Wednesday, December 29, 2010

Probe Tuning :NMR

Probe Tuning :NMR

Probe tuning must be matched to the sample in order to minimize pulse widths and maximize sensitivity.  Although the effects are small for routine proton spectra, they can be dramatic for more complex pulse sequences.  Although the tuning changes between different organic solvents usually do not justify the effort to tune, if you are changing from an organic solvent to water or D2O, retuning the probe is usually worthwhile.  If you are starting a long experiment, you will want to check the probe tuning.

The wobb command allows you to tune and match a probe in an easy way even when the coil is heavily mistuned and mismatched. First copy the parameters for the nucleus you wish to observe.
Now enter the acquisition window by clicking Acquire/Observe fid window and start the wobble routine by clicking on Acquire/Acquisition parameter setup/Tune probehead. Alternatively enter acqu and then wobb via the keyboard. The acquisition is started and after a few seconds the wobble curve is displayed and refreshed continuously. A vertical line is drawn at the center frequency
(SFOx) to provide optical information on the frequency which is to be tuned. The horizontal axis of the coordinate system is scaled in MHz and labeled accordingly. Useful information like nucleus, tuning frequency, frequency of the minimum of the wobble curve and wobble width, is displayed in the information window. Simultaneously the LED display on the preamplifier is set and refreshed accordingly. The wobble curve shows a dip downwards which changes while you turn the tune or match knobs of the probe.  Example displays in Figure. 

1. Turn the tune knob (tune sliders on the indirect probe) so that the dip moves towards the center of the screen. Keep on turning until the dip is exactly in the center across the vertical line.
2. Turn the match knob (match sliders on the indirect probe) so that the dip becomes deeper. Keep on turning until the base of the dip is at a minimum. This occurs at the zero level line for most probes.
3. On most probes the matching influences the tuning and vice versa, so repeat steps 1 and 2 until the dip is exactly in the center of the screen and its base at minimum level. Then the probe is tuned and matched.
When you are finished with tuning and matching stop wobb by activating the stop button or by entering stop via the keyboard.

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