Understanding J‐Modulation during Spatial Encoding for Sensitivity‐Optimized Ultrafast NMR Spectroscopy

Abstract

Ultrafast (UF) NMR spectroscopy is an approach that yields 2D spectra in a single scan. This methodology has become a powerful analytical tool that is used in a large array of applications. However, UF NMR spectroscopy still suffers from an intrinsic low sensitivity, and from the need to compromise between sensitivity, spectral width, and resolution. In particular, the modulation of signal intensities by the spin-spin J-coupling interaction (J-modulation) impacts significantly on the intensities of the spectral peaks. This effect can lead to large sensitivity losses and even to missing spectral peaks, depending on the nature of the spin system. Herein, a general simulation package (Spinach) is used to describe J-modulation effects in UF experiments. The results from simulations match with experimental data and the results of product operator calculations. Several methods are proposed to optimize the sensitivity in UF COSY spectra. The potential and drawbacks of the different strategies are also discussed. These approaches provide a way to adjust the sensitivity of UF experiments for a large range of applications.

Fokker-Planck formalism in magnetic resonance simulations

“…Figure 5 demonstrates the effect of diffusion and flow on the ultrafast COSY spectrum of a simple threespin system. The J-modulation effects -a quantum mechanical phenomenon that essentially requires full density matrix treatment in the spin subspace -are clearly visible [65]. Another interesting conclusion is that fast diffusion and fast flow can cancel each other's deleterious effects to some extent.…”

“…It is clear that frequency-swept pulses are easier to discretise in frequency-amplitude coordinates. A practical example of Equation (26) enabling the simulation of a sophisticated spatio-temporal NMR experiment is ultrafast NMR spectroscopy [61,65]. The processes that make ultrafast NMR possible take place in the direct product  of the one-dimensional real space  and the spin state space :…”

“…The duration of the coherence selection gradient pulses was 1 ms. Spatial encoding was achieved with a WURST pulse with a smoothing factor of 40 and a bandwidth of 10 kHz acting over a 15 ms time interval and discretised over a 1000-point time grid. Technical details about ultrafast COSY simulations in Spinach are available in[65].…”

Ultrafast double-quantum NMR spectroscopy with optimized sensitivity for the analysis of mixtures

“…In particular, because of the "constanttime" nature of the spatial encoding block, the duration of the evolution period, T has a strong impact on peak amplitudes and in turn sensitivity, as commonly found in UF NMR. 37,39 Here we, extend the framework developed for UF-COSY to assess this influence using theory and simulation. The case of an AMX system (i.e., a system of 3 weakly coupled spins I = ½), where X coupled to spins A and M, is of particular interest, as it is the minimal set of spins that displays a modulation of correlation-peak intensities by the J coupling interaction during the encoding period.…”

“…The Spinach simulation package 40 has been adapted to include the simulation of spatial encoding schemes. 37 This framework can first be assessed by comparing simulated results to analytical calculations for the AMX spin system. Figure 2 shows an example of simulated DQS spectrum; J-modulation can be obtained by integration of the peak volumes in simulated spectra obtained for a series of values of .…”