide and metal oxide-supported metal nanoparticles can adsorb and decompose chemical warfare agents (CWAs) and their simulants. Nanoparticle activity depends on several factors including chemical composition, particle size, and support, resulting in a vast number of materials with potential applications in CWA decontamination. Current instrumentation in our laboratory used to investigate fundamental gas-surface interactions require extensive time and effort to achieve operating conditions. This thesis describes the design and construction of a high-throughput, high vacuum surface analysis instrument capable of studying interactions between CWA simulants and nanoparticulate surfaces. The new instrument is small, relatively inexpensive, and easy to use, allowing for expeditious investigations of fundamental interactions between gasses and nanoparticulate samples. The instrument maintains the sample under high vacuum (10?⁷-10?⁹ torr) and can reach operating pressures in less than one hour. Thermal control of the sample from 150-800 K enables sample cleaning and thermal desorption experiments. Infrared spectroscopic and mass spectrometric methods are used concurrently to study gas-surface interactions. Temperature programmed desorption is used to estimate binding strength of adsorbed species. Initial studies were conducted to assess the performance of the instrument and to investigate interactions between the CWA simulant dimethyl methylphosphonate (DMMP) and nanoparticulate silicon dioxide.
- Determination of Extractables from Cranberry Seeds Using Supercritical CO₂Bhagdeo, Mansi Pravin (Virginia Tech, 2004-06-28)An alternative method for extraction of therapeutically beneficial compounds such as sterols, fatty acids, and tocopherols from cranberry seeds with pure SF CO2 has been provided. The supercritical fluid extraction (SFE) operating conditions such as extraction temperature, pressure of CO2, extraction time, and CO2 flow rate were optimized to maximize the extraction yield. The amount and type of SF extractables (pure CO₂) have been compared with Soxhlet extractables (hexane) to evaluate the feasibility of SFE as an alternative extraction method. The extractables obtained via hexane and SF CO₂, which were derivatized and identified by gas chromatography mass spectrometry (GC-MS), contained mostly methylated fatty acids.
- Development and characterization of a hydrogen peroxide sensor using catalase immobilized on a pyroelectric poly(vinylidene flouride) filmArney, 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.
- Development and characterization of a low power helium microwave induced plasma for spectrometric determinations of metals and nonmetalsPerkins, 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.