Abstract

During the course of investigations at this laboratory of native and modified cotton celluloses by means of wide-line NMR spectroscopy, it is frequently necessary to record the proton signal of dry cotton cellulose. The peak-to-peak line width of this signal, as measured on the first derivative of the absorption curve, which is the signal recorded, is about 12 G. The instrument employed in this work is a Varian VF-16 B NMR spectrometer. The normal mode of tuning the instrument is to display the input signal on the oscilloscope, tune out both the V-mode and U-mode signals by means of the probe paddles, and reintroduce leakage of the signal desired, in this case the V-mode. The method of detection of the signal used in the instrument results in the first derivative of the absorption being presented on the recorder when tuned to the V-mode. Alternatively, tuning to the U-mode results in the presentation of the first derivative of the dispersion being recorded. If one assumes that the V-mode and U-mode signal shapes may be adequately described as the derivatives of curves with the forms of the Bloch susceptibilities, then the following observations concerning the symmetry of the signals may be made. (Many wide-line NMR signals may not possess these properties, owing to chemical shifts or other phenomena. However, for a number of spectra frequently investigated, such as proton signals from polymeric materials, these symmetry relationships appear to be generally applicable.) The first derivative of the absorption <i>g'(h)</i> is ungerade, i.e., <i>g'</i>(−h) = − <i>g'</i>(h). The first derivative of the dispersion <i>f'</i> (<i>h</i>) is gerade, <i>f'</i>(<i>h</i>) = <i>f'</i> (<i>−h</i>). Here <i>h</i> denotes the deviation of the magnetic field from the resonance peak value. Unfortunately, the broad line width of dry cotton permits only a small segment of the input signal to be displayed on the oscilloscope and consequently the technique outlined above cannot be employed in tuning the signal. One may tune the instrument with some convenient narrow signal such as doped water and then insert the cotton sample. The wide-line signal will then normally be very close to being tuned but will have some of the U-mode signal present. Since the signal strength is normally small it is necessary to scan such signals slowly. One scan may take from 10 to 100 min. Thus, the final tuning of the signal, which can be done only by reference to the recorder presentation, may be tedious and time consuming. The purpose of this communication is to outline a method of correcting an NMR signal which may have a component of the undesirable U-mode signal present.

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