Magnetic field scan range

The original method employed was to scan eitiier the frequency of the exciting oscillator or to scan the applied magnetic field until resonant absorption occiined. Flowever, compared to simultaneous excitation of a wide range of frequencies by a short RF pulse, the scanned approach is a very time-inefficient way of recording the spectrum. Flence, with the advent of computers that could be dedicated to spectrometers and efficient Fourier transfomi (FT) algoritluns, pulsed FT NMR became the nomial mode of operation. 

Finally, the instrument can be operated in the peak-matching mode, which provides optimum mass resolving power and mass accuracy. Here the magnetic field strength is kept constant and the electric sector and acceleration voltages are scanned over a relatively small m/z range. This mode of operation is suitable when two ions that are very close in mass need to be separated or when the elemental composition of a molecule is to be determined at high resolution. 

The allowed transition in ESR is diagrammed in Figure 2. The ESR experiment is commonly conducted at a fixed frequency near 9.5 x 109 Hz by scanning through a magnetic field range until absorption of electromagnetic radiation is detected at H0. The value of H0 can then be used to calculate the electron g-factor. 

Figure 14 illustrates a recorder curve of the resonance absorption derivative of F for a sample containing 2.7 wt. % of fluorine. The time of scanning the magnetic field through resonance was 2 hours and the time constant of the spectrometer was 90 seconds in order to increase the available signal-to-noise ratio. The F resonance absorption was examined in the concentration range of 0.3 to 12.5 wt. % fluorine. 

Fig. 16.7. Abrikosov flux lattice observed by STM. Produced by a 1 T magnetic field in NbSe, at 1.8 K, the scan range is about 6000 A. The gray scale corresponds to dUdV ranging from approximately I X 10 mho (black) to 1.5 X 10 mho (white). (Reproduced from Hess et al., 1989, with permission.)...

Based on this principle, multi-nuclei instruments were initially built. They kept the frequency fixed while scanning the magnetic field over a wide range. This allowed the qualitative and global detection of elements by their characteristic resonance frequency (Table 9.2 and Fig. 9.5). 

---