Low-field NMR Spectrometer Research Articles

Evaluation of Polyethylene/Organoclay Nanocomposites by Low-field Nuclear Relaxation

Two polyethylene (PE) samples (named BF and ES) were blended with 5% organoclay by melt processing at different shear rates. X-ray diffraction (XRD) showed that the organoclay was incorporated into the PE matrices. From the results it was determined that the level of intercalation was very similar for both systems. The knowledge of molecular dynamics of PE nanocomposites is very important since it depends on the clay dispersion. Thus, the objective of this work was to employ a new technique to understand changes in the molecular mobility for these systems. The samples were characterized by proton nuclear spin-lattice relaxation time (T1H), which is measured in a low-field NMR spectrometer. The influence of the samples' preparation was evaluated through the relaxation data. For polyethylene resins, two values of relaxation parameter were found and they were attributed to mobile region (low value) and rigid region (high value). These values change after nanocomposite formation, confirming the changes in the resin matrix after organoclay was incorporated.

Consideration of some sampling problems in the on-line analysis of batch processes by low-field NMR spectrometry

A low-field medium-resolution NMR spectrometer, with an operating frequency of 29 MHz for 1H, has been assessed for on-line process analysis. A flow cell that incorporates a pre-magnetisation region has been developed to minimise the decrease in the signal owing to incomplete polarisation effects. The homogeneous esterification reaction of crotonic acid and 2-butanol was monitored using a simple sampling loop; it was possible to monitor the progression of the reaction through changes in CH signal areas of butanol and butyl crotonate. On-line analysis of heterogeneous water-toluene mixtures proved more challenging and a fast sampling loop system was devised for use with a 5 L reactor. The fast sampling loop operated at a flow rate of 8 L min(-1) and a secondary sampling loop was used to pass a sub-sample through the NMR analyser at a slower (mL min(-1)) rate. It was shown that even with super-isokinetic sampling conditions, unrepresentative sampling could occur owing to inadequate mixing in the reactor. However, it was still possible to relate the 1H NMR signal obtained at a flow rate of 60 mL min(-1) to the composition of the reactor contents.

Paramagnetic relaxation in sandstones: Distinguishing T1 and T2 dependence on surface relaxation, internal gradients and dependence on echo spacing

This work provides a generalized theory of proton relaxation in inhomogeneous magnetic fields. Three asymptotic regimes of relaxation are identified depending on the shortest characteristic time scale. Numerical simulations illustrate that the relaxation characteristics in the regimes such as the T 1/ T 2 ratio and echo spacing dependence are determined by the time scales. The theoretical interpretation is validated for fluid relaxation in porous media in which field inhomogeneity is induced due to susceptibility contrast of fluids and paramagnetic sites on pore surfaces. From a set of measurements on model porous media, we conclude that when the sites are small enough, no dependence on echo spacing is observed with conventional low-field NMR spectrometers. Echo spacing dependence is observed when the paramagnetic materials become large enough or form a ‘shell’ around each grain such that the length scale of the region of induced magnetic gradients is large compared to the diffusion length during the time of the echo spacing. The theory can aid in interpretation of diffusion measurements in porous media as well as imaging experiments in presence of contrast agents used in MRI.

Evaluation of Low-Field Nuclear Magnetic Resonance Spectrometry for At-Line Process Analysis

A low-field medium-resolution NMR spectrometer, with an operating frequency of 29 MHz for 1H, has been developed for use in process analysis. The information that is attainable at this field strength has been investigated with the use of samples from a wide range of applications including the qualitative monitoring of a benzene production process, the estimation of the average ethoxy chain length in nonyl phenol ethoxylates, and the determination of strong acid concentrations. At low field, the spectra of multi-component samples often exhibit overlapping peaks, so multivariate calibration methods are necessary for quantitative analysis. Simulated data have been used to illustrate the effects of signal-to-noise ratio (SNR), resolution, and line shape on the predictive ability of partial least-squares (PLS) calibration routines, with SNR shown to have a greater impact than resolution on accuracy and precision. PLS calibration models were applied successfully in the analysis of samples from a methacrylamide process and a simulated esterification reaction. The accuracy (<5%) and precision (<3%) were generally good for components at concentrations >1–5% w/w.

NMR Studies II. Investigation of Process Analytical NMR Techniques for the Pulp and Paper Industry1

The application of a low-field NMR spectrometer, specifically designed for industrial applications, was explored as a means of characterizing the physical properties of kraft black liquors. A variety of proton NMR parameters were determined for black liquor samples, including relative signal intensities and T2 measurements. These values were correlated against the % solids and viscosity values of these samples. Research studies demonstrated that it was readily feasible to measure % solids for a variety of kraft black liquor samples by determining the proton NMR signal intensity for each of these samples.

Comparative NMR studies of diffusional water permeability of erythrocytes from some animals introduced to Australia: Rat rabbit and sheep

The diffusional water permeability (P d) of the red blood cell (RBC) membrane of rat, rabbit and sheep, representing some animal species introduced to Australia, has been monitored, by a Mn2+-doping 1H nuclear magnetic resonance (NR) technique using a high-field spectometer operating at 400 MHz. In order to make comparisons with previous measurements on the same species (performed at 25 MHz) an analysis of the influence of instrumental parameters and of the state of blood oxygenation was performed on samples of rat and rabbit blood. It was found that by using a short interpulse delay (around 100 µs) in the Carr-Purcell-Meiboom-Gill sequence, and by performing the measurements of the transverse relaxation time of the water in the cell interior (T 2i) on packed cells (haematocrit >95%), the values of the water exchange time (T e) obtained with a high-field spectrometer are comparable with those obtained by using a low-field NMR spectrometer. The values of the diffusional water permeability (P d), calculated from the T e values, were, at 24.6°C around 10 × 10−3 cm/s in case of rat and rabbit RBC and around 5 × 10−3 cm/s for sheep RBC; at 37°C the P d values were around 16 × 10−3 cm/s for rat RBC, 14 × 10−3 cm/s for rabbit RBC and 7 × 10−3 cm/s for sheep RBC. These values are very close to the P d values previously reported for the European counterparts of these species. There were no significant differences in the P d values between laboratory rabbits and wild rabbits, or between castrated males, pregnant or nonpregnant females. These results suggest that no changes in the RBC water permeability are correlated with marked alterations in the habitat of animal species introduced to Australia or by sex hormone status. If the NMR instrument-parameters are carefully chosen then comparative studies of RBC water permeability in different laboratories, at separate locations and using different instruments are meaningful.

Design of a Low-Field NMR Spectrometer to Measure Bone Mineral

Design of a Low-Field NMR Spectrometer to Measure Bone Mineral