Development of royal HFX probe
GENERAL CONCEPTS AND THEORY
As described in , the basic concept is to use inductive coupling to split the high frequency sample coil resonance into two modes corresponding to the observe frequency for 1H and 19F. Inductive coupling was found to be superior to a circuit similar to the Haase circuit because the minimum capacitance of the variables in this circuit results in unacceptable losses when the circuit is in the uncoupled mode . The inductive and capacitive circuits are essentially equivalent in the coupled mode.
The inductive circuit used here can be described pictorially as shown in Fig.1, the two distinct modes, coupled and uncoupled, are shown as being orthogonal to each other. To implement this circuit we have constructed two coils. The first coil is similar to an Alderman – Grant coil . It serves as the sample observe coil and detects the resonance of the spins, i.e. 1H and 19F. The second coil is what we call the idler coil. This coil is used solely to couple to the sample coil and split the fundamental resonance into two modes. To accomplish this idler coil is resonated half way between the 19F and 1H resonances. In the coupled mode the magnetic axis of the idler coil overlaps with the magnetic axis of the sample coil creating two modes. The degree of separation of the two peaks depends on the degree of rotation of the idler coil which can increase or decrease the k value, the amount of coupling between the two coils. Fig.3 shows this interaction.
Fig.2 shows the sample coil and idler in close proximity to each other. The red arrow shows that we can rotate the idler coil in a manner that couples and decouples the interaction between the two coils.
PRACTICAL ASPECTS OF OBSERVING AND 1H AND 19F
Fig.4 illustrates the relationship between the idler frequency and the circuit fill factor (CFF) . The circuit fill factor is a measure of the circuit efficiency and is independent of the Q and sample fill factor for a given mode in the probe. Here one can adjust the efficiency of the coupled mode to favor one mode or nuclei over the other. Table 1 shows the PW(90) values and powers for the HFX probe in both the decoupled and coupled modes. The data here clearly indicates that the idler frequency of the circuit was set to favor 19F performance over 1H performance. For the coupled mode we wanted to keep the PW(90) values shorter for 19F so as to be able to efficiently observe the larger chemical shift region of 19F.
In order for the idler frequency to be set such that PW(90) values can be adjusted for optimal usage a mechanism to measure the idler frequency in the probe had to be devised. The idler frequency in the probe is different than when it is outside the probe due to the presence of metallic objects like the Faraday screen etc. We have also noted that the idler frequency is dependent on its orientation the sample region for the same reason.
Fig.5 shows the S11 plot of the high frequency channel when the probe is in the coupled mode (Fig.5a). Note here that when the sample coil resonance is tuned to a higher frequency the amplitude of the lower resonance decrease and also moves towards the center of the plot. Tuning the sample coil to yet a higher frequency results in a similar action with the lower resonance moving further towards the center of the plot and the amplitude moving lower, Fig.5b and Fig.5c. Here when the amplitude goes to zero this is the idler frequency because the idler is no longer interacting with the sample coil . Using this measurement method we can determine the frequency that the idler coil needs to be set at on the bench in order for it to have the correct frequency when it is in the probe. As stated earlier we usually set this frequency slightly higher so as to optimize the 19F PW(90) over that of the 1H PW(90) in the coupled mode.
Fig.6 shows the components used in the construction of the idler coil. Here the idler coil is constructed from a single copper loop that is not compensated in terms of susceptibility. The nonspin line-shape of the probe was found not to be impacted by the presence of this coil. The sapphire chip capacitor  shape was modified by the use of a rotary grinding tool so that it could fit in the available space in the probe between the sample coil and the Faraday shield. Note here that the modified chip is soldered to the chip using an alignment fixture (not shown).
TESTING AND PERFORMANCE
From the outset the goal of this HFX development project was to produce a routine NMR that had the same performance at the JEOL ROYAL Probe but with the additional capability of simultaneous 1H and 19F decoupling. Table 2 shows the data accumulated for this probe. What is remarkable here is that none of the routine performance numbers have been impacted by the addition of the idler coil. The only additional test required for the probe are those related to the coupled mode of operation. For 1H/19F decoupling, test with the diplexer combiner are needed to insure that noise from the decoupling in either 1H or 19F doesn’t leak into the other channel. See Fig.7. Fig.8 shows the nonspin line-shape for the probe on 1% CHCl3. This is important to emphasize for the routine 2D experiments that this probe enables.
Fig.9 clearly shows the decoupling capability of the ROYAL HFX probe. The 64 kHz of decoupling bandwidth indicated using adiabatic decoupling is sufficient to decouple all the 19F nuclei in this challenging molecule. Additional band width can be achieved by using the JNM-ECZ500R console sequencer. In Fig.10 we show that the noise floor for 31P and 13C observe nuclei is not increased as we turn on the decoupler channels of the spectrometer. Fig.11 shows the aromatic region 13C observe spectra of Voriconozole. With simultaneous 1H/19F decoupling the spectrum is clearly simplified allowing for structure verification. In Fig.12 we demonstrate the ability to acquire HMBC with and without 19F decoupling while observing 15N.
The new ROYAL HFX Probe provides on demand capability which is a first in NMR. Never before have so many experiments been available in one NMR probe. The ease of use and sensitivity of the probe will allow chemist greater structure elucidation capabilities in a convenient and easy to use format. The use of magnetic coupling to achieve this capability was a breakthrough. For more than 30 years probe builders have struggled to build a probe that has this on demand capability. Now with this new capability chemist can quickly determine the structure of a much wider list of molecules that extends from 31P to 15N observe nuclei.
1. Bowyer P., Finnigan J., Marsden B., Taber B., Zens A. Using magnetic coupling to implement 1H, 19F, 13C experiments in routine high resolution NMR probes. J. Magn. Reson. Vol. 261. 2015. P. 190–198.
2. Haase J., Curro N.J., Slichter C.P. Double resonance probes for close frequencies. J. Magn. Reson. 135. 1998. P. 273–279.
3. Alderman D.W., Grant D.M. An efficient decoupler coil design which reduces heating in conductive samples in superconducting spectrometers. J. Magn. Reson. Vol. 36. 1979. P. 447–451.
4. Taber B., Zens A.P. Using Magnetic Coupling to Improve the 1H/2H Double Tuned Circuit. J. Magn. Reson. Vol. 259. 2015. P. 114–120.
5. Marsden B., Lim V., Taber B., Zens A. Improving the Mass-Limited Performance of Routine NMR Probes using Coupled Coils. J. Magn. Reson. Vol. 268 2016 P. 25–35.