Tuesday, May 27, 2008
Temperature Calibration in an NMR Probe
Often it is desirable to collect NMR data at temperatures other than ambient temperature. Most NMR spectrometers are equipped with a variable temperature accessory. The user sets the desired temperature and the variable temperature unit regulates the temperature by continually adjusting the current in a resistive heater within the probe. The probe heater is inside a dewar into which a gas (air or nitrogen) is directed. The gas must be cooler than the desired temperature. It is heated by the heater and directed over the sample tube. The temperature is typically measured at the bottom of the NMR tube with a thermocouple. The set temperature may be different than the true temperature of the sample in the coil due to thermal losses, poor thermocouple calibration, or undesirable gas flow characteristics. Although the precision of the set temperature is typically 0.1 degrees Celsius, the accuracy (i.e. difference between the set temperature and the true temperature) can be several degrees Celsius. The problem is then to know the true temperature of the sample in the coil. For temperatures above room temperature, this can be done by collecting proton NMR spectra of ethylene glycol as a function of the set temperature. The chemical shift difference between the -OH and methylene protons is linearly dependant on temperature (Stefan Berger and Siegmar Braun, 200 and more NMR Experiments (2004), p. 146) and can be used to construct a calibration plot for the set temperature vs. the true temperature. The figure below shows such data collected as a function of temperature. Methanol shows similar behavior and is commonly used to calibrate temperatures below room temperature.