Spatial Encoding NMR : Methods and Application to Relaxation Measurements, Dissolution Monitoring and Ultrafast NMR

Abstract

Discrete and Continuous spatial encoding methods are described with details of understanding principles and practical implications. Step by step experimental op- timization procedure of these methods to achieve slice selection are also discussed. In the subsequent chapters we use these methods for different applications. Spin-lattice relaxation parameters of NMR active nuclei provide valuable infor- mation on molecular dynamics. Single scan selective excitation methods of mea- surement of T1 result in significant reduction of time compared to the standard inversion recovery method and are attractive tools of applications in `Real time' NMR investigations of biological and chemical processes. It is shown here that the addition of the gradient echo following the selective excitation not only significantly improves the S/N ratio, but also makes GESSIR a versatile pulse sequence. Using this sequence, T1 values ranging from 2 s to 56 s have been measured with accuracy comparable to the standard IR experiment. This indicates that it is possible to utilize GESSIR for a wide range of molecules containing protons and hetero nuclei with medium to long T1 relaxation times as a routine NMR technique. The utility of the technique for studying other relaxation parameters has also been demonstrated. It may be mentioned that for measurement of relaxation parameters routinely, a few well-chosen points are enough. A fine selection of large number of experimental points could be useful when high accuracy is required or Chapter 3. GESSIR 91 for applications where certain property of the system investigated is changing in a continuous manner spatially and requires large number of slices to be selected as discussed in the next chapter. The long duration of time-honored two dimensional experiments is reduced to fraction of seconds by employing the ultrafast encoding experiments. Main com- plications in making the UF experiments available for routine use were the limited spectral widths and resolution in both UF and conventional dimensions. Various developments have been made in the path of improvements in increasing the spectral width in UF dimension. Of these, two experimental methods that are already proposed, namely the folding of peaks into the observable spectral window and the interleaved acquisition which doubles the spectral widths in both dimensions. The integration of covariance processing with ultrafast technique yields better correlated spectrum with considerable improvement in resolution. The effectiveness of the new processing is demonstrated for UF COSY experiments which can be easily extended to other ultrafast homonuclear experiments like TOCSY, NOESY as well as multi dimensions. The proposed processing method is an initial step to work on improving resolutions of UF data and making the ease of applicability of ultrafast spectroscopy as a routine multidimensional NMR. At the same time of this work W. Qui et.al [268] proposed a processing method based on covariance and pattern recognition for improving resolutions of spatially encoded data. They used pattern recognition algorithm also for avoiding the artifacts due to very low resolution data available with the UF experiment. They implemented the method UF TOCSY spectra and shown resolution improvement with the covariance pro- cessing for relatively more number of detection gradients which is many times hardware limited. Our method of covariance data processing is essentially same as that of Qui, less number of acquisition gradients were used in our processing, linear prediction and apodization concepts were utilized and the artifacts arise due mismatch of datas with positive and negative acquisition gradients are minimized by shifting one the data. In conclusion new methods of processing/the combination of various processing methods of the ultrafast data have the scope of improving the quality of ultrafast NMR spectra.