Relaxation

In most spectroscopic techniques, how the energy absorbed by the sample is released is not a primary concern. In NMR, where the energy goes, and particularly how fast it “gets there” are of prime importance. The NMR process is an absorption process. Nuclei in the excited state must also be able to “relax” and return to the ground state. The timescale for this relaxation is crucial to the NMR experiment. For example, relaxation of electrons to the ground state in uv-visible spectroscopy is a very fast process, on the order of pico-seconds. In NMR, the excited state of the nucleus can persist for minutes. Because the transition energy between spin levels (discussed earlier) is so small, attaining equilibrium occurs on a much longer timescale. The timescale for relaxation will dictate the how the NMR experiment is executed and consequently, how successful the experiment is.

There are two processes that achieve this relaxation in NMR experiments: longitudinal (spin-lattice) relaxation and transverse (spin-spin) relaxation.

In longitudinal relaxation energy is transferred to the molecular framework, the lattice, and is lost as vibrational or translational energy. The half-life for this process is called the spin-lattice relaxation time (T1). Dissipating the energy of NMR transitions (which are tiny compared to the thermal energy of the sample) into the sample should not be a problem, however T1 values are often long. The problem arises not in where to “send” the excess energy, but the pathway along which the energy is released to the lattice. Contributing factors to this type of relaxation are temperature, solution viscosity, structure, and molecular size.

In transverse relaxation energy is transferred to a neighboring nucleus. The half-life for this process is called the spin-spin relaxation time (T2). This process exchanges the spin of nucleus A with the spin of nucleus B (A mI = -½ ® +½ as B mI = +½ ® -½. There is no net change in spin for this process. Inhomogeneity of the magnetic field or the presence of paramagnetic materials can be a large contributor to the value observed for transverse relaxation.

The peak widths in an NMR spectrum are inversely proportional to the lifetime (due to the Heisenberg uncertainty principle) and depend on both T1 and T2. For most organic solutions, T1 and T2 are long enough to result in spectra with sharp lines. However, if magnetic field homogeneity is poor or paramagnetic material (such as iron) is present the NMR signals can be broadened to the extent that the signal is destroyed or unusable.

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NMR Crosslink Density Analyzer

Combined with the sample temperature control system, VTMR20-010V-T NMR analyzer can control the environmental temperature to research change in physical properties of samples over a wide range of temperatures and rates of change. It can be used for food, energy, organic materials and other areas of research.

Functions

1. Rapid cross-link density determination of rubbers and other polymers;

2. Relaxation analysis of T2*,T2 and T1;

3. Determination of glass transition temperature;

4. Quantitative analysis of water phase with varying -temperature.

NMR Application Indexes

1. Minimum detection limit: 10 mg of  water;

2. Test range of moisture: 0.88 % – 100 %;

3. The sample temperature range: 35 0C – 150 0C(standard)/ 35 0C – 2000C (advanced), with precision ± 0.3 0C;

4. The correlation coefficient of crosslink density between benchtop NMR method and swelling method > 0.99;

5. Repeatability: RSD < 2%, reliability: RSD < 10 %. RSD: relative standard deviation

Application Direction

Determination of cross-link density of polymers (especially rubbers);

Quality control and assurance in plolymer production;

Quality inspection in polymer aging process;

Study in rubber vulcanization process and optimization of production conditions;

Research on molecular mobility of solids, semi-rigid polymers, gels, emulsions and liquids;

Imaging and determining the moisture in solid matrix;

Detection of viscosity, state and process during the epoxy resin and rubber vulcanizing;

Investigation in adhesion and activity of water of the samples;

Determination of plasticizer or rubber content of the polymers;

Determination of rubber content of the copolymer or blends;

Determination of relative content of copolymers;

Determination of solid content in rubber latex;

Research on critical water and hydration;

Rheological research on viscosity, density and the stability of materials .

Application Examples

1-the-configuration-for-the-same-type-of-equipment-could-vary-in-different-application-fields

Multinuclear and hypersensitive MRM in heterogeneous catalysis 

In many heterogeneous catalytic processes, heat transport is an important factor which, if not properly controlled, can lead to the formation of hot spots in the catalyst bed, degradation of reaction conversion and selectivity, reactor runaway and even explosion. The development of non-invasive thermometry techniques for the studies of operating reactors is necessary to advance our understanding of heat transport processes in the catalyst bed and is essential for the development of efficient and environmentally safe industrial reactors and processes.

