No description available.

Thesis (Ph. D.)--Virginia Polytechnic Institute and State University, 1993. / Vita. Abstract. Includes bibliographical references (leaves 151-154). Also available via the Internet.

Supercritical fluids were introduced as mobile phases for chromatography and extractions in the early sixties. Over the past decade the technique has received increased attention, largely owing to the introduction of several commercial instruments and to the ease of adapting available commercial equipment for use in sfe and sfc. This dissertation examines the use of supercritical CO₂ as a mobile phase for chromatography and extraction. The first chapter explores the problems and provides background for the research. The physical properties of supercritical fluids are described and pertinent recent research is identified and summarized. The following chapter presents the design of three interrelated studies that investigate in-situ concentration, quantitative aspects of sfc, and quantitative aspects of sfe. The first of the three studies examined the exploitation of the innate properties of a supercritical fluid. Because the mechanism of elution with supercritical fluid mobile phases is based on solvation, it was possible in this study to concentrate analytes at the head of the supercritical zone. Concentration was followed by supercritical fluid chromatography. This allowed the analysis of sample components at part per billion levels. The use of supercritical fluids as mobile phases for chromatography and the quantitative nature of sfc with flame ionization detection was examined in the second study. Under numerous detector and injector configurations, a distinct decrease in response factors with an increase in carbon number was evident. Results showed that the decrease in response factors was related to an increase in CO₂ flow through the detector. The magnitude of the decrease was multivariably dependent. The third study dealt with the quantitative aspects of using a supercritical Oobile phase for the extraction of PNAs and pesticides from several matrices including contaminated soil. Results showed that sfe is a reliable, easy, and efficient (> 85% recovery) method of removing trace materials from contaminated soil. Implementation of a simple resistively heated collector was used to circumvent the problems of extracting damp matrices such as soil. The results and conclusions are presented in the final two chapters. / Ph. D.

Phospholipids are commonly found in biological membranes. They have a polar head group and two ester linked fatty acids tails. Different methods such as thin layer chromatography and high performance liquid chromatography coupled with ultraviolet, refractive index, flame ionization detector, and mass spectrometry (MS) detection have long been used in the study of phospholipids. These methods were time-consuming and lacked quantitative accuracy. In this work, phosphatidylcholine, phosphatidyl-ethanolamine, phosphatidylinositol and phosphatidylserine have been studied by supercritical fluid chromatography (SFC) coupled with evaporative light scattering detection (ELSD) and mass spectrometry (MS). Four different silica-based stationary phases were studied: 2-ethylpyridine, 4-ethylpyridine, diol and conventional cyanopropyl. The influence of different mobile phase additives on the elution of phospholipids has been studied. The results have shown that isopropylamine is a better additive compared with ammonium acetate, tetrabutyl-ammonium acetate, and trifluoroacetic acid for the elution of phospholipids. All phospholipids have been eluted with baseline separation in less than 15 minutes although there is some partial overlap on the pyridine columns. The second goal for this work was fractionation of phospholipids from lecithin (a by-product from soybean) by using supercritical fluid extraction (SFE) with methanol-modified CO2. Neutral lipids were first removed from the crude sample using pure CO2. Partrial fractionation of PE and nearly pure fractionated PC were obtained by varying the modifier concentration in the extraction fluid at 460 atm and 40oC with silica gel inside the extraction vessel. A total of six components were isolated from crude soybean lecithin. / Master of Science

A low-temperature method, supercritical CO2 fluid (SCCF) technology, was applied for oxidation of metal Zn film on glass substrate at 60¢XC. In this study, Zn film was deposited by DC sputtering at room temperature and post-treated by SCCF, which is mixed with 0.15 vol % H2O. The scanning electron microscopy (SEM) images and transmission electron microscopic (TEM) indicate that high density ZnO Nanotubes were formed on the glass substrate. SCCF technology has shown successful oxidation the Zinc at low temperature for the first time.

Thesis (Ph. D.)--University of Missouri-Columbia, 2003. / Typescript. Vita. Includes bibliographical references (leaves 106-113). Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 2003. / Typescript. Vita. Includes bibliographical references (leaves 106-113). Also available on the Internet.

No description available.

The technique of extraction with supercritical carbon dioxide (SC-CO$ sb2$), which is now used routinely for the recovery of non-polar analytes from a variety of liquid and semipermeable solid media, was extended to the efficient extraction of metal ions. Measurements of the solubilities in SC-CO$ sb2$ corroborated the hypothesis that the non-polar character of a series of tetraalkylammoniun dialkyldithiocarbamates ion pairs was influenced (i) primarily by the chain length of the alkyl substituent(s) on the carbamate nitrogen and (ii) to a lesser extent, by the chain length of the alkyl substituent(s) on the ammonium counter ion. In operation, analyte metal(s) in aqueous medium was derivatized by in situ complexation with the dithiocarbamate complexing reagent and the product complex(es) was partitioned into the SC-CO$ sb2$ mobile phase then purged from the extractor by replacing the headspace with fresh solvent. A novel silica flame-in-tube interface was developed for the sensitive detection, by atomic absorption spectrometry (AAS), of As, Cd, Cu, Mn, Pb, Se, or Zn in SC-CO$ sb2$ extractor eluate. For different analyte elements, the limits of detection (LODs) ranged from sub-nanogram to low picogram if standard was flow injected into the mobile phase. These sensitivities permitted differences in the rates of mobilization of different forms of the analyte metal from various media to be explored as a technique for probing the interaction of the analyte metal with the matrix. A portion of the Zn burden in fresh bovine liver was mobilized rapidly in the absence of complexing agent and the remainder was solubilized more rapidly than the Zn in a freeze-dried reference material of this tissue. The nebulizer assembly of a conventional flame-AAS was modified to extend the range of metals amenable to on-line detection. Supplemental heating of the inlet air to 200$ sp circ$C resulted in an on line detector which provided approximately a ten-fold improvement over convention

No description available.