Reaction monitoring by low-field NMR at 45 MHz
Three reactions were monitored by low-field NMR at 45 MHz to
assess this technology and application. All of the reactions were
followed at the temperature of the magnet, 42°C, which limits this
technology and this instrument in particular to reactions occurring
at room temperature or slightly above. Reactions at lower or higher
temperature cannot be followed with this type of instrument unless
a variable temperature probe is developed or available. This work
has not been compared with other low-field benchtop NMR
systems currently commercially available with larger NMR tube
systems and wider capillaries for the NMR flow cell. Performance
among those systems and the picoSpin-45 NMR instrument is diffi-
cult to assess with no data comparison. Initially, those systems
should be able to perform similarly to the picoSpin-45, but
differences in some parameters like coil filling factors would affect
the signal-to-noise ratio to some degree.
that is lower than 10 : 1 compromises the results and is not recom-
mended. The current instrument has no automation, which makes
less likable as a PAT tool because it requires manual injections;
therefore, the instrument is not ready for online analysis unless
automation is provided. As long as the reactions are at a slower rate
than the NMR scale, monitoring reactions at a lower field is achiev-
able as demonstrated in our laboratories. Overall, monitoring
reactions at 45MHz is successful considering the points discussed
in this article.
Acknowledgements
We are grateful to Dr. Janet Cheetham, Dr. Margaret Faul, and
Dr. David Semin for their management support, interest, and
encouragement to perform this research project. We thank
Dr. John Price, Mr. Dean Artic, Mr. Chuch Miller, and Mr. Erick
Winston for their technical support with the picoSpin-45 NMR
instrument. We also want to thank Dr. Michael Bernstein, Dr.
Chen Peng, Dr. Carlos Cobas, and Dr. Santiago Dominguez for their
constructive support and discussions with Mnova software applica-
tion to monitoring reactions.
Conclusions
We have demonstrated that it is possible to follow chemical
reactions by low-field NMR. The advantages are low cost, low
maintenance, simple to use, minimal requirements from building
facilities, and without cryogenics. The reactions that can be
followed are with simple molecules as starting building blocks that
have a distinct signal, which does not overlap with solvents or other
signals from species in the reaction mixture. The reaction has to be
concentrated enough to minimize the dynamic range with
solvents. Using non-deuterated solvents is not an issue even if the
instrument lacks an internal lock while the magnet drifts. At low
field, the magnetic drift does not degrade the lineshape as it does
at high field. A low signal-to-noise ratio of 10 : 1 for the
solvent/analyte signals, respectively, is still approachable for reac-
tion monitoring at the low field of 45 MHz. However, a lower ratio
will compromise the results of the analysis and the technique.
NMR is appropriate for reactions lacking chromophores that make
other analytical techniques such as UV and IR impossible. Mnova
processing software has proved to be an excellent tool to analyze
NMR data for monitoring reactions at any field (lower or higher).
Other commercially available low-field benchtop NMR systems
would be expected to perform similarly at the same field but with
some differences depending on the NMR flow cell design.
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