Improving accuracy in DOSY and diffusion measurements using triaxial field gradients

Abstract

NMR measurements of diffusion in solution, whether primarily quantitative, or, (as in DOSY, Diffusion-Ordered Spectroscopy) qualitative, can be particularly demanding. Here we show how the use of appropriate transverse (x, y) pulsed field gradients, orthogonal to the more usual z axis pulsed field gradient applied along the long axis of the sample, can greatly reduce two important sources of systematic error in diffusion experiments. These are the extra signal attenuation caused by sample convection, and gradient-dependent signal phase shifts caused by the magnetic field and field-frequency lock disturbances generated by field gradient pulses.

Selective measurement of 1H–1H scalar couplings from crowded chemical shift regions: Combined pure shift and spin‐echo modulation approach

“…The PSSE scheme is the band‐selective analog of 2DJ‐PSYCHE, wherein the hard excitation and refocusing pulses are simply replaced by the corresponding band‐selective pulses. As the steroid type of molecules did not exhibit any significant triple quantum artifacts in the 2DJ‐PSYCHE spectra, we could get good data without employing orthogonal gradients (X and Y), as were required in the original publication, and other applications . In the BSR‐PSSE, the antiphase magnetization component associated with the spin‐echo spectra has been suppressed by using the Chirp pulse in the presence of pulsed field gradients (G1), which is akin to that reported earlier by Luy et al11a and Kiraly et al Herein, the Chirp pulse introduces an offset distribution and imposes different ZQ evolutions over the sample volume in a single scan, which leads to a complete suppression of all the ZQ/antiphase components and results in clean absorption mode spin‐echo spectra 11a.…”

Application of pure shift and diffusion NMR for the characterisation of crude and processed pyrolysis oil

“…Often, systematic errors in diffusion coefficient arise from spatial non-uniformity of the pulsed field gradients, sample convection or disturbance of the field frequency lock as extensively detailed by Kiraly et al 30 The use of transverse pulsed field gradients could help to eliminate these sources of error.…”

Practical aspects of real‐time pure shift HSQC experiments

“…For example, DMSO‐ d 6 is generally preferable to CDCl 3 as a solvent for real‐time pure shift NMR experiments. A simpler, and very effective, solution is to apply the field gradient pulses perpendicular to the commonly used vertical axis, but this requires special hardware (triaxial field gradients).…”

“…The pulse programme can be used with any number of data chunks; consecutive chunks use successive J ‐refocusing elements with gradient pulses of different strengths as set out in the pulse sequence diagram of Figure . The field‐frequency lock was gated (lock hold) to restrict sampling of the deuterium lock signal to the relaxation delay (5 s), as described by Kiraly et al This functionality can be turned off if the lack of sampling of the lock signal during the pulse sequence causes more problems than the gradient pulses applied (e.g., when working in protic solvents and/or with a small number of chunks). The carbon broadband inversion pulses (140 μs with B 1 = 17.8 kHz) and heteronuclear WURST‐40 decoupling (1.2 ms WURST‐40 pulses with a bandwidth of 10 kHz) waveforms were generated using standard Agilent procedures.…”