NMR-005: Sealable Pyrex Rotor Inserts for CP-MASS
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 Corning.
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.