Browsing by Subject "Nuclear magnetic resonance spectroscopy"
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Item Construction of a low temperature nuclear magnetic resonance force microscope(2006) Lee, Yong J.; Markert, John T.Item NMR relaxation in crude oils at elevated temperatures(Texas Tech University, 1999-08) Jacob, JamesUntil recently, most nuclear magnetic resonance (NMR) measurements in support of lab petrophysics and well logging were conducted at room temperature. However, the T2 relaxation of bulk crude oils strongly depends on temperature. Measurement of the crude oil relaxation response at reservoir temperatures is needed for optimal interpretation of log data. This work used a Can-Purcell-Meiboom-Gill (CPMG) pulse sequence to measure the NMR T2 relaxation at 2 MHz. Four stock tank crude oils were studied over the temperature range 30-100°C at a constant pressure of 300 psig. For comparison, the T2 temperature response of five viscosity standards was measured over the same temperature range and compared to the crude oil data. A two component exponential decay described all the T2 data. As expected from the temperature dependence of translational diffusion, there is not a universal curve that fits all hydrocarbon mixtures. The standards and the crude oils did not fit a single curve. The observed T2 varied directly with absolute temperature and inversely with the viscosity. For the crude oils, the log mean T2 was related to a viscosity value calculated from the stock tank oil viscosity conelation as developed by Beggs and Robinson and modified by Egbogah. This T2 relation did not agree with a widely accepted T2 conelation developed at room temperature from oil samples of differing viscosity (0.7 to 1000 cp). When the viscosity of one of the crude oils was measured as a function of temperature, that data then fit the room temperature T2 conelation. Viscosity values derived from correlations based only on API gravity are evidently insufficient to make close estimates of crude oil T2 relaxation at elevated temperatures. In the T2-viscosity plot, one of the crudes was a distinct outlier from the others. The composition of this sample suggests further investigations into the T2 relaxation of oil-based mud filtrate and mixtures of crude and oil-based mud filtrate.Item NMR structure and preliminary crystallographic studies of small protein B (SmpB) from Aquifex aeolicus(2002) Dong, Gang; Hoffman, David W., Ph. D.The tmRNA-SmpB system is a highly conserved quality control system in all prokaryotes. It has the dual function of rescuing ribosomes stalled on defective mRNA templates and tagging proteins resulting from problematic messages for degradation. Small protein B (SmpB) is an essential component of this system, lacking significant homology with any known proteins other than the same proteins from different species. The structure of the SmpB from Aquifex aeolicus was determined by multidimensional NMR techniques. It consists of an antiparallel b-barrel, with three helices packed outside the core of the barrel. While the overall structure of SmpB appears to be unique, the protein does contain an embedded oligonucleotide binding (OB) fold; in this respect SmpB has similarity to several other RNA-binding proteins that are known to be associated with translation, including initiation factor 1 (IF1), ribosomal protein S17 and the N-terminal domain of aspartyltRNA synthetase (DRS). Conserved amino acids on the protein surface that are likely candidates for direct interactions with the tmRNA and other components of the translational apparatus were identified. The presence of the two widely separated clusters of conserved surface amino acids suggests that SmpB could function either by stabilizing two distal regions of the tmRNA, or by facilitating an interaction between the tmRNA and another component of the translational apparatus. While the C-terminal ~20 amino acids appear to be unstructured, their presence may be essential for the function of SmpB in the trans-translation process. The structural model reported in this dissertation will be essential in ultimately determining the detailed mechanism by which the tmRNASmpB system performs its functions. Results of preliminary NMR perturbation studies on the complexes between SmpB and several RNA molecules are described. In addition, single crystals of the core fragment of the SmpB were obtained by the vapor diffusion method in sitting-drops at room temperature. Preliminary crystallographic analysis reveals that the crystal belongs to a tetragonal lattice, with unit cell parameters a = b = 55.0 Å, c = 65.9 Å, a = b = g = 90°. Further structural determinations by molecular rep.Item Nuclear magnetic resonance force microscopy of ammonium dihydrogen phosphate and magnetism of cobalt nanocrystals(2005) Mirsaidov, Utkur; Markert, John T.Item Nuclear magnetic resonance force microscopy: adiabaticity, external field effects, and demonstration of magnet-on-oscillator detection with sub-micron resolution(2003) Miller, Casey William; Markert, John T.Item Spectroscopic evaluation of Tb(III) chelates for potential utility as diagnostic markers(Texas Tech University, 1998-05) Hubbard, Darren ScottLanthanide spectroscopy was exhaustively studied in the early 19th Century. It was quickly learned that the fluorescence of the molecules was superb; however, the absorption cross section of the molecule was low and interest in the molecules subsided. The interest in Lanthanides resurfaced in the 1980's when chelation of a lanthanide central ion into an organic cage solved the absorption problem. The most striking features of the Lanthanide chelates were the enormous Stoke's Shift and long luminescence lifetimes. These two features enabled the investigator to sensitively probe for the solute because the signal was spectrally and temporally shifted from background noise.Item Strongly correlated systems: magnetic measurements of magnesium diboride and group IV magnetic semiconductor alloys(2007-12) Guchhait, Samaresh, 1976-; Markert, John T.Nuclear Magnetic Resonance Force Microscopy (NMRFM) is a unique quantum microscopy technique, which combines the three-dimensional imaging capabilities of magnetic resonance imaging (MRI) with the high sensitivity and resolution of atomic force microscopy (AFM). It has potential applications in many different fields. This novel scanning probe instrument holds potential for atomic-scale resolution. MgB2 is a classic example of two-band superconductor. However, the behavior of these two bands below the superconducting transition temperature is not well understood yet. Also, the anisotropic relaxation times of single crystal MgB2 have not been measured because it is not yet possible to grow large enough MgB2 single crystals for conventional NMR. Using our homemade NMRFM probe, we have set out to measure the relaxation times of micron size MgB2 single crystals to anix swer several questions relating to the anisotropy, multiband behavior, and coherence effects in this unusual superconductor. The goal of a second project is to study the effects of doping on the critical current of MgB2 superconducting wires. Ti-sheathed MgB2 wires doped with nanosize crystalline-SiC up to a concentration of 15 wt% SiC have been fabricated, and the effects of the SiC doping on the critical current density (Jc) and other superconducting properties studied. In contrast with the previously reported results, our measurements show that SiC doping decreases Jc over almost the whole field range from 0 to 7.3 tesla at all temperatures. Furthermore, it is found that the degradation of Jc becomes stronger at higher SiC doping levels. Our results indicate that these negative effects on Jc could be attributed to the absence of significant effective pinning centers (mainly Mg2Si) due to the high chemical stability of the crystalline-SiC particles. The principle goal of a third project, the study of magnetic semiconductors, is to investigate magnetic properties of Mn-implanted GeC thin films. 20 keV energy Mn ions were implanted in two samples: 1) bulk Ge (100) and 2) a 250 nm thick epitaxial GeC film, grown on a Si (100) wafer by UHV chemical vapor deposition using a mixture of germane (GeH4) and methylgermane (CH3GeH3) gases. A SQUID magnetometer study shows granular ferromagnetism in both samples. While the Curie temperature for both samples is about 180 K, the in-plane saturated magnetic moment per unit area for the first sample is about 2.2×10−5 emu/cm2 and that for the second sample is about 3.0 × 10−5 emu/cm2 . The external field necessary to saturate the magnetic moment is also larger for the second sample. These results show clear enhancement of magnetic properties of the Mn-implanted GeC thin film over the identically implanted Ge layer due to the presence of a small amount of non-magnetic element carbon.