Keeping samples sensitive to air or moisture from making
contact with atmospheric oxygen or Investigations of solid-state
samples using CP-MASS NMR techniques have become routine
in many of today's NMR laboratories. Recently, much time
and effort have been expended in enlarging the variety of
samples which can be studied using these techniques. Included
in this growing list are studies of air-sensitive samples,
such as catalysts or organometallic compounds, and semi-solid
samples, like gels or highly viscous liquids.
In the past, methods for preparing these types of samples
for CP-MASS NMR study have required heat-sealing of the
sample in a glass rotor or rotor insert 1,2. These methods,
however, were not used without difficulty. Some of the problems
involved avoiding axial dissymmetry in sealing the rotor
or insert . . . the prevention of sample destruction . .
. and the minimizing of solvent loss.
More recently, special rotors with O-ring seals have been
made available by CP-MASS probe manufacturers for the study
of air-sensitive or gelatinous samples. However convenient
these special rotors are, they often come with a substantial
price tag. Only if you are studying large numbers of such
samples is this likely to be a cost-effective method.
A Simpler Alternative An alternative, developed by scientists at Texaco,
Inc. in close cooperation with WILMAD, makes it possible
to avoid the problems in heat-sealing inserts and the high
costs of special rotors. As described originally 3, this
approach uses a small drop of epoxy applied to the constriction
of a Pyrex rotor insert to seal the sample. One advantage
of this sealing method is that it can be performed in the
inert atmosphere of a glove box, so it's ideal for air-sensitive
solids. It creates a reproducibly symmetrical seal, too.
Powdered samples can be funneled and packed into the insert
and gelatinous samples can be warmed and transferred into
the insert as a fluid via syringe. After the epoxy is set
and dry (10 minutes with quick setting epoxy), the sealed
insert is cut through the constriction with a glass saw.
In the original work, the sealed insert was placed in the
lower portion of a Otsuka/Chemagnetics Kel-F rotor for 13C
CP-MASS studies in an M-100S probe. Spinning rates in excess
of 3KHz were reported with gelatinous polysaccharide samples.
Rotor inserts are now available for all major MASS probe
types, including those from Bruker, Chemagnetics, Doty,
and Varian. Product numbers and a description of the rotor
into which each fits is provided in Rotor Insert Listings
in WILMAD's NMR96 catalog online. Can't find your rotor
in the list? Let us know. Chances are good that an insert
can be prepared for it, too. Contact WILMAD with details
of the rotor dimensions.
1 J. D. Gay, J. Mag. Res. 58, 413 (1984).2 D. Geschke and E. Quillfeldt, J. Mag. Res.
65, 326 (1985)3 P. J. Giammatteo, J. Mag. Res. 71, 147 (1987).
Kel-F is a trademark of 3M Co. Macor is a trademark of
Bruker is a trademark of Bruker Instruments, Inc.
Chemagnetics is a trademark of Otuska.
Doty is a tradename of Doty Scientific, Inc.
Texaco is a trademark of Texaco, Inc.
Varian is a trademark of Varian Associates.