gradients and the second is the continued accumulation of heat in the sample

Gradients and the second is the continued

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gradients, and the second is the continued accumulation of heat in the sample during the experiment. Convection in the sample results in signal attenuation which, if it becomes large enough, will result in a spectrum that can not be phased. Convection is such that the flow will strive to remove any temperature gradients and will thus assist in dealing with the second issue, heat accumulation. In order to reduce the induced convection and its effect one can first use a pulse sequence with convection compensation, such as the one used in this report. Another partial solution is to reduce Joule heating, one of the concerns with running eNMR. Luckily, in some systems that are sensitive to temperature variation, the change in chemical shift can be used as an internal thermometer. This is the case for BMIM TfAc and thus a study of chemical shift changes over the course of an eNMR experiment was carried out in order to investigate the extent of Joule heating. Firstly, changes in chemical shift resulting from changes in the sample temperature were detected. The sample was heated to 55 , allowed to stabilize, and then a 1D spec- trum was obtained. This was repeated at 60 and 65 . By plotting the chemical shift change of the singlet peak close to 9.8 ppm versus temperature, the shift-temperature relation was obtained from the linear fit of data. 1 6 1 4 1 2 1 0 8 6 4 2 0 -2 C h e m ic a l sh ift (p p m ) 6 5 C 6 0 C 5 5 C Figure 2.3: The spectra show that increased temperature results in lowered chemical shift for BMIM TfAc with 5 volume % of hexamethyldisilane.
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2.2. PROBLEMS ENCOUNTERED 19 Figure 2.4: Magnification of the peaks at 9.8 and 7.9 ppm from figure 2.3 to better illustrate the change in chemical shift due to temperature change. Secondly, an eNMR experiment at 110 V applied voltage at 60 was carried out with a single scan per experiment and with 16 repetitions. This voltage was selected since it corresponds to the voltage at the final point in previously performed eNMR experiments (with δ, Δand g taken from the successful eNMR experiments). By studying the chemical shift of the ionic liquids in the performed eNMR experiments, the temperature variation in the sample during an eNMR experiment could be plotted. Using the obtained cali- bration curve (chemical shift vs. temperature), the temperature variation in a complete eNMR experiment could be calculated. Figure 2.5: Plot of temperature versus scan number during an eNMR experiment on BMIM TfAc with 5 volume % hexamethyldisilane. Run at 110V supplied voltage. As can be seen from the temperature calibration in figure 2.5 , there is some heating of the sample during a single scan. The temperature profile in figure 2.5 obtained whilst doing the temperature calibration can be explained by convection. When the sample is heated, a temperature gradient is created. Until the gradient becomes large enough no convection is induced. Once the gradient becomes large enough, convection is induced
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20 CHAPTER 2. SUMMARY OF RESEARCH and the temperature in the sensitive volume decreases and at the end reaches a point where the temperature stabilizes. Increased time between individual scans and increased
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