Evaluation of Adiabatic Frequency-Modulated Schemes for Broadband Decoupling in Isotropic Liquids

✍ Scribed by Riqiang Fu; Geoffrey Bodenhausen

• Publisher: Elsevier Science
• Year: 1996
• Tongue: English
• Weight: 146 KB
• Volume: 119
• Category: Article
• ISSN: 1064-1858
• DOI: 10.1006/jmra.1996.0062

✦ Synopsis

Heteronuclear scalar couplings lead to extensive line ceed a threshold, CHIRP-95 is best used with apodization of the RF amplitude profile at the beginning and end of the broadening in high-resolution NMR spectra, unless they are frequency sweep (13, 14). In independent work, Kupc ˘e and decoupled by suitable radiofrequency irradiation schemes.

Freeman have shown that inversion can be achieved Broadband decoupling of nuclei with spin-1 2 represents an adiabatically with pulses that have an RF amplitude shaped increasing challenge for spectrometers operating at higher in the form of a sausage. The authors suggested that these and higher magnetic fields. Most decoupling methods rely pulses should be combined with a 20-step phase cycle comon a combination of inversion pulses R with phase cycles posed of a 5-step phase cycle and an MLEV-4 supercycle. and supercycles introduced by Levitt and Freeman (1) and

Because the samples and the scalar coupling constants conby Waugh (2). The decoupling performance strongly desidered in the studies of Bendall ( ) and those of Freeman pends on the performance of the basic inversion element R.

and Kupc ˘e (15, 16) could not be compared with those inves-The well-known MLEV (1), WALTZ (3), and GARP (4) tigated in our laboratory (13, 14), and because different crisequences rely on composite pulses composed of phaseteria and line-broadening parameters were used, there apshifted rectangular pulses with a constant carrier frequency.

pears to be a need for a comparison between these methods.

Such composite pulses cannot produce an adequate inversion

This Communication describes experiments and simulations over a very broad bandwidth. Frequency-switched inversion to evaluate the performance of various adiabatic frequencypulses used in the MPF decoupling schemes of Fujiwara et modulated decoupling schemes.

al. (5) greatly extend the inversion bandwidth, because the

Figure shows various frequency-modulated wave forms carrier frequency is stepped through the spectrum. Various that have proven to be useful for adiabatic broadband inverfrequency-modulated pulses have been described (6-8) for sion of longitudinal magnetization. The RF carrier, repreachieving adiabatic inversion over large bandwidths. When sented by a dotted line, is swept over a frequency range used for decoupling, such pulses have the dual advantage of Dn sweep . The simple linear chirp of Fig. with a constant increased bandwidth and improved tolerance to variations RF amplitude has the advantage that there is no need for a in RF amplitude.

linear amplifier or a programmable attenuator, but the disad-Efficient decoupling requires that adiabatic frequencyvantage that the adiabatic condition is not easily fulfilled modulated pulses be combined with phase cycles (9) and over the full frequency sweep, particularly if the RF amplisupercycles (10). Starc ˇuk et al. (11) have proposed the tude exceeds a threshold. Adiabaticity can be easily im-DAP-16 scheme in which hyperbolic secant pulses are comproved by apodizing the RF amplitude at the beginning and bined with the MLEV-16 supercycle (6, 7). In the STUD the end of the chirp (8). Many different functions can be scheme proposed by Bendall (12), similar hyperbolic secant used for this purpose, and the choice is probably not very pulses are used in combination with a five-step cycle and the critical to the outcome of the experiments. Bo ¨hlen et al. MLEV-4 supercycle. In the so-called CHIRP-95 decoupling (8) have proposed that the initial and final parts of the RF scheme (13, 14), inversion is achieved adiabatically by a amplitude profiles be multiplied by suitable segments of sine linear frequency sweep which is combined with an 80-step waves. Freeman and Kupc ˘e ( ), guided by different aescycle composed of a five-step phase cycle (9) and an MLEVthetic criteria, proposed the ''sausage'' (WURST) wave 16 supercycle (10). Normally, when the RF amplitude is form, which is obtained by multiplying the RF amplitude weak, there is no need to modulate the RF amplitude in profile by the nth power of a sine wave. When a typical CHIRP-95 decoupling. However, for RF amplitudes that exvalue of n Å 20 is used, and when the central portion of the sausage is ''stretched'' ( ), it is difficult to see a significant difference between the shapes proposed by Bo ¨hlen and by * Also a member of the