The Field-Cycling NMR Relaxometry Technique

Spin-lattice relaxation dispersions according to Eq. 30 can be recorded over several decades of the frequency with the aid of field-cycling NMR relaxometry [18-21]. Combined with ordinary high-field NMR relaxometry, the accessible ranges for protons and deuterons are 

Incrementing the relaxation interval t thus permits one to record the relaxation curve for a given relaxation flux density The spin-lattice relaxation dispersion is then scanned point by point by stepping B through a series of discrete values spread over the desired flux density (or frequency) range. 

It can be shown that relaxation losses in the finite switching intervals diminish the dynamic range of the variation of the relaxation decay and hence 

At low fields, proton and especially deuteron spin-lattice relaxation times of viscous polymer systems may easily fall short of a millisecond. That is, coming from the large polarization field of typically 0.5 T, relaxation fields that can be as low as 10 T must be reached and settled and stabilized with a desired accuracy of better than 10% within a total passage time in the order of one millisecond. The short settling time is a stringent condition which is not easy to fiilfill practically. Likewise the passage from the relaxation field to the detection field should occur in a transition time of the same order. In particular, the detection flux density needed for magnetic resonance must be hit and reproduced with an accuracy of about 10 in subsequent transients. 

It is therefore of paramount importance to check and calibrate the field cycle with the aid of a test device providing the accuracy, the time resolution and, in particular, the dynamic range required. A fast field probe placed at the sample position connected to a 12 to 16 bit transient recorder with a MHz bandwidth turned out to be a safe way for avoiding any experimental artifacts by imperfections of the field cycle. 

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