Selective-excitation-based method for measurement of NMR relaxation time.

Background: Relaxation time provides invaluable insights into the molecular structure, interactions, and dynamics in nuclear magnetic resonance spectroscopy. However, conventional relaxation-time measurement techniques produce inaccurate relaxation times when the spectral peaks overlap because of the narrow chemical-shift range and J-coupled splitting. While the combination of pure-shift methods can solve this issue, they are not widely used due to their inherent drawbacks such as low sensitivity and long acquisition time. There is a great need for a feasible and sensitive method to measure the relaxation time for overlapping peaks. (87).

Results: This study proposes a new method that combines selective excitation with a conventional relaxation-time measurement method, named GEM-IR/CPMG, to accurately measure the longitudinal and transverse relaxation times in the samples with overlapping peaks. The method has a similar acquisition time as the conventional method with small sensitivity loss. The feasibility and effectiveness of the method were demonstrated through experiments using three types of samples: 1-bromobutane, a mixture of butanol and butyric acid, and 17β-estradiol. The results show that the relaxation times measured by this method are in general agreement with the results of the conventional method. In addition, to demonstrate the advantages of the method for low-concentration samples, a sample of estradiol at 8 mM was measured with the results obtained matching the high concentration. (125).

Significance: The GEM-IR/CPMG method eliminates interference from overlapping peaks in proton relaxation-time measurement and preserves the crucial coupling information of the sample, thus allowing accurate measurement of the relaxation time. Moreover, it selectively excites the spin of interest in a single scan, demonstrating a minor loss of spectral sensitivity and facilitating the measurement of low-concentration samples, making it widely applicable to chemical analyses. (62).