NMR Relaxation Measurements Based On Real Time Pure Shift Techniques And Its Application On Complex Samples

As an effective method for material detection and analysis,nuclear magnetic resonance(NMR)has been rapidly devoloped and widely used due to its non-invasive detections of molecular-level information and other characteristics.The combination of relaxation times and other detection parameters,such as chemical shifts and spin-spin scalar(J)couplings,endows NMR spectroscopy with its superiority over other detection methods in structural study,composition analysis and molecular dynamics research.Both longitudinal spin-lattice relaxation(T1)and transverse spin-spin relaxation(T2)are essential parameters for magnetic reasonance spectroscopy and imaging,and they reveal valuable information for studying molecular dynamics in NMR applications.In general,conventional T1 and T2 relaxation measurements in commercial NMR instruments adopt inversion-recovery(IR)and Carr-Purcell Meiboom-Gill(CPMG)sequences,respectively.These two methods are based on the conventional 1D 1h NMR spectra.According to a series of acquired signals with their intensities varied as the inversion recovery delay in IR experiments or spin echo period in CPMG experiments,T1 or T2 relaxation time can be calculatedd by fitting the corresponding exponential equations.For simple samples that yield well-resolved resonances in 1D proton(1H)NMR,it is easy to assign all peaks according to corresponding chemical shift sites and track their intensity variations.However,accurate relaxation measurements from conventional 1D proton spectra are generally subject to challenges of spectral congestion caused by J coupling splittings,particularly for measurements on complex samples that contain numerous protons in different chemical environments.Besides spectral congestion,magnetic field inhomogeneity,caused by inadequate magnetic field shimming and heterogeneous sample conditions,poses another challenge for accurate relaxation measurements in practical applications.Although signal evolutions during inversion recovery delay in IR experiments for T1 measurements and spin-echo delay in CPMG experiments for T2 measurements are free of field inhomogeneity,the acquisition periods in these two relaxation experiments cannot avoid the field inhomogeneity effect.Here,we presented an NMR relaxation method,real-time ZS-IR/CPMG,based on the combination of the real-time ZS pure shift technique with IR and CPMG relaxation schemes,for T1 and T2 relaxation measurements on complex samples even under inhomogeneous magnetic field conditions.Benefitting from the slice-selective property of the ZS pure shift technique,the proposed method can avoid two major challenges in conventional IR/CPMG experiments,i.e.,spectral congestion caused by J coupling splitting and spectral line broadening originating from field inhomogeneity primarily along the static field B0 direction,thus achieving accurate relaxation measurements.Moreover,since new methods adopt real-time acquisition for pure shift extraction in a single scan,they hold the same acquisition efficiency as conventional IR and CPMG experiments.Theoretical analyses are performed to demonstrate the effectiveness of the proposed method for accurate relaxation measurements on complex samples and its practicability to nonideal magnetic field conditions.Then,experiments on three types of samples,a simple chemical solution of 1-bromobutane,a complex sample of quinine,and a real sample of azithromycin,were performed to show the abil ity of the real-time ZS-IR/CPMG for acquiring T1/T2 relaxation information from crowded and broadening NMR resonances.Both theoretical and experimental results show that the real-time ZS-IR/CPMG may provide an effective and complementary tool to conventional IR/CPMG for relaxation measurements with extensive applications to complex sample systems or even under non-ideal magnetic field conditions.