Magnetization Decay Curves Research Articles

Study of the Variation of the Electronic Distribution and Motional Dynamics of Two Independent Molecules of an Asymmetric Unit of Atorvastatin Calcium by Solid-State NMR Measurements.

Significant changes in the spin-lattice time and chemical shift anisotropy (CSA) parameters are observed in two independent molecules of an asymmetric unit of atorvastatin calcium (ATC-I) (which is referred to as “a”- and “b”-type molecules by following Wang et al.). The longitudinal magnetization decay curve is fitted by two exponentials—one with longer relaxation time and another with shorter relaxation time for most of the carbon nuclei sites. The local correlation time also varies significantly. This is the experimental evidence of the coexistence of two different kinds of motional degrees of freedom within ATC-I molecule. The solubility and bioavailability of the drug molecule are enhanced due to the existence of two different kinds of dynamics. Hence, the macroscopic properties like solubility and bioavailability of a drug molecule are highly correlated with its microscopic properties. The motional degrees of freedom of “a”- and “b”-type molecules are also varied remarkably at certain carbon nuclei sites. This is the first time the change in the molecular dynamics of two independent molecules of an asymmetric unit of atorvastatin calcium is quantified using solid-state NMR methodology. These types of studies, in which the chemical shift anisotropy (CSA) parameters and spin-lattice relaxation time provide information about the change in electronic distribution and the spin dynamics at the various crystallographic location of the drug molecule, will enrich the field “NMR crystallography”. It will also help us to understand the electronic distribution around a nucleus and the nuclear spin dynamics at various parts of the molecule, which is essential to develop the strategies for the administration of the drug.

Prediction of the stability and compatibility of Colombian heavy crude oils by 1D low field nuclear magnetic resonance relaxometry and chemometric methods

Stability and compatibility can be described by the insolubility number (IN), the solubility blending number (SBN) and the ratio (R) between them, which has become a critical index in daily refineries operation. In this work, a new methodology for predicting the stability and compatibility of heavy crude oils by 1D low field nuclear magnetic resonance (LF-NMR) relaxometry has been developed. Various chemometric models were developed to predict the IN,SBN and R properties of Colombian heavy crude oils (°API from 6 to 27), by integrating intervals of either the 1D free induction transverse magnetization decay curves (T2 FID) obtained with a low-field (LF) nuclear magnetic resonance (NMR) spectrometer, or their T2 relaxation distributions obtained via the inverse Laplace transform (ILT). Correlations between the relaxometry curves and said properties were evaluated using the principal component regression (PCR) and the partial least squares regression (PLSR) methodologies. The obtained prediction models yielded coefficients of determination (R2) above 95% with as few as 6 components and values of cross validated root mean squared error of prediction (RMSEP) as low as 0.71 for the IN, 2.37 for the SBN and of 0.03 for the R property, respectively. The best prediction models were attained using PLSR and employing the FIDs as the predictors. The use of chemometric predictive models from LF-NMR measurements represent a low-cost and faster alternative to estimate critical properties of the independent crude oils to be blended, and thus, will help in predicting the stability and compatibility of crude oil blends, which in turn may help accelerate the decision-making process in daily refineries tasks.

Application of pulsed and high-frequency electron paramagnetic resonance techniques to study petroleum disperse systems

The spectral and relaxation characteristics of “free” organic radicals (FR) and vanadyl-porphyrin (VP) complexes in various petroleum disperse systems (PDS) like bitumen, petroleum, their high-molecular components and solutions were studied using stationary (conventional) and pulsed electron paramagnetic resonance (EPR) techniques in two frequency ranges (X- and W-bands, with the microwave radiation frequencies of about 9 GHz and 95 GHz, respectively). The features of the pulsed approaches (electron spin echo, modulation of the electron spin echo signal decay, electronic relaxation times) and high-frequency EPR for PDS investigations were examined. W-band EPR allows to resolve spectrally the lines from the different paramagnetic centers and more accurately determine their spectral characteristics. It is shown that the electron spin echo can be observed at room temperatures even at high magnetic fields of 3.4 T demonstrating the potential of application of pulsed EPR techniques for the low-cost oilfield measurements. Analysis of the VP transverse magnetization decay curve permits to identify electron-nuclear interactions with the 14N and 1H nuclei in situ while in the EPR spectra these hyperfine interactions usually cannot be detected. It is found from the W-band EPR measurements that FR lineshape cannot be fitted with isotropic parameters in contrast to the established X-band results. The observed effect of increasing the rates of electronic transverse relaxation in asphaltenes is described in the framework of a model of spectral diffusion between the fast- and slow-relaxing paramagnetic centers in supramolecular complexes of asphaltenes.

