Preparing a sample
simple but follow these guidelines to avoid contamination or concentration
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
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.