Browsing by Subject "High temperature"

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    Chromatographic selectivity and hyper-crosslinked liquid chromatography stationary phases.
    (2010-01) Zhang, Yu
    The development of new stationary phases have always been of great interest in HPLC and has become increasingly important in recent years mainly driven by rapidly evolving industrial need as well as the quest for higher throughput HPLC analyses for better resolution and higher sensitivity. In this thesis, we developed a family of silica based RPLC stationary phases based on a novel hyper-crosslinked (HC) platform, prepared through a multi-layer, two-dimensional, orthogonal polymerization reaction. The resulting stationary phases showed better stability, higher efficiency and novel selectivities, which are the three essentials properties of the stationary phase that users are looking for and column developers strive to achieve. We first studied the synthesis and full characterization of a novel mixed-mode reversed-phase/weak cation exchange (RP/WCE) phase by introducing a small amount of carboxylate functionality into a hydrophobic hyper-crosslinked (HC) platform. The phase thus prepared shows a mixed-mode retention mechanism, allowing for both neutral organic compounds and charged bases to be separated simultaneously on the same phase under the same conditions. More importantly, the inherent weak cation exchange groups allow simple mobile phases to be used thereby avoiding the mass spectrometric ionization suppression problems concomitant to the use of non-volatile additives such as strong amine modifiers (e.g. triethylamine) to elute basic solutes from the strong cation exchange phases or ion pairing reagents (e.g. trifluoroacetic acid, ClO4-) to retain these solutes on conventional ODS phases. We next studied the development of a highly hydrophilic HC-OH phase prepared by hydrolyzing residual benzyl chloride groups on the hydrophobic platform. This phase is potentially useful as a candidate for use as the first dimension phase of comprehensive two-dimensional LC where column stability and low retentivity are greatly desired. We also developed a novel graphical method, the phase selectivity triangle plots, for visualizing the effect of surface chemistry (e.g. C18 vs. Phenyl vs. Fluoro) on stationary phase selectivities. The use of the new plots assists the selection of appropriate stationary phases for method development in both isocratic and gradient elution.
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    Suitability of chondrules for studying the magnetic field of the early solar system: an examination of synthetically produced dusty olivine
    (2009-12) Hu, Yifan
    Chondritic meteorites are rare, yet incredibly valuable windows into the geophysical and geochemical environment of the early solar system. Dusty olivine grains containing exsolved nanometer scale iron nickel alloy inclusions are present in many chondritic meteorites and their remanent magnetization may give insight into the strength of the solar dynamo at the time of chondrule formation. Laboratory methods for determining the paleointensity of these rare materials must be optimized prior to conducting experiments on actual meteorite samples. To this end, we have used high temperature recrystallization techniques to produce synthetic dusty olivine samples with textures remarkably similar to those observed in chondritic meteorites. The olivine grains used in these annealing experiments are from the 13 kya Haleyjabunga picritic basalt flow in Iceland and have compositions of Fo90, which closely resembles the olivine composition observed in chondritic meteorites. Samples were annealed at 1350 ˚C, 1315 ˚C and 1425 ˚C either under vacuum in the presence of graphite or under controlled oxygen fugacity using pure CO gas. The laboratory produced magnetic mineral assemblages in 4 different types of samples as well as the starting material have been characterized using low and high-temperature magnetic measurements, hysteresis loops, FORC diagrams, and scanning electron microscopy. The room temperature remanence properties of these materials have been explored using stepwise IRM and ARM acquisition and alternating field demagnetization. These synthesis techniques allow us to produce a wide range of iron nickel grain sizes with correspondingly large variations in coercivity (between 0 and 500 mT). High-temperature measurements of saturation magnetization show that all the samples reach their Curie temperatures at ~760 ˚C, consistent with kamacite, a low Ni high Fe metal alloy. Multiple experiments have shown that care must be taken to rigorously control the atmosphere in which the samples are heated and cooled in order to avoid forming trace amounts of magnetite on the surface of the samples. Future research will explore the feasibility of using modified Thellier protocols or the Shaw method to determine the paleointensity of laboratory induced thermoremanent magnetizations.