Thermal and dipolar interaction effect on the relaxation in a linear chain of magnetic nanoparticles

We perform computer simulations to study the relaxation in a one-dimensional chain of dipolar interacting magnetic nanoparticles (MNPs). Using the two-level approximation of the energy barrier, we perform kinetic Monte Carlo simulations to probe the relaxation mechanism as a function of dipolar interaction strength and temperature. The anisotropy axes of the MNPs are assumed to have random orientations. At high temperatures, the magnetization decay curve is exponential for weak dipolar interactions. It is found that dipolar interactions slow down the magnetic relaxation and increase the effective Néel relaxation time τN, which is affected by thermal fluctuations. In the weak dipolar limit, there is a perfect agreement between simulated and analytically evaluated values of τN for a wide range of temperatures. Microscopic analyses such as magnetic moments correlations and dynamic domain formation also suggest an increase in ferromagnetic coupling with an increase in dipolar interaction strength or decrease in thermal fluctuations. We believe that the concepts presented in this work are relevant in the context of applications such as data storage, digital data processing, and magnetic hyperthermia, in which the linear chain of MNPs are pervasive.

A time domain signal equation for multi-echo spin-echo sequences with arbitrary excitation and refocusing angle and phase.

Accurate T2 mapping using multi-echo spin-echo is usually impaired by non-ideal refocusing due to B1+ inhomogeneities and slice profile effects. Incomplete refocusing gives rise to stimulated echo and so called "T1-mixing" and consequently a non-exponential signal decay. Here we present a time domain formula that incorporates all relaxation and pulse parameters and enables accurate and realistic modelling of the magnetization decay curve. By pulse parameters here we specifically mean the actual refocusing angle and axis, and phase angle of both the excitation and refocusing pulse. The method used for derivation comprises the so called Generating functions approach with subsequent back-transformation to the time domain. The proposed approach was validated by simulations using realistic RF pulse shapes as well as by comparison to phantom measurements. Excellent agreement between simulations and measurements underpin the validity of the presented approach. Conclusively, we here present a complete time domain formula ready to use for accurate T2 mapping with multi-echo spin-echo sequences.

Deuteron Solid-State NMR Relaxation Measurements Reveal Two Distinct Conformational Exchange Processes in the Disordered N-Terminal Domain of Amyloid-β Fibrils.

We employed deuterium solid-state NMR techniques under static conditions to discern the details of the μs-ms timescale motions in the flexible N-terminal subdomain of Aβ1-40 amyloid fibrils, which spans residues 1-16. In particular, we utilized a rotating frame (R1ρ ) and the newly developed time domain quadrupolar Carr-Purcell-Meiboom-Gill (QCPMG) relaxation measurements at the selectively deuterated side chains of A2, H6, and G9. The two experiments are complementary in terms of probing somewhat different timescales of motions, governed by the tensor parameters and the sampling window of the magnetization decay curves. The results indicated two mobile "free" states of the N-terminal domain undergoing global diffusive motions, with isotropic diffusion coefficients of 0.7-1 ⋅ 108 and 0.3-3 ⋅ 106 ad2 s-1 . The free states are also involved in the conformational exchange with a single bound state, in which the diffusive motions are quenched, likely due to transient interactions with the structured hydrophobic core. The conformational exchange rate constants are 2-3 ⋅ 105 s-1 and 2-3 ⋅ 104 s-1 for the fast and slow diffusion free states, respectively.

