WILMAD NMR
THE IMPORTANCE OF SENSITIVITY AND MAGNETIC FIELD HOMOGENEITY IN NMR PRECISION SAMPLE TUBES
NUMBER TWO IN A SERIES OF NEWSLETTERS FOR NMR SPECTROSCOPISTS
(NOTE: To help newcomers to the rapidlydeveloping field of NMR High Resolution Spectroscopy in deciding on their needs for NMR glass items at the most reasonable cost, we include this brief discussion of the problems of sensitivity and magnetic field homogeneity)
In N M R high resolution spectroscopy, four factors point t o the importance of using sample tubes manufactured with maximum precision: (a) ease of spinning; (b) minimizing spinning side bands and, hence, some limited degree of sensitivity increase; (c) thin wall fabrication and a corresponding optimum filling factor; and (d) minimizing errors in resonance line positions when comparing lines simultaneously in separate samples. Spinning a sample increases the apparent magnetic field homogeneity only in the plane normal to the vertical (the spinning axis). Therefore, thick wall capillary tubing sometimes used to fabricate small volume N M R tubes has no specific advantage for the "ultimate in resolution" over the thin wall precision tubes except in very special instrumentât ion peculiarities. For cylindrical tubes, signal sensitivity increases rapidly with the I.D. Maximum sensitivity is obtained by achieving an optimum combination of two contributing factors; the amount of sample available to influence the receiver coil . . . and the shape and dimensions of the sample tube for maximum homogeneity during spinning. MAXIMUM SENSITIVITY WITH SUFFICIENT S A M P L E - W h e r e sufficient sample is available, maximum sensitivity is achieved by the use of a cylindrical precision tube with as large an O.D. as the receiver coil will accept under spinning conditions and the thinnest wall possible. A column of liquid the length of which measures twice the diameter of the receiver coil meets the practical goals of minimizing inhomogeneity contributions from nuclei near both ends of the column, and optimizing the number of nuclei which can appreci-
ably contribute to the intensity of the signal. In this discussion, we assume that inhomogeneities along the spin axis of an infinite •column can be corrected to the point of secondary importance. MAXIMUM SENSITIVITY W I T H O U T SUFFICIENT S A M P L E - W h e r e sufficient sample is not available and/or when inhomogeneities along the spin axis of an infinite column are primary factors, the sensitivity and homogeneity can be optimized by the use of a spherical or
cylindrical cell (See Figures 1 and 2). In a precision, spherical micro-volume cavity there are no end-effect inhomogeneities except at the opening and these are small, depending on the size of the opening. In general, if the amount of sample available is approximately equal to or less than the value 4/3 it r 3 , where r is the outside radius of the largest precision tube which can be accepted by the receiver coil under spinning conditions, then it is likely to be advantageous for both sensitivity and resolution (homogeneity) to use a spherical cell. It is assumed that the spherical cell will always be completely filled with the liquid sample under study. HIGH FREQUENCY SPECTROME T E R S - T h e 100 Mc and higher frequency spectrometers now being built provide greater chemical shifts—a very critical feature for complex biological and polymeric materials. But with increased shifts, the small undesirable effects arising from tube imperfections and asymmetric flux distributions of the applied magnetic
field unfortunately increase proportionately and sometimes even faster. These inhomogeneities become very significant in the 100 Mc and higher frequency spectrometers. Three parameters can cooperatively minimize these undesirable effects—a stable spinner, increased precision (tight concentricity tolerance and low camber) of the sample tube, and higher spinning speeds. These parameters are complimentary, for with increased sample tube precision come significantly improved spinning speeds—
and overall, a resulting decrease in side band energies, higher resolution, and improved sensitivity. Improving sample tube concentricities and lowering cambers by factors of two can often reduce the relative spinning side bands by at least a factor of five for the very high field (high frequency) spectrometers.
NEW BULLETIN AVAILABLE Our new Bulletin No. 5006 covering sample tubes, coaxial cell systems, and precision NMR spinners is now available to you without cost. This 24-page bulletin lists the various tubes, coaxial cells, and spinners available together with the dimensions and tolerances we are able to maint a i n . The brochure also gives prices for the Wilmad line of NMR glassware.
WILMAD GLASS COMPANY, INC. Pioneer in Glass for NMR Spectroscopy U.S. Route 40 and Oak Road, Buena, N. J. 08310 · Phone: (609) 692-6000 Circle No. 32 on Readers' Service Card V O L 3 9 , N O . 1 1 , SEPTEMBER 1 9 6 7
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