Line-Narrowing by Polychromatic Selective Spin-Locking in NMR

Wiame, Coline; Loutfi, Hadi; Sheberstov, Kirill; Bodenhausen, Geoffrey

doi:10.5194/mr-2026-9

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https://doi.org/10.5194/mr-2026-9

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06 Jul 2026

| 06 Jul 2026

Status: this preprint is currently under review for the journal MR.

Line-Narrowing by Polychromatic Selective Spin-Locking in NMR

Coline Wiame, Hadi Loutfi, Kirill Sheberstov, and Geoffrey Bodenhausen

Abstract. Selective Spin-Locking (SSL) of the magnetisation vectors of selected singlets or multiplets in high-resolution Nuclear Magnetic Resonance (NMR) spectra by mono- or polychromatic selective radio-frequency (RF) irradiation can reduce the line-widths to the limit given by homogeneous T₁_ρ relaxation. The resulting line-narrowing leads to an increase in signal height of spectra of ¹H, ¹⁹F, ³¹P or other nuclei that is in principle only limited by the ratio T₁_ρ/T₂*. At the same time, the scalar interactions that give rise to multiplets are decoupled, thus leading to an additional gain in signal height. Common one-dimensional (1D) experiments such as inversion recovery (IR) as well as the excitation and reconversion of Long-Lived States (LLS) by Spin-Lock Induced Crossing (SLIC) can significantly benefit from SSL.

Received: 03 Jun 2026 – Discussion started: 06 Jul 2026

Competing interests: At least one of the (co-)authors is a member of the editorial board of Magnetic Resonance.

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.

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Coline Wiame, Hadi Loutfi, Kirill Sheberstov, and Geoffrey Bodenhausen

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RC1:
'Comment on mr-2026-9', Dusan Uhrin, 19 Jul 2026 reply
The paper by Wiame et al presents a versatile approach to concurrently removing the fine structure of multiplets caused by homo- and/or heteronuclear coupling while achieving significant line-narrowing of NMR signals by eliminating the effects of inhomogeneous Bo. The paper is well-presented, with numerous examples, providing the NMR community with an excellent starting point for using this technique in a wide range of applications. As stated in the manuscript, "The data that support the findings of this study are available from the corresponding author upon request." I support the publication of this manuscript and like the authors to consider the points raised below.
The authors have provided typical parameters (pulse length, flip angle, power level, acquisition interval length, etc.), but they optimized these for different spectra (e.g., paragraphs 170 and 240). Could they provide some suggestions or rationale for choosing certain parameters? Additionally, what was the value of the after-pulse delay tau? The power deposition to the probe appears to be low; however, it would be desirable to put this into perspective using the technical specifications of current NMR probes.

Some spectra contain low-intensity peaks on each side of the collapsed singlet. Some comments on their origin would be useful. For example, in Figures 3 and 4, are these peaks present due to signal density caused by Bo inhomogeneity, but in others, such as Figures 5 and 8, perhaps not? The acquisition times used are short, hence the chunking artifacts are positioned much further away. Is it possible to link the presence of these peaks to some parameter of the pulse sequence?

The noise in the ³¹P SSL spectrum in Figure 11 is larger than in the reference spectrum. The stated peak height gains of approximately 3, 13, and 5 reflect the figure as presented, but perhaps not the SNR improvements, which should be considered when quantifying the signal enhancement.

Figure 9 demonstrates the use of polychromatic pulses, and it was interesting to see that the signals below 2.5 ppm preserved their multiplet structure, albeit with reduced intensities. The authors also demonstrated that applying SSL at frequencies 6.5 Hz apart is possible. It would be interesting to state how close ¹H multiplets can be for one of them to be collapsed by SSL, yielding a quantitative response without interference from the signal next door. Would the off-resonance signal still be a multiplet, preserving its structure?

In the latest iteration of SHARPER pulse sequences, the authors have moved away from using selective pulses during acquisition. Although functional (Uhrin and co-authors, https://doi.org/10.1039/d2cc01325h open_in_new), they unnecessarily shortened T_2eff, and a better outcome is achieved when the acquisition module consists of much shorter, hard pulses (Uhrin and co-authors, https://doi.org/10.1038/s41467-023-40130-2 open_in_new, https://doi.org/10.1021/jacs.5c11092 open_in_new). For the removal of J couplings only, typical chunk times (here AQ times) below 1 ms are used in combination with 90 or 180 deg pulses during the pulse/acquire period. It would be interesting to see how this approach compares to the SSL pulse in terms of the resulting signal linewidth, hence the signal hight and SNR.

Reflecting on the previous two points, applying SSL to a single frequency (provided that nearby signals are not affected) would present an advantage over SHARPER (even if some intensity drop is registered) as it would avoid the need for selective excitation.

In paragraph 250, the authors stated as a reason for narrowing ³¹P signals: "Surprisingly, the experimental line-widths were even narrower, since Δ1/2 = 0.7 Hz at all three sites, which may be due to the cancellation of broadening due to chemical exchange with Mg2+ ions." What is the mechanism of this narrowing? Chemical exchange usually broadens the signal. Can SSL remove exchange broadening?

In the concluding remarks, the authors list a number of areas with potential for the use of the SSL methodology, which is appreciated as it will accelerate the spread of this free-of-charge SNR enhancement methodology. As CPMG-type acquisition modules are widely used in applications where large Bo inhomogeneities need to be suppressed (e.g., the work of Prof Ville-Veikko Telkki and many others), it would be interesting to see if the SSL technique can also be applied in this field.

