Deadtime probe

Experimental strategies for overcoming the common problems of sensitivity, probe deadtime and ringing, and broad resonance lines are discussed in Chapter 3. 

The earliest ° T1 NMR solid state studies were of the common thallium salts in both the solid phase and the melt, and of thallium silicate, borate and chalcogenide glasses. The ° T1 NMR literature up to 1988 has been extensively reviewed by Hinton et al. (1988). The chemical shift data for a number of thallium compounds are presented in Table 9.11. In view of the large CSA values which can occur in T1 compounds, distortion of the lineshapes can occur by loss of signal during the probe deadtime. Some of the earlier ° T1 NMR data may be corrupted in this way, and should be treated with caution. 

Another problem in many NMR spectrometers is that the start of the FID is corrupted due to various instrumental deadtimes that lead to intensity problems in the spectrum. The spectrometer deadtime is made up of a number of sources that can be apportioned to either the probe or the electronics. The loss of the initial part of the FID is manifest in a spectrum as a rolling baseline and the preferential loss of broad components of... 

Advantages. The experiment can be carried out with a conventional fast-spuming MAS probe so that it is straightforward to implement. For recording the satellite transition lineshapes it offers better signal-to-noise and is less susceptible to deadtime effects than static measurements. As the effects differ for each value, a single satellite transition experiment is effectively the same as carrying out multiple field experiments on the central transition. 

In addition to the effects from the probe there is the electronic deadtime, including pulse ringdown (100 ns), preamplifier recovery (800 ns), filter overdrive recovery (1 p.s) and ADC conversion droop (200 ns) (Hoult 1979). Magnetoacoustic ringing (up to 200 p.s) can be very significant if careful probe design (e.g. coil wire) is not considered. Samples that exhibit peizoelectric behaviour can lead to very long response times of up to 10 ms. 

The early part of the FID is strongly influenced by the second and higher moments. Thus, it is essential to be able to record the FID with the smallest possible delay after the rf pulse and the associated deadtime. However, there are three difficulties in doing this, (a) The FID actually starts in the middle of the 90° pulse (Barnaal and Lowe, 1963) (b) there may be a significant deadtime after the pulse because of ringing in the probe and instrumental saturation of the receiver (See also V.A.4.) and (c) the S/N may be poor, particularly in samples with short Tg so that the signal is significantly reduced before you can observe it. 

An advantage of the probe-pulse method is the fact that the excitation and detection are completely decoupled. Therefore, there is no instrumental deadtime. Furthermore, sensitive optical detection is used and in a few favorable cases the spin coherence of only 10 excited molecules could still be detected. 

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