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  MONITORING PROGRESS
       
  Make pre-acquisition adjustments.
Before samples are submitted, the icons below are labeled according to their states at the end of the previous run. Once a new acquisition starts, the icon begins to blink, and the message below it changes, as the experiment initiates each stage. Once that procedure is completed, the icon's background turns dark (as seen around the traffic light below).
       
This icon represents the sample loading stage of the experiment. As the run starts, the message below this icon reads "Go to Sample n" (where n is the sample number) as the spectrometer ejects the previous tube and loads #n. Once the sample is seated properly in the probe, air is turned on in order to get the sample spinning, usually at 10-20Hz; at this point, the icon label changes to "Exec [ro acqu]". Once the spinning has stabilized at the preset speed, the message becomes "Do Qnpst", then "Qnpset done". This refers to probe tuning adjustments done if the spectrometer is equipped with a quad-nuclear probe.The NMR-CTP instrument does not have this accessory; these messages are meaningless in such cases.

(Click on the icon to learn more about this step)
The icon at right indicates the frequency locking stage of the experiment. During this procedure, the spectrometer monitors the 2H signal from the deuterated solvent used to prepare the sample. Historically, the lock signal had been used to prevent magnetic field drift, although this is less of a problem with modern superconducting magnets). Current instruments use this signal to shim, i.e., to optimize the field homogeneity (discussed immediately below), and to reference the chemical shifts in the final spectrum. The message below the icon changes from "Lock Field" at the beginning of the experiment, to "Set default shim", as it reads a file of previously set shim parameters. When the lock routine starts, the message reads "Locking Field", and changes to "Field Locked" when the procedure is complete.

(Click on the icon to learn more about this step)
During the shimming procedure, small variations in the homogeneity of the magnetic field at the sample are evened out by increases or decreases in current in several strategically placed shim coils near the probe. Inhomogeneities can be caused by sample flaws, such as undissolved materials, air bubbles, or concentration gradients, or even by changes in the positions of metal objects in the vicinity of the magnets (for systems with unshielded magnets). The shimming procedure is iterative, so this step may take a few minutes to complete. As the shimming routine starts, the icon message changes from "Shim" to "Shimming", then reads "Shimmed" when the procedure is done.

(Click on the icon to learn more about this step)

The final step before the data acquisition begins is the determination of the appropriate receiver (Rx) gain. The gain indicates how much the NMR signal is amplified. An NMR signal, or free-induction decay (FID) is composed of one or more decaying, oscillating components that are recorded as voltage (on the y-axis) versus time (on the x-axis). If the gain is set too low, much of the data-recording capability of the receiver is wasted because the signal will take up only a small part of the possible voltage (y) range. If, on the other hand, the gain is set too high, the signal will exceed the capacity of the receiver, and the signal will be clipped, or cut off, along the y-axis. Not only does this result in a loss of information, but it will also lead to distortions in the final spectrum. The icon messages during this step read "Receiver Gain", then "Auto Rx Gain", and finally, "RGA Done".

(Click on the icon to learn more about this step)