Low-field spectrometers

Such performance was beyond the capabilities of the NMR spectrometers used in the first few years of CP/MAS NMR studies of lignin. The problem of SSB strength was at first avoided by using low-field spectrometers, but this was unsatisfactory because of the poor sensitivity associated with low fields. The problem was later partly overcome by SSB-suppression pulse sequences (Dixon et al. 1981, Barron et al. 1985), or by applying correction factors to the centerband strength (Hemmingson and Newman 1985). 

Fig. 9.3.1 [Gri2] Timing diagram for operation of a low-field spectrometer. The input pulse for the transmitter is overdriven until the B magnetic field has reached its maximum value. It then falls to a maintenance level until completion of the pulse. The Q switch signal toggles the rf coil between low and high values of the quality factor. The duplexer signal switches between transmit (low) and receive (high) operation of the coil. The receiver gate is turned on just for detection of the signal.

Note that this technique was applied in the mid 1970s using low-field spectrometers. It may not be useful on high-field spectrometers. See Section 2.3.3, especially pp. 56-57. 

The a parameter is necessary for low-field spectrometers in order to correct errors from the 90° pulse. Data were also fitted by the MEM (Maximum Entropy Method) algorithm which presents the advantage of providing a continuous distribution of spin-lattice relaxation time components. 

This section discusses important details relating to H-NMR experiments and the necessary precautions to be taken before starting measurements on cement samples. The following considerations are valid for cement samples measured with low-field spectrometers and do not apply to all materials and spectrometers. 

In this section, relaxation times are discussed. The given values are characteristic for relaxation times of water in mature white cement pastes probed with low-field spectrometers (< 1.0 T or 40 MHz NMR frequency). For spectrometers with comparable frequencies, the ratios of the relaxation times should remain roughly similar if portland binders are used at... 

It should be stressed that the discussed effects, especially when referring to proton NMR, are essentially field independent, which means that the described experiments can be carried out on simple low-field spectrometers, provided that chemical resolution is not needed, which is the case for most single-component elastomers and melts. 

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