Magnetic characterization of industrial dust from Gresik, East Java, Indonesia

Industrial dust has been known to be a potential health hazard and therefore need to be closely monitored. However, in heavily industrialized area such as Gresik in East Java, Indonesia identifying the source of particular dust or differentiating one type of dust to another might be complicated. In this study, industrial dust in the form of fly ash, cement dust and lime kiln dust from various industries in Gresik were analyzed for their magnetic properties. The measurements include magnetic susceptibility, magnetic susceptibility versus temperature as well as Anhysteretic Remanent Magnetization (ARM) decay curves. These samples were also subjected to Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD) and X-Ray Fluorescence (XRF). The results, fly ash has the highest Fe content and therefore is the most magnetic. The predominant magnetic mineral is spherule-shaped magnetite. As magnetic methods are more straightforward than other type analyses, they are very prospective to be used as alternative methods for industrial dust pollution.

Palaeomagnetism of Palaeozoic glacial sediments of Northern Ethiopia: a contribution towards African Permian palaeogeography

One hundred fourteen oriented palaeomagnetic core samples were collected from 13 palaeomagnetic sites on subhorizontal to tilted glacial sediments at five localities of Northern Ethiopia. Combined alternating field (AF) and stepwise thermal demagnetization techniques were successfully applied to resolve the complete directional spectrum. A viscous remagnetization (VRM) and one stable component of magnetization were identified in most of the specimens. The VRM is removed between a temperature range of 120–350 °C and AF of up to 30 mT. Further heating until ∼650 °C results in smooth decay of the natural remanent magnetization (NRM) intensity to about 50 per cent and the rest of the NRM is efficiently removed by heating to 690 °C, while only 30–50 per cent of NRM is removed by the maximum AF available suggesting haematite as remanence carrier. Results of the magnetization decay curve plots and rock magnetic analyses using the variable field translation balance indicated the presence of magnetite with minor goethite, pyrrhotite as well. The high stability component defining a straightline segment, starting 350 °C and/or 30 mT is mostly directed towards the origin and interpreted as the characteristic remanent magnetizations (ChRMs). The direction of magnetization is determined both by best-fitting line using the least-square technique of Kirschvink and remagnetization circles of Halls for few unresolved overlapping components. The site mean directions of the sediments from two sites are normal polarity and are close to present-day field directions at the sample site. The site mean directions from 11 sites, on the other hand, are reversed in polarity with better grouping in the tilt-corrected coordinate and pass the McFadden fold test. This overall site mean direction is Dec = 143.4º, Inc = 58.8º (N = 11, α95 = 9.7º) with a corresponding mean pole position of Lat = 26.0º, Lon = 249.5º (N = 11, A95 = 13.1º). This geomagnetic pole position is later rotated into West Africa coordinates to allow for extensional rifting in the Benue Trough about an Euler pole position, at 19.2ºN, 352.6ºE through an angle −6.3º (clockwise). The resulting pole position is located at ϕs = 246.6ºE, λs = 31.8ºS (N = 11, A95 = 13.1º), this pole with its 95 per cent confidence circle intersects the 270–310 Ma, segment of the APW path for West Africa consistent with ages of between late Carboniferous and early Permian. The result also implies that the Late Carboniferous Dwyka land ice sheet had probably extended more than 1000 km further north to Ethiopia than previously known.

Counterclockwise block rotation linked to southward propagation and overlap of sub-aerial Red Sea Rift segments, Afar Depression: Insight from paleomagnetism

Thirty-four cooling units from the Pleistocene extrusive volcanic rocks exposed in regions bordered by the active Red Sea magmatic segments, in the northwestern central Afar Depression, were sampled for paleomagnetic study. Six to 12 samples were collected from each paleomagnetic site. Samples were demagnetized using Alternating Field (AF) and Thermal (TH) demagnetization techniques. The samples were then measured using the JR–6A spinner magnetometer available at the department of Earth Sciences of Addis Ababa University. The Natural Remanent Magnetization (NRM) direction reveals mostly one or two simple and straightforward components of magnetizations; the first, low-stability component is isolated by heating to 100°C–300°C or by AF of 10–30mT. The magnetization directions after these steps generally define straight lines that are directed towards the origin, and interpreted as primary NRM or ChRM (Characteristic Remanent Magnetization) and the direction of magnetization was determined. Results of the magnetization decay curve plots and rock magnetic analyses using a Variable Field Translation Balance (VFTB) indicate that magnetic mineralogy is dominated by Ti-poor titano-magnetite with magnetic grain sizes in the pseudo-single domain (PSD) range, with minor presence of goethite and maghemite in a few cases. An overall mean direction calculated for the 26 sites located within the overlap zone is obtained (D=354.4°, I=13.2°, N=26, K=43.5, α95=4.3°), and compared to the expected Geomagnetic Axial Dipole (GAD) field, based on the Apparent Polar Wander Path (APWP) Curve of the African plate (Besse and Courtillot, 1991, 2003), a difference in declination ΔD=−6.5°±4.0° is obtained. This declination difference is interpreted as counterclockwise rotation about a vertical axis, in agreement with rift propagation and right stepping overlap geometry of the Alayta–Dabbahu magmatic segments. The difference in inclination ΔI=7.8°±3.9° is believed to be related to the long-standing non-dipole field in Afar (Courtillot et al., 1984; Kidane et al., 2003).

