Radiofrequency oscillating magnetic fields

In c.w.-n.m.r. spectroscopy, a relatively weak, but rapidly oscillating, magnetic field is produced on the x axis by the application of a continuous, low-powered radiofrequency (r.f.) to the transmitter coil(s). As this radiofrequency approaches the resonance frequency, the magnetization vector is very slightly tipped out of the z axis, and precesses about this axis. When this frequency of precession is matched by the r.f. applied (the resonance condition), some of the individual, nuclear moments undergo transitions to the less-stable energy-level represented by precession about the — z direction, accompanied by absorption of energy from the transmitter coil. 

Nuclear magnetic resonance spectroscopy is a technique to study the perturbation of atomic nuclei in a static magnetic field (Rq) in the presence of a second oscillating magnetic field (in the radiofrequency range). ... 

NMR collects information concerning interactions between the nuclei of certain atoms present in the sample when they are submitted to a static magnetic field which has a very high and constant intensity and exposed to a second oscillating magnetic field. The second magnetic field, around 10 000 times weaker than the first is produced by a source of electromagnetic radiation in the radiofrequency domain. 

Figure 1. Fundamentals of ICR excitation. The applied magnetic field direction is perpendicular to the page, and a sinusoidally oscillating radiofrequency electric field is applied to two opposed plates (see upper diagrams). Ions with cyclotron frequency equal to ("resonant" with) that of the applied rf electric field will be excited spirally outward (top right), whereas "off-resonant" ions of other mass-to-charge ratio (and thus other cyclotron frequencies) are excited non-coherently and are left with almost no net displacement after many cycles (top left). After the excitation period (lower diagrams), the final ICR orbital radius is proportional to the amplitude of the rf electric field during the excitation period, to leave ions undetected (A), excited to a detectable orbital radius (B), or ejected (C).

Mq that is shrouded in the strong magnetic field Bq is too weak to be measured. So, to get a signal, resonance of the nuclei is obtained by superimposing upon 5q, in the probe area, a weak oscillating field which originates fi-om a coil that is fed with alternating current radiofrequency. 

Both reactant and product ions can be detected in a manner resembling the detection of magnetic nuclei in an NMR experiment. The ions are exposed to a radiofrequency field o)rf from an external oscillator, and while sweeping the magnetic field H, the ions absorb energy from the applied field when the resonance condition of equation 1 is satisfied (i.e., [Pg.69]

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