SOGGY: Solvent-optimized double gradient spectroscopy for water suppression. A comparison with some existing techniques

doi:10.1016/j.jmr.2006.10.014

Journal of Magnetic Resonance

Volume 184, Issue 2, February 2007, Pages 263-274

https://doi.org/10.1016/j.jmr.2006.10.014 Get rights and content

Abstract

Excitation sculpting, a general method to suppress unwanted magnetization while controlling the phase of the retained signal [T.L. Hwang, A.J. Shaka, Water suppression that works. Excitation sculpting using arbitrary waveforms and pulsed field gradients, J. Magn. Reson. Ser. A 112 (1995) 275–279] is a highly effective method of water suppression for both biological and small molecule NMR spectroscopy. In excitation sculpting, a double pulsed field gradient spin echo forms the core of the sequence and pairing a low-power soft 180°(−x) pulse with a high-power 180°(x) all resonances except the water are flipped and retained, while the water peak is attenuated. By replacing the hard 180° pulse in the double echo with a new phase-alternating composite pulse, broadband and adjustable excitation of large bandwidths with simultaneous high water suppression is obtained. This “Solvent-Optimized Gradient–Gradient Spectroscopy” (SOGGY) sequence is a reliable workhorse method for a wide range of practical situations in NMR spectroscopy, optimizing both solute sensitivity and water suppression.

Introduction

The problem of efficient, simple, and robust suppression of strong solvent resonances, in particular the strong water resonance that dominates the spectra of molecules in aqueous solution, has been a focus of attention in the NMR spectroscopy community for many years, and many different methods have been proposed. Some of these methods, such as saturation [1], [2] of the H₂O resonance during the relaxation delay that precedes the pulse sequence by a low-intensity continuous irradiation are unsuitable when the solute has protons that exchange with the solvent, as the lines from these exchangeable protons may be attenuated greatly [3] by saturation transfer [4]. The same issues arise in water-eliminated FT (WEFT) pulse sequences [5], [6]. Pulsed methods that avoid exciting the water [7], [8], [9], [10], [11], [12] avoid the saturation problem, but high suppression can be hard to achieve reliably, and the excitation profile can also result in unwanted attenuation of solute resonances nearby the water, or outside the bandwidth of the excitation.

By far, the most often-used water suppression techniques use pulsed field gradients (PFGs) to rapidly attenuate the H₂O resonance, winding any transverse magnetization into a tight spatial helix oriented along the PFG axis, and causing the integrated magnetic flux through the NMR receiver coil to nearly vanish for the water magnetization. These include gradient-enhanced coherence transfer pathway selection [13], WATERGATE [14], WATERGATE using only hard pulses and delays [15], excitation sculpting [16], WET [17], modifications of WATERGATE [18], and PURGE [19]. All these PFG-based methods have their individual strengths and weaknesses, and may not be applicable in all situations depending on the details of underlying pulse sequence and characteristics of the sample itself. In some cases, the water suppression performance can vary, or be apparently erratic using the same sequence on different samples. This may necessitate somewhat tedious trial-and-error optimization of the water suppression to get usable spectra, and is worthwhile avoiding if possible.

By analyzing the PFG-based schemes using a simple theory, and exploring the possible factors that influence performance, we arrive at an improved water suppression sequence based on the previous excitation-sculpting [16] template. This solvent-optimized double gradient spectroscopy (SOGGY) sequence seems to be a good compromise between convenience and performance in many practical situations.

Section snippets

Theory

In 1995, Hwang and Shaka [16] outlined a general and flexible way to suppress the strong water resonance using a double pulsed field gradient spin echo (DPFGSE) sequence: G₁-S-G₁-G₂-S-G₂. Here S denotes an arbitrary sequence of radiofrequency (RF) pulses and G_i are mathematically independent PFGs. The analysis, focused on a single uncoupled resonance line and neglecting any dynamics that may arise from the presence of the spatial magnetization helix itself or radiation damping (see below),

Experimental

Water suppression would certainly be simpler if it were done strictly within the digital confines of computer simulation. However it is, of course, carried out in the lab using imperfect software and hardware, and sometimes in environments that are less than perfect. All practical matters related to spectrometer performance, unexpected effects from intense lines at high fields, and the balance between convenience and robustness on one hand, and absolute performance on the other, are best

Radiation damping and anomalous refocusing

Methods that manipulate z-magnetization, such as WET [17] seem anecdotally to require extensive optimization for best performance. Such optimization is partly necessitated by having a number of adjustable parameters. But there is also another difficulty when working with repetitive sequences of the type α_i–G_i (exciting transverse magnetization and then winding it into a tight helix with a gradient) as in WET. The potential problem has been elegantly elucidated by Lin et al. [23]. Calculations

Conclusions

Solvent suppression remains an area of interest for NMR technique development, and the best compromise will be dictated by the details of the experiment, spectrometer hardware limitations, and other considerations such as the time it takes to optimize the performance on each sample, and the degree of suppression required. Many solvent suppression schemes have been proposed, and many of them are capable of excellent performance when adjusted by direct interactive observation of the residual

Acknowledgments

This research was made possible by support from the National Institutes of Health, GM- 66763, and by a UC Discovery Grant BIO05-10533. K.J.D. was supported by the National Institutes of Health, National Research Service Award 5 T15 LM007443 from the National Library of Medicine. B.D.N. thanks the UCI School of Physical Sciences for partial support.

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