Category Archives: Sensitivity

Challenges and perspectives in quantitative NMR

Giraudeau, Patrick. “Challenges and Perspectives in Quantitative NMR.” Magn. Reson. Chem., 2016, 9.

https://doi.org/10.1002/mrc.4475

This perspective article summarizes, from the author’s point of view at the beginning of 2016, the major challenges and perspectives in the field of quantitative NMR. The key concepts in quantitative NMRare first summarized; then, themost recent evolutions in terms of resolution and sensitivity are discussed, as well as some potential future research directions in this field. A particular focus is made on methodologies capable of boosting the resolution and sensitivity of quantitative NMR, which could open application perspectives in fields where the sample complexity and the analyte concentrations are particularly challenging. These include multi-dimensional quantitative NMR and hyperpolarization techniques such as para-hydrogen-induced polarization or dynamic nuclear polarization. Because quantitative NMR cannot be dissociated from the key concepts of analytical chemistry, i.e. trueness and precision, the methodological developments are systematically described together with their level of analytical performance.

Rutile dielectric loop-gap resonator for X-band EPR spectroscopy of small aqueous samples

Mett, Richard R., Jason W. Sidabras, James R. Anderson, Candice S. Klug, and James S. Hyde. “Rutile Dielectric Loop-Gap Resonator for X-Band EPR Spectroscopy of Small Aqueous Samples.” Journal of Magnetic Resonance 307 (October 2019): 106585.

https://doi.org/10.1016/j.jmr.2019.106585

The performance of a metallic microwave resonator that contains a dielectric depends on the separation between metallic and dielectric surfaces, which affects radio frequency currents, evanescent waves, and polarization charges. The problem has previously been discussed for an X-band TE011 cylindrical cavity resonator that contains an axial dielectric tube (Hyde and Mett, 2017). Here, a short rutile dielectric tube inserted into a loop-gap resonator (LGR) at X-band, which is called a dielectric LGR (dLGR), is considered. The theory is developed and experimental results are presented. It was found that a central sample loop surrounded by four ‘‘flux-return” loops (i.e., 5-loop–4-gap) is preferable to a 3-loop–2-gap configuration. For sufficiently small samples (less than 1 mL), a rutile dLGR is preferred relative to an LGR both at constant K (B1= Pl) and at constant incident power. Introduction of LGR technology to X-band EPR was a significant advance for site-directed spin labeling because of small sample size and high K. The rutile dLGR introduced in this work offers further extension to samples that can be as small as 50 nL when using typical EPR acquisition times.

Sensitivity Considerations in Microwave Paramagnetic Resonance Absorption Techniques

I recently read again through Feher’s article on EPR sensitivity. After 61 years this article is still very important when building instrumentation for EPR or DNP. It explains many EPR concepts and the relevant instrumentation using simple, easy to understand concepts.

I must read for everyone working in the field of magnetic resonance instrumentation.

Feher, G. “Sensitivity Considerations in Microwave Paramagnetic Resonance Absorption Techniques.” Bell System Technical Journal 36, no. 2 (March 1957): 449–84.

https://doi.org/10.1002/j.1538-7305.1957.tb02406.x.

Within the past few years the field of paramagnetic resonance absorption has become an important tool in physical and chemical research. In many ways its usefulness is limited by the sensitivity of the experimental setup. A typical example is the study of semiconductors in which case one would like to investigate as small a number of impurities as possible. It is the purpose of this paper to analyze the sensitivity limits of several experimental set ups under different operating conditions. This was done in the hope that an understanding of these limitations would put one in a better position to design a high sensitivity resonance experiment. In the last section the performance of the different experimental arrangements is tested. The agreement obtained with the predicted performance proves the essential validity of the analysis. This paper is primarily for experimental physicists confronted withe the problem of setting up a high sensitivity spectrometer.

Understanding Surface and Interfacial Chemistry in Functional Nanomaterials via Solid-State NMR

In recent years DNP-NMR became a very important tool for ssNMR in the area of material science. This is a very nice review illustrating the application of ssNMR to the area of material science and how DNP-NMR can help to overcome sensitivity issues.

Marchetti, A., et al., Understanding Surface and Interfacial Chemistry in Functional Nanomaterials via Solid-State NMR. Adv Mater, 2017. 29(14): p. 1605895-n/a.

https://www.ncbi.nlm.nih.gov/pubmed/28247966

Surface and interfacial chemistry is of fundamental importance in functional nanomaterials applied in catalysis, energy storage and conversion, medicine, and other nanotechnologies. It has been a perpetual challenge for the scientific community to get an accurate and comprehensive picture of the structures, dynamics, and interactions at interfaces. Here, some recent examples in the major disciplines of nanomaterials are selected (e.g., nanoporous materials, battery materials, nanocrystals and quantum dots, supramolecular assemblies, drug-delivery systems, ionomers, and graphite oxides) and it is shown how interfacial chemistry can be addressed through the perspective of solid-state NMR characterization techniques.

