小中大资料：核磁共振 NMR Basis FAQ 2 核磁共振 NMR Basis FAQ 2
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 500 Mhz spectrometer when compared to a 300 MHz spectrometer should have an advantage of (5/3) squared, or 2.8 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 indirect 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.
In order to see how some of the spectrometers compare under "real life" conditions, a dilute sample was run for 256 scans on the Inova 500 (PFG indirect detection probe), broadband Gemini, and VXR300. A D1 delay of 1 second with a 45 degree pulse was used, and 16 dummy pulses were given to bring the system to a steady state before starting acquisition. The signal to noise ratios of three resonances were then measured.
CDCl3 CH3 quarternary
VXR300 21.3 16.5 2.2
Gemini BB 16.9 15.8 4.3
Inova 500 29.9 22.4 5.9
It can be seen that there is not a large difference in the signal to noise you can expect to see on these instruments. Also remember that
if there is not much sample available, you should reduce the amount of solvent. (See How much solvent volume should I use?) A 4mm tube and rotor is available. This will allow you to use even less solvent than is necessary in a 5mm tube.
if you are interested in quarternary carbons, a longer D1 delay of 3 seconds or more is advisable.
Why are some of the peaks in my APT missing?
The APT experiment relies as much on the size of the 1JCH coupling as the number of attached protons to generate the spectral pattern. This is because the delays in the experiment are matched to the inverse of the size of 1JCH. If 1JCH is much larger than the default 1JCH of the experiment (usually set to 140 Hz which is the average of 1JCH for sp3 and sp2 carbons) then peaks will either disappear or appear with incorrect phase. Carbons that may show this behaviour are terminal ethynyl groups (1JCH = 250 Hz approx.), epoxide carbons (1JCH = 175 Hz), furan, pyrone and isoflavone carbons (1JCH = 200 Hz), 2-unsubstituted pyridine and pyrolle carbons (1JCH = 180 Hz) and 2-unsubstituted imidazole and pyrimidine carbons (1JCH > 200 Hz).
I can't lock on.
You are using a deuterated solvent aren't you?
Can you see a lock signal? If not, make sure the lock is turned off, turn the lock power and lock gain to their maximum values, and look for a sine wave by adjusting Z0. If you find a sine wave, adjust Z0 until its frequency becomes zero. Then reduce the lock power (to avoid saturating the lock) and try to lock on.
If it loses lock as soon as you try to lock on, turn the lock off and adjust the lock phase as shown in the manual.
Is your tube spinning? It might not be spinning because you inserted the tube too quickly, causing it to break. Take the tube out and check that it is in one piece. While you have it out, use a depth gauge to check that the sample is centred in the probe.
It won't shim.
Check the linewidth of the narrowest line in your spectrum. If there are some broad lines and some narrow lines, the broad lines are probably broad because they are undergoing chemical exchange, not because the resolution is poor. Broad lines may also be caused by quadrupolar broadening if your compound has a transition metal.
If you have not already done so, load the standard shims. You don't know what sort of state the previous user left the shims in! On the XL200, type homo and check that the shims agree with the values displayed on the screen. All other spectrometers in the NMR Centre have a macro rtss which loads the standard shims. This macro is equivalent to typing rts('stdshm') su. If you are not using the PFG probe on the Inova 500, you will need to read in a different shim set.
If it still won't shim, take the tube out and inspect your sample. Is the tube scratched? Is there anything floating in the sample? Is the sample centred in the coil? If you are using a small amount of solvent to improve the concentration, you may need to add some more solvent to make it easier to shim.
Do you have paramagnetic ions in your sample?
Have other people been getting poor resolution? If so, report it to a member of the NMR staff. If not, change NMR tubes, filter your sample, and try again. If changing tubes solves the problem, throw the old tube away.
Have you placed your NMR tube in an oven to dry? If so, throw the tube away as it has distorted. (Remember, glass is a liquid. It flows at high temperature). The correct way to dry a tube is via a stream of dry nitrogen through a glass wool filter.
My tube broke when I inserted it into the magnet.
During the day, phone an NMR staff member. Tell them the solvent and any hazards posed by your compound. After normal working hours tell the watchmen who will call in someone. Leave a note to warn others not to use the spectrometer. Remember - the more quickly you lower a tube into the magnet, the more likely it is to break! If necessary, use two hands on the sample eject button to make it easier to lower the tube slowly
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