Increased time between individual scans and increased air flow over the sample

Increased time between individual scans and increased

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where the temperature stabilizes. Increased time between individual scans and increased air flow over the sample will further reduce convection. Heat accumulation in the sample during an eNMR experiment is the second possible result due to Joule heating. With 30 second delays between individual scans no dis- cernable heat accumulation could be found, indeed the sample shows a tendency to cool down as the experiment progresses due to increased convection. This was investigated by use of the chemical shift change in the sample during the three eNMR experiments for BMIM TfAc. Run 1 - 3 are the same run 1 - 3 reported in the result section of this report (see figure 2.9 , figure 2.10 and figure 2.11 ). Figure 2.6: Temperature variation during eNMR experiments on BMIM TfAc with 5 volume % hexamethyldisilane. The high viscosity and low diffusion coefficient of ionic liquids in general presents another set of problems. High viscosity increases the chance of heat accumulation in the sample during an eNMR experiment whilst the low diffusion coefficients that are typical of ionic liquids present the main concern. As can be seen by equation 1.11 a small diffusion coefficient will result in a small electrophoretic mobility. This in turn (see equation 1.12 ) will result in a small phase shift if no other constants are changed from what is typically used for eNMR when investigating other samples. By increasing the constants in equation 1.12 the theoretical observed phase shift can be increased. When doing this several things need to be kept in mind. The signal attenuation of an eNMR experiment, discounting flow effects, is given by equation 1.5 on page 10 . As can be seen, increasing any parameter will result in more signal attenuation. Δ also needs to be smaller then T 1 by a significant margin, preferably no larger then 0.5 T 1 . Another important thing to consider when increasing Δ is that it will directly affect Joule heating. δ also has a upper limit, governed mainly by T 2 . The supplied electric field, E , can also be increased. The limit of how far E can be increased stems from both joule- heating and the sample itself. For sufficiently high voltage electrode reduction can take
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2.2. PROBLEMS ENCOUNTERED 21 place; their absence must be verified by comparing 1D spectra before and after eNMR experiments. One other possibility to increase the phase shift of the eNMR experiment is to increase the gradient strength used for encoding and decoding There are two limitations when increasing g . One is the signal attenuation that will grow as g 2 and the second is the limitation of the NMR probe and gradient amplifier. In order to use larger gradients, alternate setups with a micro-imaging probe and a diffusion probe were used together with a stronger external gradient amplifier (40A). Unless the probe-amplifier system is well set issues with eddy currents arise when the gradient strength is increased. The effect is significantly larger for unshielded probes as is common in micro-imaging probes. This can be countered in several ways. First one needs to create a set of pre-emphasis settings for the setup [ 42 ]. The next step is
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