Among the particularities of FFC-NMR relaxometers is the extreme dynamics of the magnet system which, while essential for the technique , makes the acquisition field somewhat unstable (noisy). Further field reproducibility problems arise also from the thermal and mechanical stresses on the magnet during operation.
All this deteriorates signal coherence between successive scans and thus reduces the efficiency of the data accumulation process (FID shortening). The close link between magnetic field stability and reproducibility and the achievable sensibility is of considerable importance because low S/N ratios still limit many of the inviting potential applications of FFC NMR relaxometry. The limit is particularly felt when measuring relaxation profiles of nuclides with low g and/or low abundance, such as 2D, 7Li, 17O, 13C, etc.
Improving the S/N ratios has been one of the principle goals of many recent Stelar efforts, such as the development of FFC magnets with ever higher maximum fields (higher initial polarization), better cooling systems (improved field stability), higher acquisition frequencies (improved probe sensitivity), solenoid probes (better sensitivity for small-volume samples), novel null-biased sequences (reduced noise propagation in data evaluation algorithms) and, last but not least, novel signal acquisition and accumulation methods implemented on the new SOC (system-on-chip) PCI board to be used on future Stelar instruments.
The features implemented on this board include a complete digital receiver capable of direct signal digitization up to 90 MHz and including a dual digital down converters in quadrature and programmable filters (CIC and FIR) which can be switched in real time even during signal acquisition. The switchover to complete digital handling, by itself, eliminates many of the artifacts typical of all types of NMR, such as those due to offsets and quadrature misadjustment.
Another avenue leading to a S/N improvement regards samples with several phases (e.g., a rigid matrix, a bulk water phase, and an adsorbed water layer) in which it is convenient to divide the FID into several temporal windows and apply in each of them quite different signal-acquisition parameters (such as the dwell time and filter width values). This is made possible by the novel timing-control features of the on-the-chip pulser sub-processor.
Moreover, we have implemented novel data accumulation modes intended to combat the field stability and reproducibility problems which, as mentioned above, are specific for FFC NMR relaxometry. In particular, the new system adopts a number of data accumulation schemes, some of which are substantially different from the usual averaging of the Cartesian components of the complex signal. Thus, for example, it is possible to average (in separate buffers) the magnitudes and phases at each FID point and thus remove most of the field instability effects and drastically extend the usable portion of the FID. It is even possible to average simultaneously the two Cartesian coordinates as well as the magnitude-phase pairs in four distinct buffers. Mathematical algorithms for proper exploitation of all this information are under development but there is no doubt that the impact on achievable S/N ratios will be quite considerable.
- G.Ferrante, S.Sykora, Technical Aspects Of Fast Field Cycling,
in Adv.Inorg.Chem., Eds. R.van Eldik,I.Bertini, 2005, 57, 405-470.