小中大资料：核磁共振 NMR Basics / FAQ 核磁共振 NMR Basics / FAQ
All the really basic NMR questions you were too embarrassed to ask!
(Although the questions below are of a general nature, the instruments mentioned are the Varian spectrometers currently used in the NMR Centre of the Australian National University).
Please email your ideas for other questions to be included in this list to Chris Blake.
There is a much more comprehensive Basics of NMR document available at the Center for Imaging Science, Rochester Institute of Technology.
If you haven't used any NMR spectrometers at the NMR Centre before.
Come down to room G48 or G42 in the Chemistry Faculties building, and ask Chris, Tin or Peta to arrange a training session for you. Even if you have used Varian spectrometers before, our setup here is likely to be slightly different, so in the long run you will save yourself time by having a short lesson. More information is available in the introduction to the NMR Centre. It is NMR Centre policy that you will not be given a license to operate a spectrometer until we assess your competence with Varian spectrometers.
How much solvent volume should I use?
To get good resolution you need at least 0.7 ml of solvent in a 5 mm NMR tube. If you have a limited amount of sample you can increase its effective concentration by reducing the solvent volume to 0.4 - 0.5 ml however you will need to spend more time shimming. If you have a sample that will give a proton spectrum in 15 minutes when disolved in 0.7 ml, there is little point reducing sample volume if it means you need to spend an extra 10 minutes shimming! On the other hand if you then want to run a carbon spectrum of that sample it would certainly be worth reducing the sample volume. For 13C HMQC or HMBC runs with very small amounts of material, a 4mm tube and rotor is available.
What does the "ADC overflow" error message mean?
The signal recieved from the NMR sample is first amplified by the reciever and then digitised by the analog to digital converter (ADC). If the signal is too strong for them to handle, either the receiver or ADC will "overflow", causing a RECEIVER OVERFLOW or ADC OVERFLOW message to be displayed. The acquired FID is likely to be clipped, resulting in a distorted spectrum. The solution is to use autogain (type gain='n' or on a Gemini GAIN=N) or to type in a lower value for the receiver gain. If overflow still occurs when the gain is set to zero, reduce the observe pulsewidth (PW) to half its present value. If overflow still occurs dilute your sample, or if the solvent signal is causing the ADC overflow use a solvent suppression technique.
How do I shim / tune the spectrometer?
First of all, let's get our terminology straight. Shimming is adjusting the resolution of the signal by optimizing the homogeneity of the magnetic field. Tuning is adjusting the impedance of the probe. A poorly tuned probe reflects a lot of the power of the pulses, so that what should be a 90 degree pulse is in reality only (say) a 50 degree pulse. Probe tuning does not affect the resolution, however the signal to noise of a standard spectrum will be worse. Also, experiments such as DEPT or COSY that rely on accurate 90 degree pulses may produce artefacts or not work at all. (Note however that it is possible to adjust the pulse width to give a 90 degree pulse on a poorly tuned probe). Poor shimming on the other hand, results in broad NMR resonances. People often talk about "tuning the resolution" which is where some confusion between shimming and probe tuning arises. Shimming is adjusting the homogeneity of the magnetic field, so that every part of the sample in the NMR tube experiences exactly the same field strength.
OK, so how do I tune the probe?
If you're using the broadband Gemini or Mercury spectrometers, you never need to tune the probe. The probes of these intruments are tuned at the factory, and further tuning is a specialised operation. For best results, you should tune all 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 disolved in chloroform and half in D2O, 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. Some recabling is required. Do not attempt to do this unless an NMR staff member has given you a lesson. This doesn't mean that Geminis have some great "automatic tuning" technology. It just means they are left in a state of tune that is good enough for the run-of-the-mill experiments they were designed for. On other spectrometers, tuning is necessary because
You can get the best possible tuning for your sample,
You may not know what nucleus the previous user left the probe tuned to, or whether he/she completely messed up the tuning,
More sophisticated experiments such as HMQC, HMBC etc. work best when the probe is tuned and short 90 degree pulses are required.
How do I tune for carbon or phosphorus?
As mentioned above, if you're using a Gemini spectrometer, you don't need to tune the probe. 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 a knack to it, 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. On the Inova spectrometers you can tune the observe and decoupler channels at the same time. On older spectrometers you need to set up for and tune protons, then set up for and tune carbon or phosphorus. (Hint: always tune the highest frequency first and the lowest frequency last).
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