the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Performance of cross-polarization experiment at conditions of radiofrequency field inhomogeneity and slow to ultrafast MAS
Andrej Šmelko
Jan Blahut
Zdeněk Tošner
Abstract. In this manuscript, we provide an analytical description of the performance of the cross-polarization experiment, including linear ramp and adiabatic tangential sweep modifications, using effective Hamiltonians and simple rotations in 3D space. It is shown that radiofrequency field inhomogeneity induces a reduction of the transfer efficiency at increasing MAS frequencies for both the ramp and the adiabatic CP experiments. The effect depends on the ratio of the dipolar coupling constant and the sample rotation frequency. In particular, our simulations show that for small dipolar couplings (1 kHz) and ultrafast MAS (above 100 kHz) the transfer efficiency is below 40 % when extended contact times up to 20 ms are used and relaxation losses are ignored. New recoupling and magnetization transfer techniques that are designed explicitly to account for inhomogeneous RF fields are needed.
Andrej Šmelko et al.
Status: open (until 21 Jun 2023)
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CC1: 'Comment on mr-2023-9', Tom Barbara, 24 May 2023
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This is a nice overview and critque of the MAS Hartman-Hahn matching problem in the presence of RF inhomogeneity. One thing I did not see was a discussion of coil technolgy where inserts can be fabricated with better axial homogeneity (variable pitch etc. etc.). This has a fairly long history going back to Haeberlen and perhaps even earlier. More recently I believe Rachael Martin worked on building coils and gave some examples.
Citation: https://doi.org/10.5194/mr-2023-9-CC1 -
AC1: 'Reply on CC1', Zdeněk Tošner, 28 May 2023
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Thank you for this important comment. A comprehensive review on the design of rf coils was given in our article by Tošner et al., 2017, DOI: 10.1016/j.jmr.2017.09.002. A similar review is given in the paper by Kelz et al. 2019, DOI: 10.1016/j.jmr.2019.06.008. For the revised version of the manuscript, we will include a paragraph that briefly describes different rf coil designs. However, a detailed quantitative analysis of the CP transfer efficiency for all coil designs is beyond the scope of the paper. The primary aim here is to include the inhomogeneity of the rf field in the analytical description of Hartmann-Hahn matching. The solenoid is the most widespread coil design that is employed by all vendors in most of the MAS solid-state NMR probes. An expansion of the presented approach to nonsolenoidal coil designs straightforward by substituting the relevant field profile into the function ξ(z). Our first suggestion for the new paragraph that shall be placed towards the end of Introduction is as follows.
In the literature, there are several strategies to design NMR coils that are compatible with MAS and provide improved rf field homogeneity compared to the standard solenoid. Variable pitch coils were proposed by Idziak and Heaberlen (1982) and recently explored by Martin et al. (Kelz, 2019), who proposed 3D-printed templates for easy manufacturing. An interesting alternative was proposed by Privalov et al. (1996) using variable ribbon width coils that improve rf homogeneity not only along the coil axis but also in the radial direction. Another type of coils was designed for the so-called E-free probes, which minimize the sample heating effects of the applied high-frequency irradiation. These coils also show improved rf field homogeneity (Krahn et al., 2008). All strategies have benefits as well as disadvantages. Variable-pitch coils provide a lower rf conversion ratio and thus lower sensitivity. E-free probes consist of separated coils for the high- and low-frequency rf channels, which potentially leads to different RF field profiles and imbalance of these channels. Worth mentioning is the recent technology of the Cryoprobe CP-MAS probe that is reported to provide excellent rf field homogeneity (Hassan et al., 2020).
Citation: https://doi.org/10.5194/mr-2023-9-AC1
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AC1: 'Reply on CC1', Zdeněk Tošner, 28 May 2023
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Andrej Šmelko et al.
Andrej Šmelko et al.
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