A robust approach to correct for pronounced errors in temperature measurements by controlling radiation damping feedback fields in solution NMR

• Published in: Journal of magnetic resonance (1997) Vol. 248; pp. 19 - 22
• Main Authors: Wolahan, Stephanie M., Li, Zhao, Hsu, Chao-Hsiung, Huang, Shing-Jong, Clubb, Robert, Hwang, Lian-Pin, Lin, Yung-Ya
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 01.11.2014
• Subjects: Algorithms; Control systems; Control theory; Dynamic frequency shifts; Error analysis; Errors; Ethylene Glycol - chemistry; Feedback; Magnetic Fields; Magnetic Resonance Spectroscopy; Methanol - chemistry; Nonlinear NMR; Nuclear magnetic resonance; Radiation damping; Reproducibility of Results; Sensitivity and Specificity; Separation; Temperature; Temperature measurement; Temperature measurements; Thermography - methods; Temperature measurements; Dynamic frequency shifts; Nonlinear NMR; Radiation damping
• ISSN: 1090-7807; 1096-0856
• DOI: 10.1016/j.jmr.2014.09.008

[Display omitted] •Strong, chaotic self-induced dynamic frequency shifts (DFS) in NMR was discovered.•Its physical origin was identified: nonlinearity in spin dynamics by feedback fields.•DFS induces significant errors in frequencies and other quantities (e.g., temperature).•A spin control scheme to mitigate the disastrous effects of DFS was demonstrated.•It showed a 10× improvement in the accuracy of measuring samples’ temperatures. Accurate temperature measurement is a requisite for obtaining reliable thermodynamic and kinetic information in all NMR experiments. A widely used method to calibrate sample temperature depends on a secondary standard with temperature-dependent chemical shifts to report the true sample temperature, such as the hydroxyl proton in neat methanol or neat ethylene glycol. The temperature-dependent chemical shift of the hydroxyl protons arises from the sensitivity of the hydrogen-bond network to small changes in temperature. The frequency separation between the alkyl and the hydroxyl protons are then converted to sample temperature. Temperature measurements by this method, however, have been reported to be inconsistent and incorrect in modern NMR, particularly for spectrometers equipped with cryogenically-cooled probes. Such errors make it difficult or even impossible to study chemical exchange and molecular dynamics or to compare data acquired on different instruments, as is frequently done in biomolecular NMR. In this work, we identify the physical origins for such errors to be unequal amount of dynamical frequency shifts on the alkyl and the hydroxyl protons induced by strong radiation damping (RD) feedback fields. Common methods used to circumvent RD may not suppress such errors. A simple, easy-to-implement solution was demonstrated that neutralizes the RD effect on the frequency separation by a “selective crushing recovery” pulse sequence to equalize the transverse magnetization of both spin species. Experiments using cryoprobes at 500MHz and 800MHz demonstrated that this approach can effectively reduce the errors in temperature measurements from about ±4.0K to within ±0.4K in general.