Solid state nuclear magnetic resonance (NMR) refers to studies of crystalline or amorphous materials using magnetic resonance. However, experiments can be performed on a variety of “other” sample types, such as semi-solids, mixed phase, or even gas phase, in EMSL-developed sealed samples. NMR experiments can be utilized to monitor in situ reactions (i.e., lignin breakdown), determine cell wall structure on plant or fungal cell walls, identify three-dimensional (3-D) structure of molecules immobilized on a surface, or characterize complex mixtures such as organic matter.
Complex mixtures are challenging to analyze with any technique. NMR can provide quantitative information on chemical functionality present in a sample, but limits on sensitivity and resolution make in-depth characterization challenging. However, established statistical methods can pull out key features to discriminate between sample groups.
EMSL currently hosts five solid state NMR spectrometers that utilize high-field superconducting magnets ranging from 7 T (300 MHz) to 20 T (850 MHz). The available probes are standard commercial or custom, homebuilt probes.
Supporting the Biogeochemical Transformations,Structural Biology, and Terrestrial-Atmosphere Processes Integrated Research Platforms, solid state NMR can be used to characterize natural organic matter, including organo-mineral interfaces. In addition:
- 13C MAS NMR can provide quantitative or semi-quantitative analysis of the chemical functionality present in a sample.
- The intrinsically nuclei-specific nature of NMR supports stable isotope probing experiments with 13C or 15N enriched tracers.
- Effects of metal binding to organic matter may be probed directly through X-nuclei or indirectly through induced NMR property changes.
- With isotopic enrichment, 3-D structures of immobilized proteins, like mineral-bound proteins, or cell walls can be determined using multidimensional experiments.
- Other complex mixtures, such as biofuels or lignin, are well suited to NMR characterization. For example, degradation reactions can be monitored in situ by examining either the solid or any dissolved intermediates or products. Changes in material can also be monitored by observing sample dynamics.
EMSL has a number of high-field, solid-state NMR instruments. The following summarizes key resources, but reach out to our subject matter experts to discuss technical needs and system suitability.
- 850 MHz Agilent VNMRS (Ellis), with five radiofrequency (RF) channels including simultaneous 1H and 19F with X, Y, and low gamma; probes include a 40 kHz MAS (1.6 mm), 3.2 mm HXY MAS, 4 mm HXY and HFXY MAS, a 5 mm HX MAS for continuous flow, and a 4 mm gradient MAS.
- 500 MHz Varian VNMRS (Shasta), with a three-RF-channel console and probes that include a 4 mm HXY MAS, a 5 mm HXY MAS, a 5 mm HX MAS with a ceramic housing, a 7.5 mm HX MAS, a 5 mm cryogenic probe capable of static NMR down to 2.2 K, and a 5 mm diffusion probe (solution).
- 400 MHz Agilent VNMRS (Karloff), with a three-RF-channel console and probes that include a 3.2 mm HXY MAS, a 4 mm HX MAS, a 5 mm HXY MAS, a 7.5 mm HX MAS, and a 5 mm cryogenic probe capable of static NMR down to 4 K.
- 300 MHz Agilent VNMRS (Mazama), with a three-RF-channel console and probes that include a 4 mm HXY MAS, a 5 mm HXY MAS, a 5 mm HX optimized for 125 kHz decoupling, a 7.5 mm HX MAS, and a 5 mm cryogenic probe capable of static NMR down to 4 K.
Tips for success
The following is a non-exhaustive list of common NMR tips. Contact us to discuss details about sample preparation and experimental design considerations.
- Natural soil samples should be screened to remove any magnetic particles.
- Ideally, for 13C at natural isotopic abundance (≈1%), samples with ≥ 5% C is desirable; observation of lower C content samples is possible but will be limited by longer experiment times and potential background signals.
- NMR background signals can include 13C, 27Al, 1H, 19F, and 91Zr; please open a discussion about any low levels of analyte, and arrangements can be made to reduce the background.
- For most organic matter studies, data analysis is based on integrating signal regions. Due to the reduced resolution (compared with solutions), solid-state NMR for these materials limits results to quantifying functional groups.
- The most common experiment on organic materials is cross polarization (CP), and it is not quantitative. There are sets of samples that allow quantitative CP; factors such as paramagnetics (i.e., FeIII or MnIII/IV ions) interfere.
- Direct polarization can be quantitative; however, it requires much longer experiment times (sometimes prohibitive), and background signals are more likely to appear.
- Some levels of paramagnetic ions are fine (≤ 5%); however, organics near the radical will be washed out from the NMR experiment.
- Rotor volumes and max spin rates are as follows:
- 1.6 mm 8 µL 40 kHz (lower for low Al rotors)
- 3.2 mm 22 µL 25 kHz
- 4 mm 52 µL 18 kHz
- 5 mm 230 µL 12 kHz
- 7.5 mm 450 µL 7 kHz
- Variable temperature experiments are possible; contact us about any needs to determine what limits the equipment may have based on your experiment’s needs (e.g., reduced background).