Preparing a sample is simple but follow these guidelines to avoid contamination or concentration errors.
Contamination -- Vials, bottles, or test tubes must be clean and dry. It will be difficult or impossible to identify unknown if there is contamination or if the unknown is not very pure. Many substances, including the solvents and test tube cleaner, must not be in contact with bare skin. Use good non-latex gloves. Always wear eye protection in the laboratory.
Solvent -- The cheapest and most commonly used solvents are carbon tetrachloride or deuterated chloroform . (Caution -- Carbon tetrachloride is a carcinogen. It should only be used in the hood. Do not get it on bare hands nor breathe the vapors.) The solvent is commonly spiked with a reference --TMS (tetramethylsilane). Other solvents need to have a 1% (w/w) concentration of TMS. The NMR spectrum will be plotted from right to left, from higher to lower frequencies. TMS produces a sharp peak and will be the furthest downfield — at the lowest frequency — and is used as the zero reference point on the spectrum.
Solids Samples -- Never prepare sample in the NMR laboratory. Place approximately 25 to 50 mg to a clean glass vial and then add about 2 mL of solvent, containing 1% TMS. Mix well until the sample dissolves. Filter out any particulate or undissolved materials. Add about 0.5 to 1.0 mL of the liquid to a clean NMR tube and cap.
Liquid Samples -- For liquid samples, place approximately 1 drop into a clean glass vial and then add about 2 mL of solvent with 1% TMS. Mix well and filter out any particulate or undissolved materials. Particulate mater must be filtered through media in a Pasteur pipet. Add about 0.5 to 1.0 mL of the liquid to a clean NMR tube and cap. NMR analysis takes place in a 5-mm diameter, thin-walled glass tube.
If this sample is too concentrated to obtain the proton NMR spectrum turn down the receiver gain (RG) PNMR. If the spectrum is still poor, pour out 50-75% of the sample (save or halogen wastes) and put enough deuterated chloroform (no TMS!) in the tube such that there is 4 cm of solution in the tube.
Cleaning NMR Sample Tubes -- To save a sample, pour the contents of the sample tube into a 5 mL conical vial (in the hood!) Rinse the tube with either diethyl ether or methylene chloride and add the rinse to the conical vial. Evaporate off the solvent in the hood. Rinse the tube twice with a small amount of acetone and dispose of the rinses in the halogen waste container. Place the tube inverted in the clean tube container. Rinse cap with acetone and place in the clean cap container. To discard the sample, pour the sample into the halogen solvent container. Rinse the tube three times with a small amount of acetone and dispose of the rinses in the halogen waste container. Place the tube inverted in the clean tube container. Rinse cap with acetone and place in the clean cap container. NMR tubes are expensive handle with care.
