CHAPTER-7

CHAPTER-7: "Field Lock
In order to produce a high resolution NMR spectrum of a sample, especially one which requires signal averaging or phase cycling, you need to have a temporally constant and spatially homogeneous magnetic field. Consistency of the Bo field over time will be discussed here; homogeneity will be discussed in the next section of this chapter. The field strength might vary over time due to aging of the magnet, movement of metal objects near the magnet, and temperature fluctuations. Here is an example of a one line NMR spectrum of cyclohexane recorded while the Bo magnetic field was drifting a very significant amount. The field lock can compensate for these variations.

The field lock is a separate NMR spectrometer within your spectrometer. This spectrometer is typically tuned to the deuterium NMR resonance frequency. It constantly monitors the resonance frequency of the deuterium signal and makes minor changes in the Bo magnetic field to keep the resonance frequency constant. The deuterium signal comes from the deuterium solvent used to prepare the sample. The animation window contains plots of the deuterium resonance lock frequency, the small additional magnetic field used to correct the lock frequency, and the resultant Bo field as a function of time while the magnetic field is drifting. The lock frequency plot displays the frequency without correction. In reality, this frequency would be "