NMR analyzer and MRI techniques are known to be able to evaluate local temperatures of liquids. However, for a multiphase gas-liquid-solid reactor the available techniques based on the liquid phase benchtop NMR signal detection are not applicable since the local liquid content in the catalyst pores varies with space and time.

We have demonstrated earlier that the direct 27Al MRI of industrial alumina-supported catalysts (e.g., Pd/Al2O3) is a potential way toward the spatially resolved thermometry of an operating packed bed catalytic reactor. Recently, we were able to implement this approach and to obtain 2D temperature maps of the catalyst directly in the course of an exothermic catalytic reaction. The images obtained clearly demonstrate the temperature changes with the variation of the reactant feed and also the existing temperature gradients within the catalyst at a constant feed.

One of the obstacles in developing novel applications of MRM in porous media is its fairly low sensitivity even if 1H signal detection is used. A number of hyperpolarization techniques are currently being developed that can enhance the NMR signal by 4-5 orders of magnitude even at intermediate (3-7 T) magnetic fields, and even more in low and ultra-low magnetic field applications that are currently gaining popularity. Parahydrogen-induced polarization (PHIP) is the only hyperpolarization technique of relevance to catalysis as PHIP effects are observed in hydrogenation reactions when parahydrogen is involved.

We have shown that PHIP can be generated not only in homogeneous hydrogenation reactions but also in heterogeneous catalytic processes catalyzed by a broad range of different heterogeneous catalysts. Thus, the development of the novel hypersensitive NMR/MRI techniques for heterogeneous catalysis becomes possible.

Also, this approach can provide hyperpolarized gases and catalyst-free hyperpolarized liquids for a wide range of novel applications of NMR and MRI in, e.g., materials science, chemical engineering and in vivo biomedical research. Applications of this hypersensitive approach to the studies of gas flow in microfluidic chips and of the hydrogenation reaction in a packed bed microreactor will be demonstrated.

This work was supported by the following grants: RAS 5.1.1, RFBR 11-03-00248-a and 11-03-93995-CSIC-a, SB RAS integration grants 9, 67 and 88, NSh-7643.2010.3, FASI 02.740.11.0262 and МК-1284.2010.3.

Laplace Inversion for Obtaining Relaxation-Chemical Shift Correlation

Kerogen is the most abundant organic matter that is dispersed in the earth’s formation and is the source of fossil fuels such as oil and gas. Large amounts of kerogen exist in the form of oil shale which is not favorable for extraction.

The largest accumulation of oil shale is located in the Piceance Basin, Colorado. The so called ‘Green River oil shale’ was deposited in a lacustrine environment and contains type-1 kerogen as an organic resource. Even though the depositional environment is known there are details of the molecular structure of type-1 kerogen which still have to be identified.

We used 13C and 15N high resolution solid state benchtop NMR spectroscopy as one of the primary methods to elucidate the structure of kerogen through measurements of chemical shift, spin-lattice relaxation time, variable contact time, and dipolar dephasing rates. Using single or double exponential fitting methods, one can extract structural information from the variation of peak intensity with mixing times used in CP/MAS or dipolar dephasing experiments.

Here we introduce a new data processing method that uses a Laplace inversion algorithm to process a set of CP/MAS NMR analyzer data with different mixing times to generate 2D NMR spectrum that gives a chemical shift in one dimension and relaxation time in the second dimension.

The relaxation-chemical shift 2DNMR can be used to determine 13C (or 15N) chemical shift, proton spin-lattice relaxation time, cross polarization time, and dipolar dephasing time constant as well as their distributions for mixture samples such as oil shale. It also improves the accuracy of deriving structural parameters of macromolecules using both 13C or 15N chemical shift and relaxation cutoffs.

Polymers Under Uniaxial Stress

Local order and dynamics in polymers under mechanical stress is studied by low-field NMR. Permanent magnets in a Halbach arrangement permit NMR investigation without the limits present in high-field NMR. In particular the confined stray field permit the application of benchtop NMR in a stretching apparatus and a rheometer.

