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.
As Figure 1 illustrates, a magnet with better B0 homogeneity will generate spectra with lines that are both narrower and more intense (a narrow lineshape), while a magnet with poorer B0 homogeneity will yield broader and less intense lines (a broader lineshape). The homogeneity therefore affects both resolution and sensitivity. In the example, two peaks that are resolvable with good B0 homogeneity gradually “smear together” as homogeneity degrades and become completely unresolved from one another which means the instrument’s resolution is decreasing. At the same time, we see the poorer B0 homogeneity causes a reduction in signal intensity, which directly affects the instrument’s sensitivity and in turn the limits of detection and quantitation (LOD/LOQ).
Both resolution and sensitivity are critical because they determine what range and type of samples and concentrations you can measure, how long measurements will take, and therefore what range of applications the instrument can be used for. System stability is important because it impacts the ability to make longer measurements over a period of time, limits the environment that the instrument can be used in, as well as the overall ease of use of the spectrometer.
Over the next few posts, I will discuss the following key performance characteristics in more detail and why you should care about them. For each performance characteristic I will provide an objective and “vendor-neutral” method for testing them.
- 1H lineshape and resolution
- 1H sensitivity
- 13C sensitivity
We will also mention some of the considerations in regards system stability and we intend to have some more detailed posts about this topic in the near future.