NMR Sample Preparation 
The following guidelines are offered so that you can make as perfect a sample as possible, the first time.  The three steps of sample preparation are concentration, clarity, and sample volume. Paying attention to these three factors when you are preparing your sample will eliminate wasted time and materials.  What is important to recognize is that if your sample is poorly prepared the resulting spectrum will likely be lower in resolution than what you might expect.  What is also important is that if you get a poor quality spectrum with 1 scan, increasing the number of scans will usually not improve it.

Concentration
Optimally, one wants a sample of sufficient concentration for the particular spectrum required. For 1H nmr spectra (1-dimension and COSY spectra) usually 10-20mg of material dissolved in a total volume of 0.6mL of solvent will be sufficient for a good quality spectrum requiring anywhere from 1-12 scans.  Strictly speaking, a deuterated solvent is not necessary because the Eft does not lock on the solvent signal.  A deuterated solvent does help to eliminate background peaks arising from the solvent.

Since C13 is about 5000 times less sensitive than 1H, one will require significantly more sample.  If the sample being measured is a liquid it is permissible to measure the liquid directly without a solvent.  If the sample is a solid, then 200mg dissolved in a minimum volume of 0.6mL solvent should provide a good spectrum in about a minute.  If only 100mg of material is available, then this concentration will provide a good spectrum in about 4 minutes.  Remember that in order to double the signal to noise ratio, the measurement time will increase by 4 times.

Clarity
Once you have the sample in solution, you should check it for suspended particles such as bits of filter paper or other fibers or powders.  Hold the tube up to a bright light a look for diffractions off the suspended particles.  The suspended particles will cause broadening of the lines and give poorer resolution spectra.  To remove the suspended particles, filter the sample through a pipette with a small plug of glass wool packed into it.  When preparing the pipette containing the glass wool, be sure that the glass wool is not packed too loosely nor is the plug of glass wool too large (see photo).  After filtering the sample through the glass wool, the sample should be free of particulate matter. 

Quantity
The NMR detector is situated so that a volume of about 50-80uL is detected about 15mm from the bottom of the tube.  It is therefore important that the volume of the sample be kept to a minimum, no more than 0.6mL of sample volume.  If one uses more than 0.6mL of solvent, the sample will be more dilute than desired and the data acquisition may require more time because of a dilute solution.  Samples lower in volume are also undesirable because the meniscus of the sample will distort the magnetic field and the close the meniscus is to the detector coil, the more difficult the shimming or field optimization of the sample.

Sample Labeling
If you are measuring more than one sample, it is wise to label the tube using a Sharpee marker on the top of the tube or on the tube cap or a small label.  Do not use a label which has a flap because the sample needs to spin when it is in the magnet and this can interfere with good spinning.