Temperature dependence of magnetic anisotropy constant in cobalt ferrite nanoparticles

The temperature dependence of the effective magnetic anisotropy constant K(T) of CoFe2O4 nanoparticles is obtained based on the SQUID magnetometry measurements and Mössbauer spectroscopy. The variation of the blocking temperature TB as a function of particle radius r is first determined by associating the particle size distribution and the anisotropy energy barrier distribution deduced from the hysteresis curve and the magnetization decay curve, respectively. Finally, the magnetic anisotropy constant at each temperature is calculated from the relation between r and TB. The resultant effective magnetic anisotropy constant K(T) decreases markedly with increasing temperature from 1.1×107J/m3 at 5K to 0.6×105J/m3 at 280K. The attempt time τ0 is also determined to be 6.1×10−12s which together with the K(T) best explains the temperature dependence of superparamagnetic fraction in Mössbauer spectra.

Determination of the Temperature Dependence of the Magnetic Anisotropy Constant in Magnetite Nanoparticles

The temperature dependence of the effective magnetic anisotropy constant, K(T ), of Fe3O4 (magnetite) nanoparticles is obtained based on SQUID (superconducting quantum interference device) magnetometry. The variation of the blocking temperature, TB , as a function of particle radius, r, is first determined by associating the particle size distribution and the anisotropy energy barrier distribution deduced from the hysteresis curve and from the magnetization decay curve, respectively. Finally, the magnetic anisotropy constant at each temperature is calculated from the relation between r and TB . The resultant effective magnetic anisotropy constant K(T ) decreases markedly with increasing temperature from 5.9 × 10 J/m at 5 K to 1.1 × 10 J/m at 280 K.

Temperature dependence of magnetic anisotropy constant in manganese ferrite nanoparticles at low temperature

The temperature dependence of the effective magnetic anisotropy constant, K(T), of MnFe2O4 nanoparticles is obtained based on superconducting quantum interference device magnetometry measurements. The variation of the blocking temperature, TB, as a function of particle radius, r, is first determined by associating the particle size distribution and the anisotropy energy barrier distribution deduced from the hysteresis curve and magnetization decay curve, respectively. Finally, the magnetic anisotropy constant at each temperature is calculated from the relation between r and TB. The resultant effective magnetic anisotropy constant K(T) decreases markedly with increasing temperature from 8.5 × 104 J/m3 at 5 K to 0.35 × 104 J/m3 at 125 K.

Structural and geochronological implications of the Fentale Volcanics at a nascent passive margin of the Main Ethiopian Rift: Constraints from magnetostratigraphy study at the Kereyou Lodge, Ethiopia

We sampled twenty-four cooling units for magnetostratigraphic study from a 110 m thick Early Pleistocene volcanic section near Kereyou Lodge in the Main Ethiopian Rift. Seven samples were used from each site for analysis of remanent magnetization using thermal and alternating field demagnetization techniques. The Natural Remanent Magnetization (NRM) direction comprises two vector components in most samples. The first, low-stability component is isolated generally by heating to 100 °C–250 °C or by demagnetization using an alternating field intensity of 10–15 mT. After these steps most magnetization directions define straight lines directed towards the origin, and record the primary NRM or ChRM (Characteristic Remanent Magnetization). Magnetization decay curve analyses show the possible magnetic minerals to be titanomagnetite, magnetite, titanohematite and hematite. Paleomagnetic site mean directional analyses revealed that upper 18 flows are of reversed polarity, and the bottom 4 flows are normal. Using available age information the normal interval is correlated to the Olduvai Subchron on the Geomagnetic Polarity Time Scale (GPTS) of Cande & Kent (1995) and the flows at the polarity transition are dated to be 1.77 Ma. The overall declination and inclination differences calculated ΔD = −4.8° ± 4.4°, ΔI = 0.8° ± 4.2°, are consistent with the 7° counterclockwise reported (Kidane et al., 2009). Site-mean inclinations show a systematic change with flow position. Inclinations get progressively shallower with depth for the top 17 volcanic flows and progressively steeper for the bottom 4 flows consistent with progressive backward and rift ward block tilting mechanisms respectively.

