Low Field NMR Spectroscopy and Applications

Since the inception of NMR, the field has been marked by rapid change and frequent innovations as evidenced by Nobel Prizes in physics, chemistry and medicine:

  • 1943 – Stern – discovery of magnetic moment of a proton
  • 1944 – Rabi – resonance method for recording magnetic properties
  • 1952 – Bloch & Purcell – new NMR measurements
  • 1991 – Ernst – high-resolution NMR
  • 2002 – Wuthrich – determining 3D structure of biological macromolecules
  • 2003 – Lauterbur & Mansfield – discoveries in MRI

Prior to 1961, NMR spectrometers were not commercially available. They were made in specialized physics laboratories and operated exclusively by trained experts. These spectrometers generated an external magnetic field with permanent iron magnets. This technology was found to have a theoretical limit of about 100 MHz, and therefore was soon made obsolete in favour of super-conducting technology.

Why? Higher field = greater resolution (now ranging up to 1000 MHz!!) = improved structural elucidation. Regardless of these undeniable benefits, there are some downfalls to HR spectrometers – namely high cost and major steric restrictions. These spectrometers require dedicated, controlled locations and trained maintenance staff to perform weekly cryogen fills. As such, despite the benefits that NMR could add to industrial, online applications, the ubiquitous technique remains largely underrepresented in industry.

 

Source: Dalitz, F.; Cudaj, M.; Maiwald, M.; Guthausen, G. Prog. Nuc. Mag. Reson. Spec. 2012, 60, 52
[http://dx.doi.org/10.1016/j.pnmrs.2011.11.003]

Although not as beneficial in chemical analysis, there are a number of applications where medium-field spectrometers can provide more than sufficient resolution for NMR spectroscopy – for instance, QA/QC and reaction monitoring. This has sparked a renaissance in permanent magnet technology development as permanent magnets are better poised to facilitate the incorporation of NMR technology into routine industrial analysis – in field laboratories or even trucks.

Some potential example applications where medium-field NMR spectrometers have shown utility:

  • QA/QC: Determination of % oxygenates added to fuel (e.g., methanol, ethanol, methyl tert-butyl ether (MTBE) [from Skloss, T. W. et. al. Anal. Chem. 1994, 66, 536]
  • RXN Monitoring: Determination of the rate constant of the acid-catalyzed acid esterification. NMR was found to be the most accurate & quickest method (versus raman, near infrared (NIR) in the reaction of crotonic acid & 2-butanol [from McGill et. al. Analyst. 2002, 127, 287]

Further reading: Will 60 MHz NMR work for your application?