Spectral windows, widened

Standard sets of acquisition parameters include typical transmitter offset values. If wider spectral widths are required, simple trial and error with concentrated samples or standards permits the operator to widen the spectral window in a selective manner. For example, the operator may suspect the presence of highly deshielded H signals and wish to open the H spectral width from the usual 10 ppm (4,000 Hz at 400 MHz) to 15 ppm (6,000 Hz). In order to add all of the additional 5 ppm (2,000 Hz) worth of sw capacity to the downfield (high-frequency) end of the spectral range (i.e., from 10-15 ppm), the transmitter offset value is increased by approximately 2.5 ppm (1,000 Hz at 400 MHz). This procedure also keeps the transmitter offset positioned in the middle of the widened spectral window. 

Fortunately, in proton spectroscopy it is generally possible to detect the presence of an aliased peak because of its esoteric phase which remains distorted when all others are correct. In heteronuclear spectra that display only a single resonance such phase characteristics caimot provide this information as there is no other signal with which to make the comparison. In such cases it is necessary to widen the spectral window and record any movement of the... 

The range of chemical shifts over which NMR spectroscopic signals appear is dependent on the nucleus. The most commonly observed nucleus is H and, in organic compounds, a spectral window from 5+15 to 0 usually encompasses most signals. In inorganic compounds, the window may have to be widened if, for example, H nuclei attached to metal centres are to be observed, or if... 

An additional consequence of the widened spectral windows at high spectrometer field Is the requirement for enhanced pulse power. In order to assure uniform excitation across the full spectral width the rf pulses have to be sufficiently short. This Is particularly critical to achieve with hlgh-Q multlnuclear probes, and a fully satisfactory solution to this problem has at this time not been fomd. In Figure 5 the transmitter rf amplitude distribution for a... 

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