Download Applications of Nuclear Magnetic Resonance Spectroscopy in by L. M. Jackman, S. Sternhell, D. H. R. Barton and W. Doering PDF

By L. M. Jackman, S. Sternhell, D. H. R. Barton and W. Doering (Auth.)

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The contents of the stores are then converted back to analogue voltages and recorded as for a normal spectrum. The result is a spectrum with a signalto-noise ratio which is improved by the square root of the number of scans. The frequency intervals into which the spectrum is divided must be less than the desired resolution. It is also essential that the data stored in a given memory address corresponds in each scan to the same point in the spectrum. For this reason, afield-frequencylocked spectrometer, preferably with automatic homogeneity control, or a permanent magnet instrument must be used.

Clearly, the smaller the volume of homogeneous field required the higher will be the effective homogeneity. Thus considerations of resolution demand small samples. This requirement has to be balanced against the demands of sensitivity which is related to the number of nuclei of a given type in the sample under investigation. In other words, we have generally to compromise between the conflicting requirements of resolving power and signal strength. We have seen that homogeneity in the X and Z directions (Fig.

1-2-8. f. (100 Hz). In practice, it is possible to modulate simultaneously at more than one frequency. As we are only interested in the relative positions of resonance absorptions in a spectrum (p. 12) we wish to measure the position of a given absorption relative to a reference or control absorption which is observed at a constant field-to-frequency ratio. The control absorption can be derived from a separate sample (external lock system) or from a reference compound added to the sample under investigation (internal lock system).

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