Recovery of bulk proton magnetization and sensitivity enhancement in ultrafast magic-angle spinning solid-state NMR

A large portion of the magnetization in a CP experiment remains unused after an experiment and different strategies exist to make better use of the proton magnetization. Here the authors show their results of testing 7 different cp schemes. Although not directly related to DNP these techniques are still very valuable to increase the sensitivity of an NMR experiment especially in combination with DNP.

Demers, J.P., V. Vijayan, and A. Lange, Recovery of bulk proton magnetization and sensitivity enhancement in ultrafast magic-angle spinning solid-state NMR. J Phys Chem B, 2015. 119(7): p. 2908-20.

http://www.ncbi.nlm.nih.gov/pubmed/25588120

The sensitivity of solid-state NMR experiments is limited by the proton magnetization recovery delay and by the duty cycle of the instrument. Ultrafast magic-angle spinning (MAS) can improve the duty cycle by employing experiments with low-power radio frequency (RF) irradiation which reduce RF heating. On the other hand, schemes to reduce the magnetization recovery delay have been proposed for low MAS rates, but the enhancements rely on selective transfers where the bulk of the (1)H magnetization pool does not contribute to the transfer. We demonstrate here that significant sensitivity enhancements for selective and broadband experiments are obtained at ultrafast MAS by preservation and recovery of bulk (1)H magnetization. We used [(13)C, (15)N]-labeled glutamine as a model compound, spinning in a 1.3 mm rotor at a MAS frequency of 65 kHz. Using low-power (1)H RF (13.4 kHz), we obtain efficient (1)H spin locking and (1)H-(13)C decoupling at ultrafast MAS. As a result, large amounts of (1)H magnetization, from 35% to 42% of the initial polarization, are preserved after cross-polarization and decoupling. Restoring this magnetization to the longitudinal axis using a flip-back pulse leads to an enhancement of the sensitivity, an increase ranging from 14% to 21% in the maximal achievable sensitivity regime and from 24% to 50% in the fast pulsing regime, and to a shortening of the optimal recycling delay to 68% of its original duration. The analysis of the recovery and sensitivity curves reveals that the sensitivity gains do not rely on a selective transfer where few protons contribute but rather on careful conservation of bulk (1)H magnetization. This makes our method compatible with broadband experiments and uniformly labeled materials, in contrast to the enhancement schemes proposed for low MAS. We tested seven different cross-polarization schemes and determined that recovery of bulk (1)H magnetization is a general method for sensitivity enhancement. The physical insight gained about the behavior of proton magnetization sharing under spin lock will be helpful to break further sensitivity boundaries, when even higher external magnetic fields and faster spinning rates are employed.

Sensitivity enhancement and contrasting information provided by free radicals in oriented-sample NMR of bicelle-reconstituted membrane proteins

Tesch, D.M. and A.A. Nevzorov, Sensitivity enhancement and contrasting information provided by free radicals in oriented-sample NMR of bicelle-reconstituted membrane proteins. J Magn Reson, 2014. 239(0): p. 9-15.

http://www.ncbi.nlm.nih.gov/pubmed/24355622

Elucidating structure and topology of membrane proteins (MPs) is essential for unveiling functionality of these important biological constituents. Oriented-sample solid-state NMR (OS-NMR) is capable of providing such information on MPs under nearly physiological conditions. However, two dimensional OS-NMR experiments can take several days to complete due to long longitudinal relaxation times combined with the large number of scans to achieve sufficient signal sensitivity in biological samples. Here, free radicals 5-DOXYL stearic acid, TEMPOL, and CAT-1 were added to uniformly (15)N-labeled Pf1 coat protein reconstituted in DMPC/DHPC bicelles, and their effect on the longitudinal relaxation times (T1Z) was investigated. The dramatically shortened T1Z’s allowed for the signal gain per unit time to be used for either: (i) up to a threefold reduction of the total experimental time at 99% magnetization recovery or (ii) obtaining up to 74% signal enhancement between the control and radical samples during constant experimental time at “optimal” relaxation delays. In addition, through OS-NMR and high-field EPR studies, free radicals were able to provide positional constraints in the bicelle system, which provide a description of the location of each residue in Pf1 coat protein within the bicellar membranes. This information can be useful in the determination of oligomerization states and immersion depths of larger membrane proteins.

