How to Find the Central Frequency of NMR Analyzer?

Basic principle of NMR analyzer :

The spin frequency of hydrogen atoms in a certain magnetic field obeys the following rule. γ*B₀=ω , where B₀ is the magnetic field strength of the NMR analyzer, γ is the magnetogyric ratio of hydrogen and ω is called larmor frequency (also called central frequency) which is the resonance frequency of hydrogen atoms in a certain magnetic field. If the RF frequency is equal to larmor frequency, the resonance phenomenon of hydrogen atoms can occur. However the material of our magnet is very sensitive to temperature. Even a little variation of temperature can cause the variation B₀ which also causes the variation of central frequency. Therefore, at the very beginning of the NMR experiment, we need adjust the RF frequency to central frequency and this step is called finding central frequency.

Fig 1: larmor frequency

The detail about how to find central frequency is explained below.

Step 1: Put the standard oil sample which without impurity;

Step 2: Click the bottom in the toolbar, sampling 10 seconds;

Step.3:.Click the bottom in the toolbar, the offset frequency will change automatically. If the real part is separate from image part, you find the correct offset frequency.

Notice:

a) NMR analyzer software can find the central frequency correctly only if the temperature of the magnet is kept at the working temperature, SF is set to the value setting by factory and SF1+O1 is close to the resonance frequency. If the value of SF1+O1 is about 40 KHz far from central frequency, this function will be invalid.
b) The value of SF should be consistent with the factory settings.
c) You can observe the real part (the red curve), the imaginary part (the green curve) and magnitude (the blue curve) of the signal by clicking the options in the checkbox which is above the Figure.