The major drawback of low-field NMR analyzer, the lack of chemical shift resolution, is not a problem, because in the study of known materials properties other than their chemical composition are of interest.  Mechanical stress on elastomers results in partial chain ordering and consequently reduced chain mobility.

The resulting stronger residual dipolar couplings are manifested in the stronger buildup of double quantum coherences and in a shortening of the slower component of the transverse relaxation time. After releasing the load the return to the dipolar couplings and the relaxation times of the unextended sample is followed on a time constant of tens of minutes The crystalline and amorphous fractions of semicrystalline polymers are distinguished by their transverse relaxation times.

To localise the stress effect in the rf coil, the diameter of the rod under study is reduced in the portion located in the rf coil. Under mechanical load there is a significant shortening of the transverse relaxation time as well as an increase in the residual dipolar coupling which are determined from the build up of double quantum intensities.

The shortened relaxation times return to values close to those found in unloaded samples, when the load is kept constant. The time constant of this relaxation appears to be longer than that found in mechanical stress relaxation experiments.

The interaction with paramagnetic moieties in the fillers in polymer nanocomposites has a strong impact on the longitudinal relaxation time. Delaminating filler particles under mechanical stress results in a shorter T1 of the protons in the polymer, because the contact area between the filler and the polymer increases.

Flow and Diffusion Measurement with MR

Nuclear magnetic resonance (NMR) non-invasively accesses many parameters in contrast with other commonly used measurement methods, whether they are non-invasive or not.
These parameters can be divided roughly into three classes of information: chemical, physical, and spatial.Chemical includes benchtop NMR spectroscopy, the workhorse in analytical chemistry and in structural biochemistry but, to date, there has been relatively little overlap between this class and this conference.
Physical information accessible with NMR analyzer includes molecular structure, phase transition, diffusion, and flow.Both chemical and physical information can be combined with spatial information to produce maps of such information.In addition, flow and diffusion, by their nature, involve spatial information.
Such spatially resolved information is the main emphasis of this meeting.In this lecture, I shall review NMR flow and diffusion measurements.What is needed for such measurements is the presence of a known gradient of the static magnetic field strength in which the experiments are conducted.
When a nuclear spin moves in the field gradient, its precession rate changes and this can be detected to yield the displacement of the spin in the time required to do the experiment–times measured in milliseconds.The dependence of such displacements as a function of measurement time results in identification of the nature of sample motion, i. e., whether it is flow or diffusion.
We will start with basic principles and go on to examples with emphasis on gaining physical background knowledge that may aid in understanding flow and diffusion presentations during this meeting.Some references to this subject are listed below.The last three are based on previous ICMRM conferences, specifically in 1991, 1997, and 2009.

Mobile NMR

Mobile NMR started in the well logging industry. Early on, benchtop NMR devices were developed to be deployed inside the borehole to characterize the fluids of the well downhole [1]. The successful NMR analyzer well-logging devices measure distributions of relaxation and diffusion parameters in the stray field of permanent magnets .

The same principle is followed with the much smaller NMR-MOUSE , which has a higher field, a stronger gradient, and a smaller sensitive volume. It is used for nondestructive materials testing of large objects such as rubber tires, polymer pipes, and objects of art . Today, the NMR force microscope is the smallest stray-field NMR device .

While the sensitive volume of stray-field NMR devices can be shaped with proper magnet design , the sensitivity can significantly be improved only by enlarging the sensitive volume and the field strength, a strategy which returns mobile NMR to the roots of NMR spectroscopy and imaging with closed magnets.

In fact, a variety of desktop MRI magnets has been pioneered by Kose , and the first miniature spectroscopy magnet has been developed by McDowell . Today miniature NMR gadgets are targeted for specific detection of biomarkers , and the first desktop NMR spectrometers for chemical analysis by high-resolution NMR appear on the market .

The advances in mobile NMR are expected to benefit from the progress in developing widely applicable and miniaturized hyperpolarization methodologies, alternative detection schemes, microfluidic components for sample preparation and handling, the use of high-TC superconducting magnets, and the development of user-friendly apps for different types of information-driven measurements by untrained NMR consumers.