Use of fluoroptic thermometer temperature sensing for high-resolution temperature-controlled NMR applications

Abstract

Many NMR experiments, especially kinetic studies of biological transformations using cryoenzymology techniques as performed in this laboratory ( I-5)) require precise sample temperature control. Temperature measurement in a superconducting NMR system has traditionally required use of a dimetal thermocouple placed in the temperature control airstream near the point at which it enters the sample chamber. Such systems almost invariably cause distortion in the local magnetic field, even when thermocouples are prepared from supposedly nonmagnetic metals, and extensive tuning is usually required after thermocouple installation or adjustment. Another problem is the difficulty in correlating temperature inside the sample with that of the incident airstream. Sample heating due to absorption of decoupler energy and temperature deviations from measured values as temperature variance from ambient increases are common difficulties. A second thermocouple inserted directly into the sample tube can give true sample temperature, but tends to distort the magnetic field more drastically than external temperature sensors and as a rule cannot be used with a spinning sample, limiting resolution to lower nonspinning levels. A Luxtron fluoroptic thermometer recently purchased by our laboratory which uses ratio of emissions of fluorescent lines of rare earth phosphors in a sensor at the terminus of a plastic-clad silica optical fiber to determine temperature eliminates these problems. The nonmetallic, nonelectronic design of the sensor device makes the system ideal for temperature control in NMR studies, but use in a modern spectrometer in the manner required was not possible without substantial modifications of the normal configuration. The temperature measurement and control system described below was designed to provide instantaneous, accurate temperature measurements inside a spinning sample tube without degradation of sensitivity or resolution. Spectra were recorded on a Bruker WM-300 wide-bore spectrometer equipped with either a Bruker 5 mm 13C/‘H probe or a homemade 5 mm ‘H/13C probe. Temperature