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Friday, May 24, 2019

Fast 2D Data Collection - NOAH and NUS

One always strives to collect high quality 2D NMR data in a short period of time.  This is particularly important for samples of limited stability or perhaps for monitoring chemical reactions.  High magnetic fields and cryogenically cooled NMR probes have allowed for a higher signal-to-noise-ratio for a given quantity of sample, thereby reducing data collection time as a fewer number of scans are required.  Gradient enhanced 2D NMR data collection gained widespread use in the 1990s.  This represented a tremendous time saving as multi-step phase cycles required for coherence selection could be reduced or eliminated as they were replaced by pulsed field gradients.  Some pulse sequences which required 16 scans per increment to accommodate the necessary phase cycle could be run with a single scan for every increment with the use of pulsed field gradients, thus reducing the data collection time by a factor of 16.  Now, 2D data collection with coherence selection via pulsed field gradients is considered "conventional".  More recently, Non-Uniform Sampling (NUS) was introduced.  Data collection with this technique samples only a limited number of increments in the t1 domain.  The unsampled increments are calculated based on the sampled increments prior to Fourier transformation. The data collection time is reduced in accordance with the number of increments not sampled.  Recently, Kupce and Claridge1,2 have developed a technique where multiple 2D methods are concatenated in a single super pulse sequence employing a single relaxation delay. They have called the technique NOAH (NMR by Ordered Acquisition using 1H detection) The time saving of the NOAH technique compared to individually collected 2D spectra results from waiting a single relaxation delay for all experiments rather than a single relaxation delay for each separately acquired spectrum. The data for each spectrum is acquired in separate memory blocks which are separated after data collection allowing the data for each 2D method to be processed individually.  Very recently, both NUS and NOAH have been used together to further reduce data collection times3.  A comparison of the time saving is shown in the figure below for a sample of sucrose in DMSO-d6 collected on a Bruker AVANCE III HD 600 NMR spectrometer equipped with a cryoprobe.
All spectra were collected with 2 scans and a 1 second recycle time.  Individually, both NOAH and NUS offer a significant time saving but when used together they permit very fast, high quality data collection.  A COSY, edited HSQC and HMBC can be collected in a total time of only 4 minutes and 8 seconds.  Other ultra-fast techniques have been developed by others where an entire 2D spectrum is collected in less than one second.

1. Eriks Kupce and Tim D. W. Claridge.  Chem. Commun. 54, 7139 (2018).
2. Eriks Kupce and Tim D. W. Claridge. Angew. Chem. Int. Ed., 56, 11779 (2017).
3. Maksim Mayzel, Tim D. W. Claridge and Ä’riks Kupce. Bruker User Library (2018).