Premises of FFC NMR relaxometry

In a conventional NMR instrument the resonant magnetic induction of a nuclide immersed in a field with magnetic induction B occurs at the Larmor frequency coi = jB, where y is the gyromagnetic ratio of the nuclide. Since the NMRD profiles cover several orders of magnitude of the field value B, they necessarily include sections with very low B values. 

A field-cycling experiment requires that the sample be subject at different times, and for variable durations, to different values of the magnetic field B. From the instrumental point of view, there are basically just two ways to achieve this goal. 

The first method consists in mechanical translation of the sample between areas with different field intensities (15,16,50-64). However, such mechanical shuttling methods are inherently slow. It follows that they are applicable only to samples with long relaxation times, limited essentially by the shortest possible time it takes to move the sample from one position to another which is typically about 50 ms. 

The second field-cycling method (65-67) uses electronic modulation of the current flowing through the coil of an electromagnet. This technique, commonly called Phst Field Cycling (FFC) NMR relaxometry, permits much faster variations of the field induction and thus extends the applicability of the field cycling approach to very short relaxation times Ti, at present, down to fractions of a millisecond. 

The instrumental aspects which will be discussed in the following sections concern primarily the fundamental characteristics and the functional behavior of those parts of an FFC NMR relaxometer which are characteristic of the FFC technique (Fig. 2). Subsequently, some of the methodological aspects of FFC NMR relaxometry, such as different data acquisition sequence and data accumulation and evaluation methods will be discussed. 

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