While low-field NMR has extremely favourable accessibility and affordable characteristics, the most common question we get asked about our family of benchtop NMR spectrometers is with respect to any trade-offs that come from moving to lower-field. For either structural elucidation or quantification (qNMR) applications, there are two main obstacles imposed at low-field: (i) resolution/ dispersion; and (ii) what I will call spectral complexity. While resolution and peak overlap can contribute to spectral complexity, this it is primarily due to second order effects.
Resolution and dispersion are related, but subtly different. Resolution refers to the ability to fully separate two peaks. Dispersion, on the other hand, refers to the spread of an individual peak over the chemical shift axis (the x-axis). To illustrate: at 60 MHz, a 1 Hz wide peak would cover 0.02 ppm, whereas at 100 MHz, a 1 Hz wide peak would cover only 0.01 ppm. This means that as the field strength increases, each peak appears narrower. Correspondingly, resonances are more likely to be resolved even if they have similar chemical shifts.
To illustrate the dispersion/resolution phenomena, the low-field benchmark compound of the ibuprofen proton NMR is shown in figure 1 acquired at 60 and 100 MHz. In the 60 MHz spectrum, the methine peak at 1.77 ppm, overlaps with both of its neighbours, so although the peak multiplicity allows the user to infer the structure from the spectrum, it is not totally clear – the septet is not fully observed and there is also error in the integration. In the 100 MHz spectrum, however, there is a clear separation between the methine peak and its methyl neighbours. The fine structure of the septet can be observed in the expected 1:6:15:20:15:6:1 Pascal’s triangle ratio.