A Simple and Inexpensive Modification for Coaxial NMR Cells Precision-made coaxial nmr cells'provide reliable data when external standards are employed hut are relatively expensive for general laboratory utility. Modifications on the basic coaxial design have been used in our lahoratory in an effort to circumvent this problem including the manufacture of Pyrex or Teflon spacers, taping the reference tube coaxially with Teflon tape, or creating a spherical bulb2 at one end of the reference tuhe. These expedients were deficient in one way or another. For example, machining Teflon spacers to the strict geometric requirements for a coaxial tube is very tedious and nearly impossible, Pyrex spacers generally allow too much play in the reference tuhe and suffer from variability in cell wall thickness, and creating a spherical hulh2 of the precise diameter which precludes leaning of the reference tube is time eonsuming-retrieval of this tight fitting reference tube is an additional problem. These deficiencies create a fundamental problem anathema toprecision nmr work in that thesample and reference will not experience tbesame external fields even though the nmr eell may be spinning rapidly.3 A simple and inexpensive modification for coaxial nmr cells that has proved very satisfactory in our lahoratory has been to employ sections of various gauge, Teflon t u b i n 6 which encircle the reference tube a8 shown in thediagram. Twosmall lengths (about 10mm) of 12-gauge tubing (containing similar lengths of 14-gauge tubing) are positioned within a piece of 10-gauge tubing so as tosurround the reference tuhe (Kimax Capillary Tubes: 0.9-1.1 ID X 90mm) above the resonating area. It is generally easier to place the reference tube into the concentric tubings first then push the whole unit firmly dawn the inside of the nmr tube. The length of the 10-gauge tubing can be such that asmall amount exceeds the opening of thenmr tube and yet doesnot prevent capping the top. (Forsealed nmr tubes a shorter length of tubing can be used). In this way reference tubes can he easily retrieved and employed again. This modification is simple and insures a reliahle coaxial nmr eell design for use with external standards. In addition, the small cost4 (about $0.09/ft) of the various Teflon tubings makes this modification both inexpensive and practical for general laboratory use. The author wishes to thank Leslie H. Simons for informative discussions and helpful suggestions. Wilmad Glass Co., Inc., Buena, New Jersey. Hinton. J. F., and Amis, E. S., J. CHEM. EDUC., 43,443 (1966). Zimmerman, J. R., and Foster, M. R., J.Phys. Chem., 61,282 (1957). 'The Polymer Corp. of Pennsylvania, Reading, Pennsylvania. The University of Texas at Austin Austin. 78712
Harold C. Nelson
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A) IO-gauge Teflon tubing. B) 12-gauge Teflon hbing m i n i n g 149uge Teflon tubing. O 5mm OD nmr tube, D) Reference tube.
