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Sample Preparation

The samples for NMR-spectroscopy must be handed to the NMR department dissolved and filled into a 5 mm NMR tube.

As solvents all deuterated solvents such as deutrochloroform, deuteroacetone, deutero-THF, deuteromethanol and deutero-DMSO are suitable. You have to take into account, that the viscosity of the solvent influences the quality of the spectra. Therefore, spectra measured in DMSO result into a line broadening.
The use of deuterated solvents is not only reasonable in order to suppress the proton signals of the solvent, but is as well important for the lock. Measurements in not deuterated solvents such as tetrachlorocarbon are not possible.
If measurements in tetrachlorocarbon are necessary, an additional deuterated solvent has to be used for the lock. This solvent can be added directly to the sample or in a sealed capillary.

MestReNova and other NMR software offer to possibility to use the NMR-solvent in order to calibrate the NMR shift.
However, the NMR shift (in proton and carbon NMR) is defined in relation to TMS. The NMR shift of any signal depends on temperature and pH. All measurements on the 300 MHz and the 600 MHz NMR are carried out at 25 °C. MestReNova does (to my best knowledge) not give any information on the temperature they use for referencing to DMSO, methanol, acetone … and so on. Therefore no exact calibration of 5the shift is possible.
The chemical shift of TMS is defined to be zero at any temperature.
Another problem is the pH dependence of the chemical shift. In the presence of an acid acetone, methanol, DMSO, … may be in a protonation-deprotonation-equilibrium which influences the shift of the solvent as well.

As a consequence one should ALWAYS USE TMS AS STANDARD! Using the solvent as standard should be avoided.

Air bubbles, fibres and suspended solids cause line brodening and problems with the phase. Potentially filtration of the sample is necessary in order to remove suspended material.
Method 1 (glass wool):
Some glass wool is placed in the tip of a glass disposable pipette (the glass woll can be stuffed into the tip using a second dispoable piptette) and this pipette one places this pipette directly on top the NMR tube. The sample is filtered directly into the sample tube.
Method 2 (Kleenex-tissue):
Prepare a small square of Kleenex-tissue (cut, don not tear in order to avoid fibres)and fold the piece of tissue around the tip of a glass pipette. Fill the pipette usimg the tissue as filter, discard the tissue and fill the solution directly from the glass pipette into the NMT tube.

In general it is true, that the signal to noise ratio increases with increasing concentration. Doubling the concentrations yield a twice as high signal to noise ratio. It is possible to increase the signal to noise ratio by increasing the length of the experiment. In order to double the signal to noise ratio you have to increase the time of the experiment to the forthfold. Therefore to low concentrations have to be avoided, because thes result in inappropiate long times for the exeriments. This is true especially for 13C{1H}-NMR.
On the other hand one has to avoid to high sample concentrations, because they cause line broadening. This is ussually no problem in 13C{1H}-NMR, but it may cause loss of information in 1H-NMR.
There is no optimum concentration for both experiments. For a typical molecul with a molecular weight of 200 g/mol, 10 mg of compound in 0.7 ml solvent are sufficient for 1H-NMR, however for 13C{1H}-NMR 30 to 50 mg should be used.

The optimum filling level of a NMR-tube is 4.5 to 5.5 cm. A higher filling level yields to an unnessarry dilution of the sample, a lower filling level causes problems during the measurement.

The NMR tubes must have a minimum length of 17.5 cm (6 1/8 inch), otherwice they can not be handled by the autosampler. This would lead to a disruption of service.
NMR-tubes have to be free of grease and chemical substances on the outside. Grease and other substances may damage the probe, might harm the staff and lead to additional signals and can conterminate the probe.
Therefore one should clean the NMR-tubes using isopropanol.

Paramagnetic contaminations lead to dipol-dipol-interactions, which cause line broadening and thus should be avoided.
Paramagnetic contaminations can be caused by the use of metalic spatulas or by needles of syringes, especially in the presence of acids.

Air bubbles disturb the measurements. They have to be removed by slight tapping of the NMR-tube.

Due to safety considerations only samples in undamaged NMR-tubes can be accepted. Otherwice the caps do not fit well and a safe sample handling is not assured.
The Erlenmeyer flask, the bottle or ..., in which the sample tube is handed in must be undamaged as well. Sharp edges of tin cans are not acceptable.
The NMR-tube must be free of toxic substances on the outside. If samples are especially toxic, this has to be marked.

Sample flasks, which tend to tumble over, are not allowed. Flasks, which are not high enough or have a small footprint are not suitable.

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