Generating Parahydrogen-Induced Polarization Using Immobilized Iridium Complexes in the Gas-Phase Hydrogenation of Carbon–Carbon Double and Triple Bonds

Skovpin, I.V., et al., Generating Parahydrogen-Induced Polarization Using Immobilized Iridium Complexes in the Gas-Phase Hydrogenation of Carbon–Carbon Double and Triple Bonds. Appl. Magn. Reson., 2012. 44(1-2): p. 289-300.

http://dx.doi.org/10.1007/s00723-012-0419-5

Immobilized iridium complexes synthesized using [Ir(COD)Cl]2 by anchoring on hydrous and anhydrous silica gels were studied in terms of generating parahydrogen-induced polarization (PHIP) in the gas-phase hydrogenation of propylene and propyne. Distinguishing differences in the hydrogenations of carbon–carbon double and triple bonds were found. It has been shown that in the double bond hydrogenation both catalysts are very active even at 25 C. The reaction yield in continuous flow experiments is more than 70 %, whereas the obtained PHIP degrees are very low. In the case of the triple bond hydrogenation, a more or less active hydrogenation reaction was observed at relatively high temperatures (&70–80 C) for the catalyst immobilized on anhydrous silica, while the catalyst immobilized on hydrous silica was inactive at these temperatures. Contrary to the double bond hydrogenation, the triple bond hydrogenation provided significant signal enhancements observed in 1H nuclear magnetic resonance spectra for the signals corresponding to protons of vinyl fragments of product propylene in both PASADENA and ALTADENA experiments. The catalyst, however, is not stable under the triple bond hydrogenation reaction conditions, and deactivates within several minutes. It was also found that at higher temperatures (100–120 C), this atalyst demonstrates a reactivation most likely associated with the reduction of Ir(I) that results in the ormation of Ir(0) surface metal nanoparticles.

Generating Parahydrogen-Induced Polarization Using Immobilized Iridium Complexes in the Gas-Phase Hydrogenation of Carbon–Carbon Double and Triple Bonds

Skovpin, I.V., et al., Generating Parahydrogen-Induced Polarization Using Immobilized Iridium Complexes in the Gas-Phase Hydrogenation of Carbon–Carbon Double and Triple Bonds. Appl. Magn. Reson., 2012. 44(1-2): p. 289-300.

http://dx.doi.org/10.1007/s00723-012-0419-5

Immobilized iridium complexes synthesized using [Ir(COD)Cl]2 by anchoring on hydrous and anhydrous silica gels were studied in terms of generating parahydrogen-induced polarization (PHIP) in the gas-phase hydrogenation of propylene and propyne. Distinguishing differences in the hydrogenations of carbon–carbon double and triple bonds were found. It has been shown that in the double bond hydrogenation both catalysts are very active even at 25 C. The reaction yield in continuous flow experiments is more than 70 %, whereas the obtained PHIP degrees are very low. In the case of the triple bond hydrogenation, a more or less active hydrogenation reaction was observed at relatively high temperatures (&70–80 C) for the catalyst immobilized on anhydrous silica, while the catalyst immobilized on hydrous silica was inactive at these temperatures. Contrary to the double bond hydrogenation, the triple bond hydrogenation provided significant signal enhancements observed in 1H nuclear magnetic resonance spectra for the signals corresponding to protons of vinyl fragments of product propylene in both PASADENA and ALTADENA experiments. The catalyst, however, is not stable under the triple bond hydrogenation reaction conditions, and deactivates within several minutes. It was also found that at higher temperatures (100–120 C), this atalyst demonstrates a reactivation most likely associated with the reduction of Ir(I) that results in the ormation of Ir(0) surface metal nanoparticles.

Drastic sensitivity enhancement in 29Si MAS NMR of zeolites and mesoporous silica materials by paramagnetic doping of Cu2+

Inagaki, S., et al., Drastic sensitivity enhancement in Si MAS NMR of zeolites and mesoporous silica materials by paramagnetic doping of Cu. Phys Chem Chem Phys, 2013.

http://www.ncbi.nlm.nih.gov/pubmed/23824320

The paramagnetic doping of Cu2+ in both mesoporous silica materials and microporous silicate crystals (zeolites) can be used effectively to enhance the signal intensity of 29Si solid state magic angle spinning NMR, as a result of shortening of the spin-lattice relaxation time, T1, by the paramagnetic effect, because of the Cu2+ electronic relaxation time in the range of 10-8 s. This leads to drastically reduced data-collection times, typically 80-fold shorter than that in mesoporous silica. We found that the estimated range of the paramagnetic effect of Cu2+ doping in porous silicates was at least 1 nm.

Drastic sensitivity enhancement in 29Si MAS NMR of zeolites and mesoporous silica materials by paramagnetic doping of Cu2+

Inagaki, S., et al., Drastic sensitivity enhancement in Si MAS NMR of zeolites and mesoporous silica materials by paramagnetic doping of Cu. Phys Chem Chem Phys, 2013.

http://www.ncbi.nlm.nih.gov/pubmed/23824320

The paramagnetic doping of Cu2+ in both mesoporous silica materials and microporous silicate crystals (zeolites) can be used effectively to enhance the signal intensity of 29Si solid state magic angle spinning NMR, as a result of shortening of the spin-lattice relaxation time, T1, by the paramagnetic effect, because of the Cu2+ electronic relaxation time in the range of 10-8 s. This leads to drastically reduced data-collection times, typically 80-fold shorter than that in mesoporous silica. We found that the estimated range of the paramagnetic effect of Cu2+ doping in porous silicates was at least 1 nm.

Have a question?

If you have questions about our instrumentation or how we can help you, please contact us.