M. R. Bendall

Depth and refocusing pulses for use with inhomogeneous radiofrequency coils in nuclear magnetic resonance spectroscopy

In a pulsed nuclear magnetic resonance method of analysis, in which a single radiofrequency pulse is applied using an irradiation coil which provides an inhomogeneous radiofrequency field across the sample volume, the inhomogeneity of this field ensures that some sample regions experience a radiofrequency pulse which is closer to a π/2 pulse angle than for other regions, resulting in a larger signal intensity from the former regions as compared to the latter. If a second pulse is applied after the first, prior to acquisition of the signal, and if the phase of this pulse is alternated through the four phases: 0°, 90°, 180° and 270°, during a series of transients, then, provided the receiver phase is changed by 180° when the second pulse phase is changed by 90°, the signal intensity will accumulate from regions where the second pulse angle is π radians and there will be discrimination against regions where the second pulse angle differs markedly from π radians. This discrimination towards certain sample regions can be improved by adding further pulses after the second pulse provided the phase of each additional pulse is alternated through all four 90° phase shifts, during a series of transients, independently of the phase alternation of the other pulses, and provided that for each individual 90° phase shift, the receiver phase is changed by 180°. In its simplest form, such a series of pulses may be represented by θ;(2θ[±x,±y]) n indicating n phase alternated pulses additional to the first excitation pulse, where these additional pulses are twice as long as the first pulse.

A pulse sequence for use in performing nuclear magnetic resonance spectroscopy

A liquid sample comprising a system made up of two types of heteronuclei is pulsed in a particular manner in a nuclear magnetic resonance (NMR) experiment so that the resulting NMR signal from the one type of heteronucleus depends on the scalar-coupled interaction with the other type of heteronucleus. The sequence of radiofrequency pulses is such that the two types of heteronuclei interact via the phenomenon of polarization transfer and by the phenomenon of the correlated motion of the two types of heteronuclei in the transverse plane of the doubly rotating reference frame. The combination of these two phenomena in the one pulse sequence provides NMR signals which are easily made less dependent on the exact magnitude of the heteronuclear scalar coupling constant. The coupled NMR signals for polarization transfer from a system of multiple spin-half nuclei have multiplet components in the normal ratio. Consequently the pulse sequence provides an improved method for obtaining chemical structural information. Applications include the provision of accurate edited subspectra; polarization transfer between any number of one type of heteronucleus of any spin number and any number of a second type of heteronucleus of any spin number; and two-dimensional NMR spectroscopy.

Signal-to-noise considerations and θ-pulse defects when editing 13C spectra. Comparison of DEPT and SEMUT

Abstract A 13 C spin-echo sequence with a single variable θ proton pulse coincident with the carbon refocusing pulse has been proposed which provides complete editing of a 13 C spectrum into separate quaternary, methine, methylene, and methyl subspectra. This sequence, named SEMUT, has been found to be as accurate as the DEPT sequence with regard to errors in the subspectra resulting from divergence of the single-bond 13 C 1 H coupling constant from the value assumed in setting the DEPT and SEMUT delay periods. In this paper, however, it is found that the similarity in the signal dependence of methine and methyl groups as a function of the θ pulse, and the additional complexity of the linear combinations of SEMUT spectra required for editing, leads to higher noise levels in the final subspectra for the SEMUT method as compared to the DEPT/q-only method. It is also shown that the basic SEMUT method is more sensitive to inhomogeneity and missetting of the θ pulse. Such inaccuracies, combined with the more complicated linear combinations, will make basic SEMUT more difficult to use. The problem of sensitivity to θ pulse missetting and inhomogeneity can be alleviated by a modification to the basic SEMUT sequence which unfortunately also removes its advantage as a minimal pulse number sequence not requiring phase control, but which gives very good insensitivity to moderate inhomogeneity. It is also found that other recently proposed editing methods have little, if any, more to offer than DEPT.