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
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.
A rough comparison was obtained using a standard menthol sample, and
measuring the signal to noise of the CDCl3 solvent and one of the menthol
| CDCl3 || CH3 |
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?)
- if you are interested in quarternary carbons, a longer D1
delay of 3 seconds or more is advisable.
- If the signal to noise of your carbon spectrum is too low,
try running a short and/or long range proton-carbon 2D correlation
experiment. It has been known for a long time that this can give
dramatic improvements in S/N. See J. Am. Chem. Soc. 101, 4481 - 4484 (1979).