NMR Spectrometers: Resolution and Error Correction

The Lorentzian peaks in an NMR spectrum are defined by their position, amplitude, and full width at half maximum.

The peak width of a properly prepared sample in a perfectly homogeneous field is governed by the spin–spin relaxation time alone.

However, the applied magnetic field, B₀, is rendered inhomogenous by the presence of the sample and the probe. This results in broadened peaks with extraneous side-bands and poor resolution.

Magnetic field inhomogeneities are corrected by a process called shimming.

During shimming, current is passed through a set of shim coils surrounding the probe, generating small magnetic fields that enhance or oppose B₀ in the vicinity of the sample.

These shim coil fields are manipulated to obtain the most uniform magnetic field across the sample.

Shimming, along with optimized pulse sequence parameters and proper sample preparation, ensures good peak shape, high signal-to-noise ratio, and maximum resolution.