Radiofrequency complications

Al MAS NMR has been demonstrated to be an invaluable tool for the zeoHte sdentist It provides a simple and direct way to quantify the proportions of A1 in four [Al(4)j, five [Al(5)j and six [Al(6)j coordinations. Quantitative determination of these species is an important issue in catalysis, and major effort is devoted on this topic. As mentioned already, for A1 only the central transition (-i-half to —half selective exdtation ) is detected. The central transition is unaffected by first order quadmpolar interaction, but the presence of second order effects causes broadening and complicates the quantitation of the A1 species. Usually hydrated samples and short radiofrequency pulses are employed for quantitative determination of framework and extra framework aluminum species. It is uncertain whether hydration changes the coordination of A1 species. Certain extra framework A1 can have very large quadmpolar interactions resulting in very broad lines ( NMR invisible ) [155, 202]. Unlike Si NMR, Al has a short relaxation time due to its quadmpolar nature, and the Al NMR spectrum with good signal to noise can be obtained in a relatively short time. 

Zhou L, et al, Thromboembolic complications of cardiac radiofrequency catheter ablation a review of the reported incidence, pathogenesis and current research directions. J Cardiovasc Electrophysiol 1999 10(4) 61 I —620. 

Despite numerous applications, conventional CRAMPS still remains one of the most demanding solid state NMR experiments as it requires the use of specially prepared spherical samples to minimise radiofrequency inhomogeneity effects and the careful calibration and setting of pulse widths and phases. Further modifications of the experiment that do not require the complicated and extended set-up procedures have been suggested recently. These are known as rotor-synchronised CRAMPS, which combines a new multiple pulse sequence [21], and its modification which uses a standard WHH-4 sequence at ultrafast MAS frequencies (e.g. 35 kHz) [22]. 

The lifetime of the ordered state is finite due to the finite rate of the dipolar spin-lattice relaxation. For H0VO4 this lifetime exceeds an hour (Clark et al. 1987), however it is significantly shorter (to about 200 s) in experiments with radiofrequency and modulating fields, and consequently the measurements become complicated (Bleaney et al. 1987). 

Hindricks, G., The Multicentre European Radiofrequency Survey (MERES) complications of radiofrequency catheter ablation of arrhythmias. The Multicentre European Radiofrequency Survey (MERES) investigators of the Working Group... 

---