The T-lock: automated compensation of radio-frequency induced sample heating

Abstract

Modern high-field NMR spectrometers can stabilize the nominal sample temperature at a precision of less than 0.1 K. However, the actual sample temperature may differ from the nominal value by several degrees because the sample heating caused by high-power radio frequency pulses is not readily detected by the temperature sensors. Without correction, transfer of chemical shifts between different experiments causes problems in the data analysis. In principle, the temperature differences can be corrected by manual procedures but this is cumbersome and not fully reliable. Here, we introduce the concept of a "T-lock", which automatically maintains the sample at the same reference temperature over the course of different NMR experiments. The T-lock works by continuously measuring the resonance frequency of a suitable spin and simultaneously adjusting the temperature control, thus locking the sample temperature at the reference value. For three different nuclei, 13 C, 17 O and 31 P in the compounds alanine, water, and phosphate, respectively, the T-lock accuracy was found to be < 0.1 K. The use of dummy scan periods with variable lengths allows a reliable establishment of the thermal equilibrium before the acquisition of an experiment starts.

A new broadband homonuclear mixing pulse for NMR with low applied power

“…The long mixing duration means that the main worries in this experiment are sample heating and probe arcing. Generally, experiments that are sensitive to sample heating from RF pulses, 6 and high bandwidth applications in protein and small molecule NMR, can also be improved by using NF4.…”

“…[1][2][3][4][5][6] However, total correlation spectroscopy (TOCSY) mixing pulses require high RF power to cover the large ranges of chemical shift frequencies encountered in practice. 7 This problem is exacerbated at high field, since bandwidth increases proportionally to field strength.…”

Analytical optimization of active bandwidth and quality factor for TOCSY experiments in NMR spectroscopy

“…The total correlation spectroscopy (TOCSY) experiment is used extensively in NMR studies 1–6 . In TOCSY, spin angular momentum is shared via J -couplings between coupled nuclei.…”

“…The key limitation of the technique is the high radiofrequency (RF) power required to correlate spins over the large bandwidths of chemical shift offset frequencies encountered in typical experiments 7 . Use of high RF power for prolonged periods of time can lead to sample heating and cause severe damage to the NMR probe and preamplifier when the hardware is operated outside the specifications 6,8 . This problem is exacerbated in high field magnets, because the chemical shift offsets (and therefore RF power requirements) are proportional to the field strength.…”

HNCA-TOCSY-CANH experiments with alternate 13C-12C labeling: a set of 3D experiment with unique supra-sequential information for mainchain resonance assignment

“…However, often severe cases of overlap still exist, or ambiguities arise because the different pulse sequences may lead to slightly altered sample temperature, which increases the chance of incorrect assignments. In future, the latter problem might be eliminated with the development of the “ T -lock”, which automatically compensates the radio-frequency induced sample heating (Hiller et al 2009). In cases where carbon signals overlap, especially for Gly, which lacks C β , relying on the better dispersed 15 N can be advantageous.…”