Stability and Benchtop NMR

NMR spectrometers are perhaps most commonly used to generate qualitative data – you know – determine relative integrations, elucidate a structure, or even just assess a reaction’s progress.  For inquiries of this ilk spectrometer performance – including sensitivity and resolution/lineshape are important.  Why?  Well, ultimately because these parameters control both the speed and the confidence that you can generate data.  Basically they facilitate your quest to deduce and/or confirm your molecular structure.

An NMR spectrometer is also a pretty powerful quantitative tool to determine sample composition, evaluate purity and calculate important reaction constants (ΔG,ΔH, ΔS, etc.).  Instrumental requirements to generate quantitative data can be more stringent.  Not only are spectrometer performance metrics important, but also other factors – such as the accuracy, precision and repeatability – also become important.  These correlate to the stability of a magnetic field.

Most NMR users are familiar with high-field supercon magnets that afford extremely stable external magnetic fields capable of acquiring data for extended periods of time.  If the external field is generated by permanent magnets (e.g., NdFeB or SmCo), as is the case with benchtop NMR spectrometers, the field can be less stable. Permanent magnet materials have characteristic temperature coefficients – meaning that a spectrometer’s magnetic field will respond to changes in temperature.  Although NdFeB has more favourable remanence and coercivity properties that ultimately afford a higher magnetic field (why it was chosen for the NMReady-60), this alloy also has a less favourable temperature coefficient and, therefore, must be very, very carefully engineered.  If not properly controlled, environmental fluctuations can create inhomogeneity and time-variation in the external magnetic field, and this can visibly broaden spectral lines, especially when data are signal-averaged.

This is easily the biggest concern we get from people interested in exploring the efficacy of benchtop NMR in their teaching, research or analysis application.  To overcome this challenge, we have several methods in place to stabilize the magnet and minimize magnetic and thermal fluctuations induced by its external environment.  One such precaution is to operate the NMReady-60e slightly above room temperature (~30 oC) and rigorously to control its internal temperature with feedback loops.  This minimizes the adverse effects of room temperature fluctuations to the instrument’s performance and generates a sufficiently stable field, whose residual fluctuations are further mitigated with a frequency-agile lock.[1] This lock enables signal averaging without inducing line broadening.

Shimming is an electronic means to correct spatial non-uniformities that are present in the magnetic field.  Although we do encourage regular shimming to maintain an optimally uniform magnetic field, it is not necessary to interrupt long run experiments.  We have acquired spectra over the course of 24 hours without experience a decrease in the quality of observed spectra.

If you would like more information and/or have any questions please don’t hesitate to contact us!

[1]Typically we recommend a deuterium lock but there are non-deuterated options available to those that would like them.