Approach to High-Resolution ex Situ NMR Spectroscopy

Abstract

Nuclear magnetic resonance (NMR) experiments are typically performed with samples immersed in a magnet shimmed to high homogeneity. However, there are many circumstances in which it is impractical or undesirable to insert objects or subjects into the bore of a high-field magnet. Here we present a methodology based on an adaptation of nutation echoes that provides resolved spectra in the presence of matched inhomogeneous static and radiofrequency fields, thereby opening the way to high-resolution ex situ NMR. The observation of chemical shifts is regained through the use of multiple-pulse sequences of correlated, composite z-rotation pulses, producing resolved NMR spectra of liquid samples.

High-resolution NMR of anisotropic samples with spinning away from the magic angle

“…The idea, first proposed by Andrew [4], of spinning the field at the magic angle represents an alternative for static samples located inside (in-situ) or outside (ex-situ) of the magnet and has recently been the focus of considerable attention [5,6]. Here the constraint of reaching the magic angle represents serious engineering problems, since in the case of spinning the field electromagnetically or rotating a permanent magnet, the required power increases substantially with the angle [7].…”

Two-Dimensional High-Resolution NMR Spectra in Matched B0 and B1 Field Gradients

“…A brief survey of the experimental setup is given in Ref. (19). In order to emulate exsitu conditions inside the magnet bore, we produced a static field gradient by applying current to the x-gradient coils of the probehead.…”

“…between nuclei of the same type, as in Fig. 1e, a refocused COSY spectrum is obtained, as described previously (19).…”

“…2b were tested with a sample of ethanol in a tube of 10mm diameter, arranged as shown in Ref. (19). A static B 0 field gradient of 0.4 mT/cm was applied.…”

“…In a recent publication we presented an approach towards high resolution ex-situ NMR spectroscopy (19) that is based on nutation echoes (9,10,20) due to rf field gradients which are perpendicular to the static field B 0 and proportional to the static field gradient. For sample regions where the static field and the rf field (B 1 ) gradient are matched, or where the effective rf gradient can be matched to the static field by appropriate composite pulses, the linebroadening caused by B 0 inhomogeneities can be refocused during the acquisition period by applying a train of rf gradient pulses (see below).…”