Improved Interpretation of Nuclear Magnetic Resonance T1 and T2 Distributions for Permeability Prediction: Simulation of Diffusion Coupling for a Fractal Cluster of Pores

NMR relaxometry is a powerful tool for inferring porosity and permeability data. In practice, measured magnetization decay curves are inverted for relaxation time distributions. Subsequently, one presumes a linear relationship between the pore radius distribution and the T1 and T2 distribution, for longitudinal and transverse magnetization, respectively. The fundamental equations used are based on a pore model, in which pores are assumed to be isolated from each other with respect to the NMR process and have smooth walls. The present study is based on a geometrical pore space model with connected pores and structured pore walls. The physical processes of surface relaxation, irreversible dephasing of magnetic spins and diffusive proton exchange between pores, are described by a system of differential equations. The solution yields a set of exponential functions representing the relaxation time distribution. We describe the difference between the distributions obtained for diffusion coupling and for isolated pores. With diffusion coupling on, the spectral width of the T1 distribution is strongly reduced, which indicates that the influence of large and small radii according to the T1-pore radius relationship is mixed to some extent. For a fractal pore space structure, where large pores are surrounded by adjacent minor pores, the T1 distribution does not resolve these substructures. Nevertheless, permeability values calculated from the logarithmic mean relaxation time T1,LM are quite the same for diffusion coupling and for isolated pores. The T2 distribution for diffusion coupling is little constricted and gives a better resolution of the pore wall structures than the corresponding T1 distribution. The permeability values from T2 distributions agree with the values from longitudinal magnetization, provided that we use a corrected relaxation time T2,corr, accounting for the dependence of the surface relaxivity ρ2 on pore radius. The study shows that radius distributions calculated from a T1 and from a T2 distribution differ from one another and both present an altered image of the true pore radius distribution. In practice, this has no serious influence on estimating permeability of medium- to high-permeability sandstones with the currently applied methods. The presented methodology of calculating the NMR response of pore space models with diffusion coupling may facilitate understanding porosity-permeability relationships of different rock types such as carbonate rocks with micro-porosity.

Proton NMR relaxation study of swelling and gelatinisation process in rice starch–water samples

Proton transverse magnetization decay curves of rice flour starch–water samples were measured and analysed for the presence of four components in the relaxation curve. T2 values were interpreted on the basis of the diffusive and chemical exchange model that provided evidence for extra granular bulk water and three more water populations whose relaxation rate is governed by diffusive and chemical exchange with starch components. The analysis of relaxation data provided information on dynamics of water molecules as well as on the size and dispersion of diffusive domains. Furthermore, by measuring solid to liquid ratio, transverse and longitudinal relaxation curves of starch–water mixtures at increasing temperatures – from 20 to 77°C – swelling and gelatinisation processes were monitored.

Proton NMR transverse relaxation measurements to study water dynamic states and age-related changes in Mozzarella di Bufala Campana cheese

Proton transverse magnetization decay curves of Mozzarella di Bufala Campana samples were measured and analysed for the presence in the sample of four components, namely “serum water”, “entrapped water”, “junction zone water” and “fat”, characterised by different transverse relaxation times, T 2. T 2 values were interpreted on the basis of the diffusive and chemical exchange model that allowed information to be obtained on the size of diffusive domains and dynamics of the water molecules. Furthermore, T 2 values were measured as a function of the aging time of Mozzarella samples, as produced by different cheese-makers. A decrease of “serum water” T 2 value with aging time was found and this may be used to monitor the evolution of Mozzarella samples up to seven days after manufacture.

