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Technical Notes
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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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