NMR Probe

How do I tune the probe? If you're using a Gemini 200 spectrometer, you never need to tune the probe. The probe of the broadband Gemini is tuned at the factory, and further tuning is a specialized operation. The H/C Gemini should only be tuned by staff members.

For best results, you should tune other spectrometers before acquiring a spectrum. Frequency, solvent and sample height all affect probe tuning. If you were running a set of similar samples in the same solvent, you might only bother to tune the probe before running the first spectrum. If however, half your samples were dissolved in chloroform and half in D ₂O, you might run all of the chloroform samples and then quickly adjust the tuning after inserting the first D₂O sample. Tuning involves setting up for the nucleus of interest and minimizing the reflected power shown on the meter on the magnet leg. Never attempt to do this unless an staff member has given you a lesson. This doesn't mean that Gemini's have some great "automatic tuning" technology. It just means they are left in a state of tune that is good enough for routine experiments. On other spectrometers, tuning is necessary because you can get the best possible tuning for your sample. What nucleus did the previous user tuned the probe? This information is recorded in the instrument log.

How do I tune for carbon or phosphorus? As mentioned, if you're using a Gemini 200 spectrometer, you don't need to tune the probe. In the past one Gemini is tuned for protons (room 130) and the other for carbon (room 38) or protons.

First, check whether the probe you are using requires a tuning stick to be inserted. Tuning sticks are kept separate from the probe, and have a small capacitor on the end to change the tuning range of the probe. If a tuning stick is required, select the stick for the observe frequency and screw it gently all the way into the probe. You can find the observe frequency by setting up for the nucleus of interest and reading the value of sfrq from the dg display. Then make the cable connections for tuning, and adjust both the tuning and matching rods. These two tuning rods affect each other, so it is usually necessary to go back and forth between them to get a good minimum. There is a bit of time required, so persevere! (Hint: make the tuning worse with one rod, then better with the other. Each dual operation should result in better tuning than before). Also note that if you are decoupling protons while observing carbon or phosphorus, it is a good idea to check the proton tuning. If the probe is poorly tuned to protons,  some decoupler power may be reflected, resulting in an improperly decoupled spectrum.

CAUTION: If you turn a stick with too much force it may strip wires and destroy or severely damage a probe.

Which spectrometer should I use for carbon?  When measuring carbon spectra, the main concern is usually signal to noise. You would expect higher field spectrometers to have a decisive advantage - for example a 400 Mhz spectrometer when compared to a 200 MHz spectrometer should have an advantage of (4/2) squared, or 2 times the signal to noise. However, there are other considerations, including for example the type of probe. An indirect detection probe has the proton observe coil on the inside (that is, closer to the sample than the coil used for carbon). This improves the proton signal to noise, however if you use an inverse detection probe for directly observing carbon, the signal to noise will of course be worse than a standard probe which has the carbon coil on the inside. Regardless of the probe design, carbon and protons use different coils, and since the electronic circuit for the two nuclei is different it makes no sense to compare proton signal to noise on two instruments and extrapolate the results to carbon. Also, signal to noise tests are usually performed by collecting a single scan on a concentrated sample, however this does not give the best indication of the results obtainable on "real" samples where the sample is scanned for several hours. When a sample is repeatedly pulsed, the relaxation times of the various carbons must be taken into consideration. Nuclei take longer to relax at higher fields, so the gain in signal to noise is less than expected. Also note that carbons that do not have directly bonded protons (i.e. carbonyls and quaternaries) have much longer relaxation times than protonated carbons.