The molecular structures and dynamics of solids, liquids, gases, and mixtures are of critical interest to diverse scientific fields. High-temperature, high-pressure in situ MAS NMR enables detection of the chemical environment of constituents in mixed phase systems under tightly controlled chemical environments.
This non-destructive method can be used to analyze complex systems at controlled temperatures and pressures over time, and is therefore highly useful for investigating key scientific questions. These methods allow NMR experiments to be conducted under tightly controlled and specialized conditions, to solve problems under environments relevant to the scientific hypothesis being explored. The multi-nuclear nature of NMR makes it suitable for numerous samples.
Coupled with this method, it can provide insight into an array of systems, spanning fields such as ketolysis, geochemistry, and biology. For pre-treatment of the sample solid, weigh approximately twice the mass of the solid sample that is desired for the NMR experiment, and place the solid sample into a quartz sample tube plugged with quartz wool. Connect the isolation valves to the tube for the pretreatment of interest, and fix the end of the tube onto the gas isolation valve in the open position.
Then, place the tube into the cool furnace and begin the treatment. After the treatment, stop the flow and vacuum, and turn off the temperature controller. Disconnect the quartz tube from the treatment system, and quickly seal the sample with the isolation valves to maintain the desired sample environment.
Then, transfer the tubes and closed valves to the antechamber. To load the samples into an NMR rotor, first, place the rotor into the holder to maintain directionality, and place the sample funnel into the bore of the rotor. Remove the isolation valve from the sample tube, and pour a small quantity of solid material into the funnel.
Tap the powder down into the funnel using a packing rod to lightly direct the sample into the rotor as necessary. When the desired quantity of sample has been loaded, a microsyringe can be used to slowly inject the desired volume of a liquid sample into the center of the rotor before placing the cap onto the top of the rotor, and turning the cap counterclockwise with the rotor cap bit to engage the O-ring between the rotor and cap. To charge the NMR rotor with the chemicals of interest, place the sealed rotor onto the rotor stage and tighten the nut by hand to secure the rotor in place.
Lower the rotor stage into the lower section of the high-pressure exposure device, and use an Allen wrench to turn one of the screws 90 degrees to secure the rotor stage into the bottom of the exposure device. Place the top section of the NMR loading device into and on top of the bottom section, with the NMR cap bit aligned with the top of the cap head of the NMR rotor. Place the two clamps over the top of the lip where the upper and lower sections of the exposure device meet, and latch the clamps into place.
Tighten the six bolts on the top of the upper section of the exposure device to engage the ceiling surface between the upper and lower sections, and connect the thermocouple on the upper section of the NMR exposure device to the gas line inlet and outlets. Apply vacuum to the system to purge the high-pressure loading chamber of ambient gases. Open the gas source valves on the high pressure syringe pump, and run the program set on the pump while monitoring the real pressure inside of the exposure device.
To open the NMR rotor, rotate the external screw mechanism, which is coupled to the interior NMR cap bit in the clockwise direction to allow the gas of the desired pressure to enter the NMR rotor and equilibrate. To reseal the NMR rotor, rotate the external screw mechanism counter-clockwise. A viewing window will assist in determining when the rotor is closed.
Then, open the exposure device gas outlet valve to slowly depressurize the system. To conduct a magic angle spinning NMR experiment, place the NMR rotor into the NMR coil on the NMR probe, and raise and lock the probe into place in the magnet bore. Use the magic angle spinning control box to adjust the sample to the desired rotor spinning rate, and initiate sample spinning.
Use the computer to begin the tuning match sequence on the desired channel, and adjust the tuning match settings on the probe to optimize the probe electronics, then, collect the magic angle spinning NMR data. In this representative analysis, some insight into the operational reaction mechanism for the conversion of ethanol to butenes can be observed. Butyraldehyde formation, coupled with the simultaneous disappearance of peaks characteristic of crotonaldehyde, followed by butyraldehyde subsequent consumption and the concomitant appearance of peaks characteristic of n-butenes, suggests that butyraldehyde is an intermediate in the formation of n-butene.
In situ, high-temperature, high-pressure magic angle spinning NMR can also be used to better understand the evolution of chemical species for biological applications. For example, as this representative carbon-13 magic angle spinning NMR spectrum of vape juice solution shows, parent glycerol is present at 63 and 73 parts per million. As time progresses at 130 degrees Celsius in an oxygen environment, the toxins acrylic acid and formic acid with formaldehyde appear at 175 and 164 parts per million respectively.
The oxidation product carbon dioxide is observed at 125 parts per million. And most importantly, even at such low temperatures, acetyl species of formaldehyde and acetaldehyde are observed between 50 and 112 parts per million. The addition of parent glycerol to formaldehyde and acetylaldehyde generates new hemi-acetyl species at 105 and 112 parts per million, which act as aldehyde carriers and can self-interact and dehydrate to generate new acetyl species.
Numerous other peaks between 50 and 80 parts per million correspond to the many other chemical environments of the hemi-acetyls and acetyls. In addition to working safely, it is important to maintain the environment within the rotor during transfers, particularly when removing the chamber top, as the rotor cap will still be engaged. Since NMR is non-destructive, additional characterizations, such as x-ray diffraction, gas chromatography/mass spectrometry, and x-ray photoelectron spectroscopy, can be pursued, enabling comparison of the NMR results with complimentary information on the same sample.
These measurements obtained at elevated temperatures and pressures have enabled researchers to address challenging problems in a way not previously possible, by looking at complex systems under relevant conditions.
