Thursday, December 10, 2009
Variable Temperature NMR - Thermal Equilibrium
When doing variable temperature NMR, students often ask me how long they should wait for thermal equilibrium in their sample before collecting NMR data. The answer depends of course on the amount of gas flow around the sample and the temperature difference between the current and desired sample temperature. The position of the thermocouple in an NMR probe is typically right below the sample. It takes time between when the thermocouple reports the desired temperature and when the sample is at the desired temperature. During this time there is a large thermal gradient across the sample as well as convection currents which will affect the line width of NMR resonances. These effects are demonstrated in the figure below. For this measurement, the temperature of the probe was set to 50°C with an air flow of 800 L/hour. Once the thermocouple read 50°C, a sample of D2O was placed in the probe and 30 minutes was allowed to pass, after which the sample was presumed to be at thermal equilibrium. The lock was established and the magnet was then shimmed. The sample was removed and allowed to sit at room temperature for 30 minutes. It was then reintroduced to the probe at 50°C. 1H NMR spectra of the residual HDO were then collected at 30 second intervals for a period of 10 minutes. As soon as the room temperature sample is reintroduced to the warm probe, it begins to warm up. During the this time, the thermal gradients and convection currents are large and the line width is adversely affected. As the sample temperature approaches 50°C the thermal gradients are smaller and the line becomes narrower. After approximately 6 minutes the width of the line changes very little. The sample appears to be at thermal equilibrium after 10 minutes.