A simple semiempiric model for NMR relaxometry data of hydrating cement pastes

In this contribution, results from NMR relaxometry experiments on hydrating white cement pastes are presented. Nonexponential effects are discussed and quantified by stretched-exponential analysis. From the magnetization decay curves in transverse relaxation experiments, the degree of hydration is estimated and a semiempiric model is proposed describing the relationship between the microstructure development in the hydrating cement and the degree of hydration.

Nuclear magnetic resonance dephasing effects in a spherical pore with a magnetic dipolar field

The NMR dephasing behavior of the nuclear spins of a fluid confined in a porous material can be investigated by Hahn spin echoes. Previous experimental results on water in a magnetically doped clay have shown a nonmonoexponentially decaying magnetization, which can be understood neither by the known dephasing rate of freely diffusing spins in a uniform gradient nor by spins diffusing in a restricted geometry. For a better understanding of NMR measurements on these systems, a systematic survey was performed of the various length scales that are involved. The standard length scales for the situation of a uniform gradient are diffusing length, structure length, and dephasing length. We show that for a nonuniform gradient, a new length scale has to be introduced: the magnetic-field curvature length. When a particle diffuses less than this length scale, it experiences a local uniform gradient. In that case the spin-echo decay can be described by the so-called local gradient approximation (LGA). When a particle diffuses over a longer distance than the structure length, the spin-echo decay can be described by the motional averaging regime. For both regimes, scaling laws are derived. In this paper, a random-walk model is used to simulate the dephasing effect of diffusing spins in a spherical pore in the presence of a magnetic dipole field. By varying the dipole magnitude, situations can be created in which the dephasing behavior scales according to the motional averaging regime or according to the LGA regime, for certain ranges of echo times. Two model systems are investigated: a spherical pore in the vicinity of a magnetic point dipole and a spherical pore adjacent to a magnetic dipolar grain of the same size as the pore. The simulated magnetization decay curves of both model systems confirm the scaling laws. The LGA, characterized by a nonmonoexponential magnetization decay, is also investigated by calculating the spatially resolved magnetization in the pore. For this regime, the magnetization is found to be inhomogeneously distributed within the pore, whereas it is homogeneously distributed in the motional averaging regime.

Entanglement Effects in Polyethylene Melts: 13C NMR Relaxation Experiments

13C NMR T1, NOE, and T1ρ were measured for two entangled polyethylene melts (M = 16K and 37K) at 5 and 75 MHz 13C Larmor frequencies from the melting point up to 530 K. Two models for entanglement dynamics are tested against the experimental data. This comparison shows that entanglements not only stretch out those relaxation times longer than the entanglement time τe but also have a spatial effect in restricting the motion of entangled points on the time scale of τe. For polyethylene, the amplitude of residual orientation on the time scale of τe is lower than the theoretical prediction for a Gaussian chain of the entanglement length with both ends fixed. The opposite conclusion was drawn from recent double quantum NMR experiments on polybutadiene. The magnetization decay curve for the T1ρ experiment is nonexponential, indicating that the loss of orientation on long time scales depends on position along the chain backbone.

NMR of multipolar spin states excitated in strongly inhomogeneous magnetic fields.

The possibility of exciting and filtering various multipolar spin states in proton NMR like dipolar encoded longitudinal magnetization (LM), double-quantum (DQ) coherences, and dipolar order (DO) in strongly inhomogeneous static and radio-frequency magnetic fields is investigated. For this purpose pulse sequences which label and manipulate the multipolar spin states in a specific way were implemented on the NMR-MOUSE (mobile universal surface explorer). The performance of the pulse sequences was also tested in homogeneous fields on a solid-state high-field NMR spectrometer. The theoretical justification of these procedures was shown for a rigid two-spin 1/2 system coupled by dipolar interactions. Dipolar encoded longitudinal magnetization decay curves, double-quantum and dipolar-order buildup curves, as well as double-quantum decay curves were recorded with the NMR-MOUSE for natural rubber samples with different crosslink density. The possibility of using these multipolar spin states for investigations of strained elastomers by NMR-MOUSE is also shown. These curves give access to quantitative values of the ratio of the total residual dipolar couplings of the protons in the series of samples which are in good agreement with those measured in homogeneous fields.