With regard to referencing prior work, since one of the applications of SSL presented in this paper is to LLS, work achieving similar outcomes in the LLS space could be mentioned. See Bodenhausen and co-authors, DOI: 10.1103/PhysRevLett.109.04760, and Levitt and co-authors, https://doi.org/10.1039/c2cp42553j open_in_new.

The approach taken by Andrew Simpson and co-workers (DREAMTIME, https://doi.org/10.1002/anie.202110044 open_in_new), a multi-focusing approach to increasing NMR sensitivity, should certainly be mentioned and discussed.

Reply
Citation: https://doi.org/10.5194/mr-2026-9-RC1
RC2:
'Comment on mr-2026-9', Dusan Uhrin, 19 Jul 2026 reply
The paper by Wiame et al presents a versatile approach to concurrently removing the fine structure of multiplets caused by homo- and/or heteronuclear coupling while achieving significant line-narrowing of NMR signals by eliminating the effects of inhomogeneous Bo. The paper is well-presented, with numerous examples, providing the NMR community with an excellent starting point for using this technique in a wide range of applications. As stated in the manuscript, "The data that support the findings of this study are available from the corresponding author upon request." I support the publication of this manuscript and like the authors to consider the points raised below.
The authors have provided typical parameters (pulse length, flip angle, power level, acquisition interval length, etc.), but they optimized these for different spectra (e.g., paragraphs 170 and 240). Could they provide some suggestions or rationale for choosing certain parameters? Additionally, what was the value of the after-pulse delay tau? The power deposition to the probe appears to be low; however, it would be desirable to put this into perspective using the technical specifications of current NMR probes.

Some spectra contain low-intensity peaks on each side of the collapsed singlet. Some comments on their origin would be useful. For example, in Figures 3 and 4, are these peaks present due to signal density caused by Bo inhomogeneity, but in others, such as Figures 5 and 8, perhaps not? The acquisition times used are short, hence the chunking artifacts are positioned much further away. Is it possible to link the presence of these peaks to some parameter of the pulse sequence?

The noise in the ³¹P SSL spectrum in Figure 11 is larger than in the reference spectrum. The stated peak height gains of approximately 3, 13, and 5 reflect the figure as presented, but perhaps not the SNR improvements, which should be considered when quantifying the signal enhancement.

Figure 9 demonstrates the use of polychromatic pulses, and it was interesting to see that the signals below 2.5 ppm preserved their multiplet structure, albeit with reduced intensities. The authors also demonstrated that applying SSL at frequencies 6.5 Hz apart is possible. It would be interesting to state how close ¹H multiplets can be for one of them to be collapsed by SSL, yielding a quantitative response without interference from the signal next door. Would the off-resonance signal still be a multiplet, preserving its structure?

In the latest iteration of SHARPER pulse sequences, the authors have moved away from using selective pulses during acquisition. Although functional (Uhrin and co-authors, https://doi.org/10.1039/d2cc01325h open_in_new), they unnecessarily shortened T_2eff, and a better outcome is achieved when the acquisition module consists of much shorter, hard pulses (Uhrin and co-authors, https://doi.org/10.1038/s41467-023-40130-2 open_in_new, https://doi.org/10.1021/jacs.5c11092 open_in_new). For the removal of J couplings only, typical chunk times (here AQ times) below 1 ms are used in combination with 90 or 180 deg pulses during the pulse/acquire period. It would be interesting to see how this approach compares to the SSL pulse in terms of the resulting signal linewidth, hence the signal hight and SNR.

Reflecting on the previous two points, applying SSL to a single frequency (provided that nearby signals are not affected) would present an advantage over SHARPER (even if some intensity drop is registered) as it would avoid the need for selective excitation.

In paragraph 250, the authors stated as a reason for narrowing ³¹P signals: "Surprisingly, the experimental line-widths were even narrower, since Δ1/2 = 0.7 Hz at all three sites, which may be due to the cancellation of broadening due to chemical exchange with Mg2+ ions." What is the mechanism of this narrowing? Chemical exchange usually broadens the signal. Can SSL remove exchange broadening?

In the concluding remarks, the authors list a number of areas with potential for the use of the SSL methodology, which is appreciated as it will accelerate the spread of this free-of-charge SNR enhancement methodology. As CPMG-type acquisition modules are widely used in applications where large Bo inhomogeneities need to be suppressed (e.g., the work of Prof Ville-Veikko Telkki and many others), it would be interesting to see if the SSL technique can also be applied in this field.

With regard to referencing prior work, since one of the applications of SSL presented in this paper is to LLS, work achieving similar outcomes in the LLS space could be mentioned. See Bodenhausen and co-authors, DOI: 10.1103/PhysRevLett.109.04760, and Levitt and co-authors, https://doi.org/10.1039/c2cp42553j open_in_new.

The approach taken by Andrew Simpson and co-workers (DREAMTIME, https://doi.org/10.1002/anie.202110044 open_in_new), a multi-focusing approach to increasing NMR sensitivity, should certainly be mentioned and discussed.

Reply
Citation: https://doi.org/10.5194/mr-2026-9-RC2

Coline Wiame, Hadi Loutfi, Kirill Sheberstov, and Geoffrey Bodenhausen

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Short summary

Selective Spin-Locking (SSL) of the magnetization vectors of selected singlets or multiplets in high-resolution Nuclear Magnetic Resonance (NMR) spectra by mono- or polychromatic selective radio-frequency (RF) irradiation can reduce the linewidths to the limit given by homogeneous T_1ρ relaxation in the rotating frame.


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