Time-Domain Frequency Correction Method for Averaging Low-Field NMR Signals Acquired in Urban Laboratory Environment
QIU Long-Qing1,3, LIU Chao1,3,4, DONG Hui1,3, XU Lu1,3, ZHANG Yi2,3, Hans-Joachim Krause2,3, XIE Xiao-Ming1,3**, Andreas Offenhäusser2,3
1State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Shanghai 200050 2Peter Grünberg Institute (PGI-8), Forschungszentrum Jülich, Jülich, D-52425, Germany 3Joint Research Laboratory on Superconductivity and Bioelectronics, Collaboration between CAS-Shanghai and Forschungszentrum Jülich, Shanghai 200050 4Graduate University of the Chinese Academy of Sciences, Beijing 100049
Abstract：Using a second-order helium-cooled superconducting quantum interference device gradiometer as the detector, ultra-low-field nuclear magnetic resonance (ULF-NMR) signals of protons are recorded in an urban environment without magnetic shielding. The homogeneity and stability of the measurement field are investigated. NMR signals of protons are studied at night and during working hours. The Larmor frequency variation caused by the fluctuation of the external magnetic field during daytime reaches around 5 Hz when performing multiple measurements for about 10 min, which seriously affects the results of averaging. In order to improve the performance of the averaged data, we suggest the use of a data processor, i.e. the so-called time-domain frequency correction (TFC). For a 50-times averaged signal spectrum, the signal-to-noise ratio is enhanced from 30 to 120 when applying TFC while preserving the NMR spectrum linewidth. The TFC is also applied successfully to the measurement data of the hetero-nuclear J-coupling in 2,2,2-trifluoroethanol.
2012, Vol. 29 
