Abstract :
[en] Nuclear Magnetic Resonance (NMR) spectroscopy is widely employed for analysis due to its inherent advantages compared to other techniques, mainly mass spectrometry. NMR provides a substantial amount of information, is non-destructive, cost-effective, reproducible, non-specific and requires few sample preparation. Collectively, these advantages position NMR as a key method for a diverse array of applications. The principal NMR pulse sequence used is the one-dimension (1D) proton experiment because of its rapidity and the ease of extracting quantitative information. However, the spectra generated by this pulse sequence can suffer from overlapping signals, impeding the identification and quantification of distinct peaks. One strategy to tackle this limitation is the implementation of multidimensional experiments. These methods consist in acquiring multiple 1D spectra and therefore impose a huge time increase, which might reduce their appeal to researchers. The pursuit of ways to expedite these multidimensional experiments has thrived over the past decades, and we identified two promising techniques: Non-Uniform Sampling (NUS) and Ultra-Fast (UF). We have implemented these methods for two-dimensional (2D) experiments: H-H COSY, H-C-HSQC, and H-C HMBC.
NUS consists in acquiring only a fraction of the 1D spectra necessary for constructing the 2D spectrum. The missing information is subsequently recalculated to generate the 2D spectrum that contains the same information amount as a spectrum acquired without NUS. NUS is defined by a percentage that indicates the amount of recorded data. A lower percentage results in less recorded data, shortened experiment time and a higher amount of reconstructed data. This technique is applicable to all pulse sequences, is easy to implement and offers a time reduction. However, a limitation arises when the NUS percentage becomes too low. As the percentage is lowered, the generation of artefacts becomes more pronounced. We successfully employed 10% of NUS without problems for HSQC and with minor issues for COSY and HMBC, where a few artefacts were present. NUS proves to be a promising method for reducing the experimental time by a factor of 10. Even more interesting, the time saved can be repurposed to gather additional information, resulting in a spectrum with an improved signal-to-noise ratio within the same experimental duration.
UF is a complex method in which the 1D spectra necessary for constructing the 2D spectrum are not acquired sequentially over time, but rather simultaneously across space. This is achieved by dividing the sample into distinct regions, each one used for generating a single 1D spectrum. All these 1D spectra are detected simultaneously, and a dedicated processing algorithm is applied to reconstruct the 2D spectrum. This technique offers a significant time reduction, allowing experiments lasting hours to be acquired in under a minute. However, several limitations temper the use of this method. Firstly, due to the sample being partitioned into regions, UF is much less sensitive compared to a “conventional” experiment. Secondly, the division into regions imposes limitations on the spectral window that can be analysed. While it is possible to expand this window, it would result in an increase in time. Consequently, UF is well-suited for H-H COSY experiments but remains constrained in its application to H-C HSQC or H-C HMBC pulse sequences because of the too wide spectral window of 13C. Lastly, this approach generates artefacts that can hinder the spectrum interpretation. In summary, UF is a promising technique offering an unparalleled time reduction, but its applicability is restricted by certain drawbacks.
