Spurious ringing

Of the three common probe parts which can ring, the body is the easiest to deal with. A good choice of the probe body material usually will take care of the problem. The following table of room temperature parameters of metals 

Furthermore, p decreases rapidly with decreasing temperature for pure metals making aluminum even less attractive at low 

and p values are taken from Handbook of Chemistry and Phy sics, 50th ed. J 

A second possible cause of spurious ringing which is limited only to some solids is piezoelectric resonance of the sample. (Of course the cause need not be limited to a sample. More on this later.) A piezoelectric crystal has the property that a mechanical deformation of the material takes place in the presence of an electric field. Therefore, an acoustic ringing can be induced in a piezoelectric material located within an NMR probe by the rf electric field associated with the rf magnetic field. Such a ringing due to piezoelectricity has the distinguishing property that it is independent of the applied magnetic field intensity in contrast with the magnetic field induced effect described in the first part of this section. 

Gibson and Raab (1972) found that piezoelectric resonances of the NMR sample can be attenuated in one of two 

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The zincblende (ZB), or sphalerite, structure is named after the mineral (Zn,Fe) S, and is related to the diamond structure in consisting entirely of tetrahedrally-bonded atoms. The sole difference is that, unlike diamond, the atoms each bond to four unlike atoms, with the result that the structure lacks an inversion center. This lack of an inversion center, also characteristic of the wurtzite structure (see below), means that the material may be piezoelectric, which can lead to spurious ringing in the free-induction decay (FID) when the electric fields from the rf coil excite mechanical resonances in the sample. (Such false signals can be identified by their strong temperature dependence due to thermal expansion effects, and by their lack of dependence on magnetic field strength). 

As hinted above, piezoelectric resonance can occur in some probe parts besides the sample so that it could even affect probes for solution NMR. Therefore, if there seems to be a problem with a magnetic field independent spurious ringing, check for possible piezoelectricity of a component in the probe. Remember that quartz is very piezoelectric. In addition to the obvious uses as tubes and whatnot, quartz is also used sometimes as a filler in epoxy. 

P. A. Speight, K. R. Jeffrey, and J. A. Courtney, "A probe modification for pulsed nuclear magnetic resonance to eliminate spurious ringing, J. Phys. E 7, 801-802 (1974). 

Practically therefore a primary break of variable pitch does not seem to be needed, a ring mt cury teeak of fixed pitdi ( f[Pg.1]

According to Ring [22], Equation 3.3.38 is mathematically well behaved. The equation has no spurious roots and maximum or minimum. Also, the fraction of liquid vaporized, m2,v/mi, varies between 0 to 1 and is linear. 

The techniques discussed in this paper can be used not only for solving the problem of magneto-acoustic or piezo-electric ringing, but also for cancelling the residual ringing in the coil, the secondary ringing caused by the Q-switch system or any other spurious signals. 

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