Tuning the 500 Probe

Why would you want to tune the probe?
1) You want to detect a nucleus other than proton or ¹³C.

Why would the probe not be already tuned when you login?
1) The instrument was not left in the standard setup, tuned to proton on one channel, and carbon on the other on an ethylbenzene sample in chloroform.

2) Even assuming that the instrument was in its standard setup, there are reasons that the probe would not be tuned for your sample. Tuning is very dependent upon temperature and solvent/ionic strength. If your solvent is something other than chloroform, the probe will not be tuned for your sample. Some solvents are close to enough to chloroform that you may not see an effect (toluene); some solvents are so far from chloroform that you may not see anything in your spectra (D₂O with high salt). Also, if you have alot of sample (~>20 mgs), the tuning will not be the same as for a weakly concentrated sample.

Should you tune the probe every time you use the instrument?
Tuning should not really be necessary, if you are working with sample similar to the standard and acquiring proton or carbon spectra. If the spectra do not look good, then checking the tuning is a good idea.