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Guidelines for NMR Measurements SS 2003

Dr. Andreas Schäfer
NMR Department
Institut für Chemie, FU Berlin
April 15, 2003

Dear Colleagues,

The number of jobs carried out by the NMR service last quarter increased by 30%.

1H

13C/Dept.

Night

2D

Hetnuc/deep

temp

250 MHz

3800

500

28

   

500 MHz

650

750

72

370

70

Because samples are submitted on an irregular basis, the NMR service department had to measure more than 100 spectra in one day several times. Since we want to perform all the measurements and return your samples as fast as possible despite this large quantity of samples, we had to come up with some rules for our service department. These requirements are not meant in any way to annoy you or cause you trouble. They should guarantee a faster carrying out of your requests. If the number of measurements that need to be done decrease again to a manageable amount, we will try our best to accomodate you again more flexibly.

Sample Concentration

Samples have to be submitted with a concentration which is suited for the required experiment. For the acquisition of routine carbon 13 spectra we cannot afford to do more than 400 scans (approximately 15 min real measurement time). The signal to noise ratio increases only with the square root of the measurement time. For this reason a useful carbon 13 spectrum will hardly be obtainable after half an hour, if it does not display a clear signal after 15 minutes. If there is only a small amount of substance available, you should use only as much deuterated solvent as necessary (see below). Too much solvent will probably not improve your spectra but it does decrease the number of molecules which can be detected by the rf. coils of the probehead. If you cannot get a meaningful carbon 13 spectrum, we can offer you an overnight measurement or a HMQC spectrum, which is easier to obtain.

In order to avoid long delays for your colleagues, we have to reject a sample from which we do not get any useful signals after 100 scans. If you expect the measurement of your sample will require more than 400 scans, we ask you to make an appointment. Material, which is not dissolved, does not increase the concentration but messes up the homogenity of the local magnetic field and consequently the line shape of your spectrum.

Amount of Solvent

We have to spend a lot of time to achieve a good homogenity of the local magnetic field. For this reason we need a minimum amount of deuterated solvent, which should be 0.6 ml (4 cm) for measurements on the AC250. At 11 Tesla (ECP500, AMX500) serious shimming has to be done and the tube should be filled up to 5 cm. If your tubes are not properly filled, you may obtain broad signals or observe a splitting of peaks. Please shake the sample thoroughly before submission. There can be regions of different concentration in the sample, which will spoil the measurement. If the amount of solvent is insufficient, proper shimming could take an hour in which case we will have to reject your sample as well.

Sample Tubes

We ask you not to use sample tubes which are broken or too short. Sometimes the homogenity seems too bad for an unknown reason and shaking of the sample is advantageous. If we do this for a broken tube, we might spill substance. Sample tubes might also get tilted during handling. There are some tubes which do not fit in the spinners. During routine work we cannot try a lot of different spinners. If your sample tubes are too narrow, we will have to reject your sample. Broken sample tubes can be given to the glassblower. Anyway, 5 of the sample tubes that fit our needs cost only 7 Euro.

Submission of Samples

Because some information about your sample is needed for carrying out the required experiments, we ask you to fill out the forms thoroughly. Of course, we do not need to know the molecular weight. We do need to know which experiment is to be performed though.

If neither 1H nor 13C is marked and no other experiment is given, we cannot accept your sample. If only 13C is marked together with integrals, we will assume that you expect us to measure a 1H spectrum. Since it is easy to mix up the submission boxes, we ask you in addition to mention 500 Mhz if you would like a measurement at 11 Tesla. Knowing which solvent you used in your sample is very important because this saves a lot of time for us! Therefore we have to reject orders where there is no or a wrong solvent. Please do not submit more than three samples with one form. They might get mixed up. A trial structure of your molecule helps us to pay attention to quarternary carbons. Also giving the amount of material can give us an idea how much measurement time will be needed.

File transfer

Data measured on the AC250 and saved on the NMR workstation are kept in the archives for ten years. The same holds true for data acquired on the spectrometers AMX500 and ECP500. If you expect us to save your data on the NMR workstation, you should mention a shortcut of your name in connection with a number from 1 to 10. Please ask whether your shortcut is used by someone else. Of course, we need a number for each experiment. If you do not give us sufficient numbers, we will save the data under NMR ### and write down ### for the corresponding extension on the returned form. Students cannot get an account of their own due to the limited disk space on the AC250 and automatic account on the large spectrometers.

The NMR staff has to handle several samples at the same time. To make sure that samples do not get mixed up, you should mark your sample tube with the same sample ID as given on the form. This sample ID will show up on your spectra. If the probe ID on the form and on the tube differ, we will use the one given on the tube. Sometimes the shortcut and probe ID are hard to decipher, in which case we can only make a guess.


www@chemie.fu-berlin.de Letzte Änderung: 2003-07-30