Today I am going to discuss two papers that explore NMR without a big and expensive magnet!
The first paper is
"Ultra-low-field NMR relaxation and diffusion measurements using an optical magnetometer"
Ganssle PJ, Shin HD, Seltzer SJ, Bajaj VS, Ledbetter MP, Budker D, Knappe S, Kitching J and Pines A.
Angew Chem Int Ed Engl. 2014 53 9766-70
The authors design and demonstrate an ultra low field (ULF) NMR capable of performing industrially relevant measurement (T1, T2 and D) for the characterization of mixtures of hydrocarbons and water. The authors claim that their instrument is the first step towards a compact, inexpensive and robust NMR sensor which operates at the Earth's magnetic field.
How does this work?
Conventional NMR detectors use a coil to detect transverse magnetization. The ULF NMR uses a magnetometer like what is in your cell phone as a compass. The specific magnetometer is optically detected (the author's do not really explain the detector in this paper). I don't pretend to understand all the details of this detector, which is called a spin exchange relaxation-free (SERF) configuration magnetometer. The important thing to understand is that the detector measures longitudinal magnetization (along the z direction), in contrast with traditional NMR coils. In fact, during acquisition, a series of 180 degree pulses are applied to sample to flip the spins between the +z and -z direction. The authors convert the "average magnitude of change in magnetometer signal in response to a pi pulse" into a sensible signal.
The authors want to design "a NMR sensor which operates at the Earth's magnetic field", but for now they have to make a few compromises to get a prototype. First, there isn't much longitudinal magnetization in samples polarized by the Earth's magnetic field, so the authors apply a "pre-polarizing" 2 T field. Second, the sample chamber is not really at the Earth's magnetic field, which (according to Wikipedia is at 25–65 uT (microtesla). By the way, a strong refrigerator magnet has a field of about 10 mT. At any rate, the chamber is designed to have no magnetic fields - there is some sort of special shielding to remove the Earth's magnetic field. Then a weak "bias" magnetic field (50 uT in this case) is applied to the chamber. This field is designed to be turned off during detection (the SERF detector has optimal response at zero field) and during pulses. The reason for turning off the field during pulses is so that "the device does not need to be re-tuned when the bias field is changed." Overall, their set up looks like the following:
The pulse sequences used to measure T1, T2 and D are the following:
If you study these pulse sequences, they will make a lot of sense. The T1 measurement does not use the inversion recovery pulse sequence, instead longitudinal relaxation time constant is measured "as an exponential decay of the spin magnetization as a function of an increasing delay time between the sample pre-polarization and measurement." You also see the unusual detections strategy. Longitudinal magnetization is flipped between -z and z by a pi pulse. The T2 measurement is a standard CPMG train. "The inter-pulse spacing (tau) is held constant and the number of pulses (n) is varied." Diffusion is CPMG with a gradient.
One thing I find pretty cool about this set up is that it uses conventional 5 mm NMR tubes!
What do the authors measure?
They record the relaxation time constants (T1 and T2) of some common solvents: water, methanol, ethanol and hydrocarbons at 0.5 G and 37 C. They also make mixtures of hydrocarbons and water. How did they do that? These liquids are immiscible! In fact, they used "a coaxial insert which separated liquid in a smaller, 3.3 mm NMR tube from the liquid in a standard thin walled 5 mm NMR tube." I'm not sure I would call that a mixture, but why split hairs! The data looks like the following:
Finally the authors can make 2D plots of T2 v T1 or T2 v D to demonstrate how readily these solvents can be distinguished based on these relaxation properties:
The authors conclude that this device is an "important first step towards the development of compact, inexpensive devices which can take advantage of the Earth's high homogeneous ambient magnetic field." They acknowledge "the limitations present in these experiment ... are artifacts of the design of the device" but feel that they make a "compelling case for future research".
I think the device described in this publication is interesting and important. I have a few questions, though. How long does it take to make each T1, T2 and D measurement shown in figure 3 (each dot on the curve)? Can T1, T2 and D distinguish miscible liquids, such as water-methanol or water-ethanol? I work at a University and the proof of the vodka is a major concern on football days! Finally, how big is the instrument, really? It is hard for me to get a feel for the size based on Figure 1. It looks almost as big as a superconducting NMR magnet.
The second paper is
"Scalable NMR spectroscopy with semiconductor chips"
Dongwan Ha, Jeffrey Paulsen, Nan Sun, Yi-Qiao Song and Donhee Ham
In contrast with the first paper, which described an unconventional detection scheme, this paper describes a miniaturized NMR system that is ~12 cm^3, weighs 7.3 kg and can perform 1D 1H, multi-pulse and heteronuclear experiments. By contrast, a commercially available system like the Picospin45 (http://picospin.com/products/picospin-45/) is 20.3 cm x 14.6 x 29.2 cm, weighs 4.7 kg and can only do 1D 1H. The major innovation by the authors is the miniaturization of the electronics into a 4 mm^2 chip. The technical details are best understood by an electrical engineer, so I won't explain too much. The net result is a system that looks like the following:
One of the advantages of the design is pulse programming. The ability to control pulses and delays enables the authors to go beyond traditional 1Ds and do multidimensional NMR. Figure 4 shows the JRES and 2D phase sensitive COSY on neat ethanol and 1.5 M alanine dissolved in D2O (acquisition times are 15 and 73 minutes, respectively).
The authors can also collect HSQC and HMQC on 13C enriched methanol in 17 and 34 minutes, respectively. Note that there is no decoupling during acquisition (t2), meaning the peaks are split by the 1JCH.
The authors round the paper out with a relaxometry experiment on a crude oil sample. They also introduce a clever processing hack to handle temperature fluctuations, which present a "significant obstacle towards portable NMR." Their solution deserves a longer explanation in this blog post, but lets face it, it is getting way too long.
To wrap up this post, I'll note that I am not convinced that either system discussed above or a commercial benchtop NMR (from PicoSpin, Magritek or Nanalysis) can ever replace a trusty 400-600 MHz NMR for resonance assignment in organic chemistry or biochemistry. I will concede that not all chemical characterization requires a 400-600 MHz NMR. To paraphrase John Edwards from Process NMR associates at the MestreNova Users meeting prior to the ENC this year "if your spectrum looks like crap at 400 MHz, it won't look too much worse at 90 MHz, so why waste time on a superconducting magnet." The authors of the papers I reviewed today present two novel NMResque instruments capable of making measurements nearly identical to a high field superconducting system. The trick will be to continue to develop these systems and find applications where these systems outperform conventional systems.
Ganssle, P., Shin, H., Seltzer, S., Bajaj, V., Ledbetter, M., Budker, D., Knappe, S., Kitching, J., & Pines, A. (2014). Ultra-Low-Field NMR Relaxation and Diffusion Measurements Using an Optical Magnetometer Angewandte Chemie International Edition, 53 (37), 9766-9770 DOI: 10.1002/anie.201403416
Ha, D., Paulsen, J., Sun, N., Song, Y., & Ham, D. (2014). Scalable NMR spectroscopy with semiconductor chips Proceedings of the National Academy of Sciences, 111 (33), 11955-11960 DOI: 10.1073/pnas.1402015111