Wednesday, July 30, 2008
Tuning Problems for Samples of High Ionic Strength
Tuning and matching an NMR probe are essential in getting optimum NMR results (also, see this link). The pulse widths and power levels called up by the spectrometer for use with parameter sets are based on the NMR probe being optimally tuned and matched. When the probe is not optimally tuned and matched, the pulse widths and power levels stored in the parameter files of the spectrometer are no longer calibrated correctly (eg. what the spectrometer calls a 90 degree or 180 degree pulse may actually be an 80 degree or 160 degree pulse). The tuning and matching of the NMR probe are affected by the sample inside the coil. It is therefore good practice to tune and match the NMR probe for each sample to insure that the pulses will be correctly calibrated for your NMR measurement. Problems may arise with samples of high ionic strength. These samples very strongly affect the tuning and matching of a probe especially at higher frequencies. The effect can be so large that the probe cannot be tuned and matched optimally. In fact, several times per year, NMR users will report to me that they are unable to tune the NMR probe on their sample. In such cases the pulses should be recalibrated for the specific sample or (if this is not possible) on a sample of similar ionic strength. This effect is illustrated at 300 MHz in the figure below. The top panel is a tuning curve for the probe containing a sample of water after the the tuning and matching were optimized. The middle panel shows the tuning curve for the probe after the water sample was replaced with a concentrated solution of NaCl. Note the gross mismatching. The bottom panel shows the tuning curve for the concentrated NaCl solution after the probe was retuned and rematched. It should be noted that the matching is not as good as that for the water sample and therefore the pulses stored in the spectrometer parameter files will not be correctly calibrated for this sample. Again note, that this problem is more pronounced at highr frequencies.