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Technical Notes 
 
Metrolab 2025 NMR Teslameter (Gaussmeter), 1060 & 1062
Deuterium Probes and their residual H signal
 
The Metrolab 1060 and 1062 range 6, 7 and 8 NMR Probes use a heavy water (D20) liquid sample in a glass vial. The sample has a small amount (less than 1%) of H20 contamination.

Early in 2000, Michael Duffy improved the rf performance of the range 8 NMR Probe. The result of this has improved the sensitivity for both the proton and deuterium resonances by over 300 percent. In the past, the lower proton resonance signal from the deuterium probes may have been too weak to 'lock' the NMR Teslameter (Gaussmeter). With the probe improvement and the probable improvement in unshimmed magnet uniformity it is likely that both the proton and deuterium resonances will be seen by the automatic tracking circuits of the NMR Teslameter (Gaussmeter).

The Gamma for H (protons) is about 42.57608MHz/T while the Gamma for D (deuterons) it is about 6.53569MHz/T. That is, in a fixed 1T field the protons will resonate at 42.57608MHz and the deuterons at 6.53569MHz. At a fixed rf frequency of 6.53569MHz, the deuterons will resonate when the field is 1T, and the protons will resonate when the field is lowered to (6.53569/42.57608) = 0.153506T (or there abouts!). If the deuterons in the sample resonate at 10T, the residual protons in the sample will resonate at 1.53506T. Thus, when a deuterium probe is in a magnet that is swept up in field, the proton resonance will occur first. At a field about 6.5 times higher, the deuterium resonance will occur. The H20 'contamination' is, in fact, used to test the probes at a field that is approximately 6.5 times lower than the design field.

With the improved probe performance, it is more likely that the 2025 NMR Teslameter (Gaussmeter) may 'lock' on to the proton signal of the NMR Probe while ramping a magnet up to field. That is, the NMR Teslameter (Gaussmeter) may display a magnet field value of 10.0T when it is really only 1.53T.

It is important to be aware of this probe behaviour, and take care in interpreting the field readings when magnet ramping. One way to be sure that the field reading is correct is to cross reference it with the power supply setting.

This dual resonance, follows directly from the sample impurity and the 2025 has no mechanism to distinguish between the proton and deuteron NMR resonances.

Please contact GMW if you have any questions or problems.

Ian J. Walker, September, 2001
 
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