In this post, I’m going to discuss a specific test for evaluating the resolution or lineshape of a benchtop NMR system. This test is measured on proton (1H) as the NMR spectrum is sensitive to the spectrometer resolution and we can make the measurement with a single scan. Resolution and lineshape refer to the width of a particular NMR spectral line, measured at 50% and 0.55% of the height of the line, as explained below. The smaller the linewidth value, the better the resolution.
1H lineshape and resolution
The information content of any NMR spectrum depends on the ability to observe and resolve different signals, or peaks, in the spectrum. It is easier to distinguish two sharp (narrow) peaks close together, than two broad peaks. The key technical factor defining the sharpness of the lines in the spectrum (the lineshape) is the homogeneity of the magnetic field generated by the magnet. Although NMR systems utilize an array of coils (“shims”) to further improve the B0 homogeneity, the achievable lineshape and resolution are strongly influenced by the inhomogeneity of the magnet itself. The process of calibrating the field and optimizing the B0 homogeneity is usually referred to as shimming. The homogeneity and resolution will gradually degrade over time, so you should carry out the shimming procedure whenever you want to ensure you have the best resolution your spectrometer is capable of. You should also do the shimming procedure immediately before making the linewidth test described here.
Measuring 1H lineshape and resolution
The way to measure instrument resolution is to collect a spectrum containing a naturally sharp line. The standard approach for is to use a sample containing chloroform in acetone-d6 – the chloroform signal from this sample has a very narrow natural linewidth.1 Figure 1 shows a 1H NMR spectrum sample of a 20% chloroform in acetone-d6 NMR reference sample collected on an 80 MHz benchtop NMR spectrometer. (https://www.sigmaaldrich.com/catalog/product/sial/611859)
Figure 1. 80 MHz 1H NMR spectrum of 20% chloroform in acetone-d6 (“lineshape”) sample
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.