J. F. Hinton and E. S. Amis Universitv, of Arkansas Fayetteville, Arkansas ~
II
An Inexpensive coaxial NMR cell
The madvisib~lity . .... of using internal standards for nuclear magnetic resonance studies of aqueous solutions as well as organic solutions has been discussed by several author^.'.^.^ Since there is no generally accepted internal standard for measuring chemical shifts in aqueous solutions, coaxial NMR cells have been developed permitting the use of external references. The construction of such-a cell appears in the literature4 and there is one commercially availablej coaxial NMR cell. This paper describes an easily constructed and inexpensive coaxial NMR cell. This cell would seem to have a number of advantages. It is easy to construct, inexpensive, and there is no loss of reference solution due to evaporation in the NMR instrument. Furthermore, the same reference solution can he used repeatedly, and reference solutions can be degassed if necessary. The coaxial cell has been tested against the commercial cell and the results found to be indistinguishable
in. tact with the flame, and hold approximately of the heated end over the flame while rotating the tube evenly. (See Fig. 2.) Internal pressure causes the end of the tuhe to expand into a sphere. I n making the spherical end, it is most important to rotate the tube steadily over the flame in order to prevent lean-
Construction
The cell consists of two parts: a 5 mm VarianeNMR tube and an inner tube made from standard 1.5 X 2.0 X 100 mm capillary tubing as shown in Figure 1. The
Reference Compound Figure 1.
NMR Tube
Coaxial NMR cell.
small sphere a t the end of the inner tuhe permits reproducible positioning of the reference compound. The inner tube containing the reference solution is constructed in the following manner. Seal both ends of a capillary tube. Using a "soft" flame of a methaue-oxygen torch, place the tip of one end of the tube into the flame again until the melting point is reached. Remove, the tube from direct conThis work has been supported by the United States Atomic Energy Commission, Contract Number AT-(4C-1)-2069. L B ~ ~ n ~A. ~A,,~ AND - BGLICK, ~ , R. E., J . Am. C h m . Soe., 78, 1071 (1956). Z~MMEBMAN, J. R., AND FOSTER, M. R., J. P h p . Chern., 61, 282 11957). -... QVIsT, M. S., AND FORSLIND, E., Acta chern. Scand., 16, 2069 (1962). ' MORIN,M. G., PAULET~, G., AND HOBBS,M. E., J. P h ~ s . C h a . , 60, 1594 (1956). Wilmd Glass Company, Landisville, N.J. 6Vmian Associates, Palo Alto, Cald. ~
Figure 2.
ConstrucHon of inner reference tube.
ing. The sphere can he made so that it just fits into the 5 mm NMR tube. After the sphere has been made, break open the other end of the tube and introduce the reference solution into the tube with a capillary pipette. Then flush the tube with nitrogen. Freeze non-aqueous references such as benzene, cyclohexane, or dioxane with dry ice before resealing the open end of the tuhe. In sealing the open end, only the very tip of the tube need come in contact with the flame. After construction of the inner tube, place the solution to be studied in the 5 mm NMR tuhe and put the inner tuhe inside. (Push into position with a solid glass rod.) One of the main advantages of this cell, other than ease of construction, is its low cost. The complete cell can be made for the cost of a Varian 5 mm. NMR tuhe (present cost $2.20) plus the cost of one capillary tuhe (less than one cent). This compares most favorably with the present cost of about $13.00 for the commercially available 5 mm od precision coaxial cell. The use of a coaxial cell such as that described in this paper is particularly advantageous for kinetic studies in which the sample tube must remain in the NMR probe for an extended period of time. We have found that reference compounds such as benzene and cyclohexane volatilize out of the commercial cell within about an hour a t our probe temperature, 3 7 T . Volume 43, Number 8, August 1966
/
443
