W. M. Keck Solid State Nuclear Magnetic
Resonance (NMR) Facility
A multi-nuclear facility dedicated to the analysis of bio-geo-cosmo-
Former Geophysical Laboratory Fellow (now Professor at Seoul National University) Dr. Sung Keun Lee tuning the 2.5 mm probe prior to running 17O MQMAS experiments on his 17O enriched silicate glasses.
One of the hats I wear at the Geophysical Laboratory is that of the principal investigator in charge of the W. M. Keck solid state NMR facility at the Geophysical Laboratory. This major instrumentation originated from the generous support of the NSF Major Research Infrastructure (MRI) Program, the W. M. Keck Foundation, and the Carnegie Institution of Washington. The instrument, a Varian-Chemagnetics CMX Infinity 300 was installed in 1998 and has been used continuously to augment research projects spanning Astrobiology, Organic Geochemistry, Biogeochemistry, Marine chemistry, molecular paleobotany and paleontology, experimental high temperature and high pressure Geochemistry, and cosmochemistry.
The NMR laboratory is well equipped to perform a broad range of experiments, the instrumental details are…
• Super Conducting Magnet: Our NMR employs a 7.05 Tesla (300 MHz 1H) Oxford wide bore magnet with Resonance Research Shim controller. The choice of this field was derived as a compromise whereby we could provide first rate solid state capabilities for 13C and 15N (where advantages in sensitivity gained by moving to high field are significantly offset by requiring higher speed MAS, hence complicating cross polarization derived experiments) and yet still provide useful capability for various quadrupolar nuclei, e.g. 17O, 23Na, and 27Al. Now, relative to high field solid state NMR (e.g. fields in excess of 14.1 T) at 7.05 T there remains considerable line broadening associated with higher order quadrupolar effects. However, in the case of many quadrupolar nuclei we have found that in many experiments (e.g. multiple quantum magic angle spinning) at 7 Tesla actually provides considerable information due to the enhanced quadrupolar broadening associated with the lower field (Check out some of the papers by former post Doctoral Fellow Sung Keun Lee for a demonstration of some of these “low” field advantages). We also find (as Jaeger and others have shown) that considerable information is also available in the satellite transition sidebands. Of course in the case of quadrupolar nuclei, spectral data acquired at different field strengths is always preferable to a single field; thus we maintain valuable collaborations with other NMR laboratories with higher field strength systems.
The main cabinets of the CMX Infinity NMR.
• RF: The GL CMX infinity is a three channel spectrometer equipped with one high power narrow band amplifier (CMA) for 1H and 19F and two high power broad band (AMT) amplifiers capable of exciting resonances spanning a low frequency limit of 15N (~ 32 MHz) and a high frequency limit of 31P. Also included are a pair of PTS frequency synthesizers, a three channel pre-amplifier and receiver, a 400 MHz oscilloscope, and a Wavetek sweep generator for probe tuning.
• Probes: We currently have five solid state MAS probes to serve a broad range of scientific inquiries. This provides for considerable versatility. Magic angle spinning speed is controlled automatically with a microprocessor MAS speed controller yielding ± 1 Hz.
7.5 mm double resonance probe-vespel housing: This probe supports large volume (~ 500 mg capacity) zirconia rotors. The maximum spinning speed is 7.0 KHz. We typically use this probe for 15N, 29Si and 31P (when a low concentration) experiments targeted at elucidating the molecular structure of organic solids and silicate glasses. We also have the “Magic Angle Turning” accessories to allow for stable low frequency spinning for the isolation of isotopic and anisotropic signal via the elegant Grant et al. MAT experiments.
Figure Above: The 7.5 mm probe is particularly excellent when signal is low and fast MAS is not required. Above we show variable contact time 1H-29Si CPMAS experiments revealing the differential rate of polarization transfer to Si (Q) molecular species with differing numbers of non bridging oxygens (e.g. Cody et al GCA 2004-2005). A short contact times Q2 and Q3 are the most intense at longer contact times Q4 is the most intense peak. The change in cross-polarization dynamics is a complex function of glass composition revealing hidden larger scale structural variation that could not be inferred from the distribution of Qn species alone.
5.0 mm double resonance probe-vespel housing: This is our workhorse for 13C analyses. With a maximum spinning speed of 12 KHz we are able to move the spinning sidebands completely outside the spectral window for carbon, affording high quality spectra. We typically employ the variable amplitude cross-polarization MAS experiments. In order to minimize background (from teflon) in single pulse experiments, we have a number of boron nitride inserts that can be used with the zirconia rotors.
Figure Above: Variable amplitude 1H-13C Cross Polarization NMR spectra of spruce at three different stages of degradation by the fungal micro-organism Gloephyllum trabeum; a ‘brown rot’ fungus. Note the selective loss of cellulose and hemicellulose indicated by the reduction in intensity of polysaccharide secondary alcohols.
Figure (above): 13C solid state NMR reveals substantial differences in the electronic environments of carbon in cellulose and hemi-cellulose. The shift in frequency of the anomeric carbon is particularly helpful in allowing one to distinguish and quantify the relative distribution of these two important biopolymers in biological materials.
5.0 mm CRAMPS probe-vespel housing: This probe is dedicated to Combined Rotation Multi-Pulse, CRAMPS, excitation experiments for solid-state proton NMR.
4.0 mm Triple resonance probe-vespel housing: This probe has a maximum MAS frequency of 18 KHz. A given sample can be excited by three frequencies simultaneously (e.g. 15N, 13C, with 1H decoupling). We are using this probe to support experiments such as TRAPDOR, REDOR, and solid state HETCOR as well as other useful experiments.
2.5 mm double resonance probe-vespel housing: This is a great probe!!! The maximum MAS speed is 30 KHz (nearly 2,000,000 rpm!)-this makes this probe ideal for solid state 1H and 19F. The extremely small rotor size results in a terrific filling factor, thus plenty of RF power delivery to the sample. Consequently we find this probe is ideal for 17O (enriched) and 27Al MQMAS experiments.
Figure Above: The 2.5 mm probe is excellent for providing substantial RF power that aids experiments like the mutiple quantum – single quantum (MQ) MAS experiment. The contour plot shown above is an 17O MQMAS spectrum of albite glass acquired by Dr. Sung Keun Lee (former GL Fellow and now Assistant professor at the Seoul National University) using our 2.5 mm probe. Clearly defined are the two oxygen environments.
Figure Above: The 2.5 mm probe is very nice for 1H and 19F NMR where fast MAS (wr/2π up to 30 kHz) is required to minimize broadening associated with homodipolar coupling. The 1H spectra on the left were acquired as part of a study of the structure of hydrated sodium silicate glasses (Cody et al. in press), the 19F NMR spectra on the right were acquired as part of a study on the effects of fluorine on the structure of sodium aluminum silicate glasses (Mysen et al. 2004)
• Console: Communication with the NMR relies on a Sun Ultra-5 workstation running Spinsight Software.
• Variable Temperature: Our NMR is equipped with a variable temperature controller, allowing for high temperature (~200 C) and low temperature (liquid Nitrogen temperature) experiments.
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