back
NMR-004: Sample Solvents in
NMR Spectroscopy
It isn't surprising
that considerable attention is directed toward carefully
choosing solvents for NMR experiments. Today, solvents must
provide an effective medium in which to dissolve a sample
to the required concentration and filling-factor . . . must
contribute no interfering resonances . . . and must provide
a lock signal to assure spectrometer stability. This presents
stringent challenges for manufacturers of NMR solvents when
added to the obvious need for chemical purity. To make your
use of solvents in NMR easier, we provide below an explanation
of the important factors to consider when choosing and handling
deuterated solvents in NMR.
Purity
Proton and 13C NMR provide the most challenging applications
for solvents, since most of the contaminants found in solvents,
like water or organic chemicals, contain protons and/or
13C atoms that create impurity signals in the NMR spectrum.
In addition, the isotopic purity of deuterated solvents,
which easily degrade through exchange with the protons in
water in the atmosphere, must be maintained at high levels.
When you buy NMR solvents, it pays to use a reliable supplier
like WILMAD or reputable manufacturer like Aldrich. Choosing
the correct isotopic purity of deuterated solvent in proton
NMR is also critical to avoid excessive protic solvent signals.
The table below provides a useful guide to selecting isotopic
purity of the NMR solvent for your samples. You should make
adjustments for the molecular weight of your samples when
using this table.
Sample Mass
MW=250 |
Molar Sample
Concentration |
Min. Solvent
Isotopic Purity |
| 12.5mg |
0.1M |
99.0% |
| 6 |
0.05 |
99.5 |
| 2.5 |
0.02 |
99.8 |
| 1 |
0.005 |
99.95 |
Higher isotopic purities (>99.5%) are best purchased
in sealed glass containers, like ampules or the Aldrich
Sure-Seal® closure. Other closures may not maintain
isotopic purity as well and can sometimes contribute small
amounts of organic contamination to non-aqueous solvents,
especially over extended periods. Deuterium isotopic purity
isn't quite as critical for solvents in 13C NMR, so 99.5
or 99.0% is usually acceptable. But with small amounts of
sample, 13C concentration in the solvent becomes more critical.
Then, 13C depleted solvents are an important consideration.
You can apply the same table, below, to 12C purity of the
solvent. But since 13C depleted solvents are usually available
in only one isotopic purity, your decision is typically
restricted to deciding when to use 13C depleted solvent.
If you find your sample size is, for example, less than
10mg of a MW250 compound, you should consider using 12C
enriched (or 13C depleted) NMR solvents or applying a more
sophisticated experiments to your sample, such as DEPT or
INEPT.
When you open the container in which a solvent is delivered,
the solvent will rapidly absorb moisture from the atmosphere,
compromising deuterium purity and adding unwanted contaminant
to the solvent. TMS bottles should be warmed to room temperature
before opening to avoid condensing water onto the cold inner
surfaces of the bottle and cap. Always recap TMS tightly.
We recommend that deuterated solvents with >99.8% isotopic
purity be opened only in a glovebox or glovebag under a
dry atmosphere. Most solvents can be dried over molecular
sieves; Linde 3A and 4A, 1.6mm are acceptable. Avoid bead-shaped
sieves; they don't do as well in drying solvents.
Make sure that all the glassware you use is dry, too. Since
water chemisorbs to glass and even vigorous oven drying
doesn't remove the last traces, we recommend you equilibrate
residual water through D2O exchange. It's important to recognize
that oven drying NMR tubes can irreparably distort precision
NMR tubes. Rather, in a dry atmosphere, soak all glassware
with a quantity of D2O that covers the working area of the
glassware, rinse with sieve dried deuterated Acetone, and
dry in vacuo or under a stream of dry nitrogen. Cap all
NMR tubes and store all glass in a very dry atmosphere or
in a D2O saturated atmosphere (e.g. in a desiccator). WILMAD
long-tip NMR pipettes help minimize the area of glass that
must be equilibrated with D2O, because they allow you to
place the sample directly into the bottom portion of a preconditioned
NMR tube. Large bottles of D2O can be kept isotopically
pure even with a septum affixed on top. Use Teflon-sheathed
Silicone septa (good resealability) and always flush a carefully
dried syringe with dry nitrogen before using. Remember to
inject the bottle with a volume of dry nitrogen equal to
the volume of solvent to be withdrawn. Careful, firm application
of a septum cap under dry atmosphere in a glovebox or glovebag
will help avoid contamination when you first open a capped
bottle of deuterated NMR solvent. For the study of nuclei
other than 1H and 13C, most any solvent that dissolves the
sample can be used. The addition of a sufficient quantity
of a miscible deuterated solvent to provide a lock signal
is necessary to assure spectrometer stability. Combinations
of protic and deuterated solvent are frequently used to
control solvent costs while providing a sufficient source
of deuterium lock signal.
Sure-Seal is a Trademark of Aldrich Chemical Co.
Linde is a Trademark of Linde Corp.
|
