When evaluating a benchtop NMR instrument there are several key performance characteristics that have a very significant impact on how the instrument will perform in your lab. These key performance characteristics are:
The spectral resolution, which is directly related to the magnet and determines the width or shape of the NMR lines (often called lineshape), and in turn the ability to separate signals in the spectrum
The sensitivity which determines the limits of detection (LOD) and quantitation (LOQ), and in turn how long sample measurements take
The stability of the magnet and instrument over time, which impacts the ability to make longer measurements, and the overall ease of use of the spectrometer
Before I examine these performance characteristics in more detail, it’s worth emphasising from the outset that the biggest aspect of a benchtop NMR system’s design that dictates how well the system performs is the “quality” of the magnetic field produced by the magnet. By “quality” we are referring to how uniform the magnetic field is over the sample volume, often referred to as the B0 homogeneity. To illustrate the importance of this key aspect of magnet design, Figure 1 shows a series of spectra collected under varying degrees of B0 homogeneity.
Figure 1. Effect of static magnetic field (B0) homogeneity on the NMR spectrum. As the homogeneity gets worse, both the resolution and sensitivity are negatively affected. The series of spectra on the left are shown at the same scale and show how the 2 peaks can no longer be resolved, and the signal intensity decreases. The series on the right is the same spectra shown with the peak intensities normalised which how the signal-to-noise ratio is decreasing (the noise is increasing) when the field is less homogeneous.