Modulated gradient NMR

Fig. 4. Frequency-domain modulated gradient NMR rf and gradient pulse sequences, showing the (actual) gradient modulation wave form Git), the time integral of the effective gradient wave form Fit), and the spectrum of Fit). H )P directly samples the diffusion spectrum. The wave forms and spectra are for (a) double lobe/dc rectangular modulation, (b) single lobe/ac rectangular modulation, and (c) single lobe/ac sawtooth-shaped phase modulation. Note that pulse sequences (b) and (c) sample the diffusion spectrum at a single frequency.

Callaghan, P. T., and Stepisnik, J. (1995a). Frequency-domain analysis of spin motion using modulated gradient NMR. J. Magn. Reson. A 117, 53-61. 

Fig. 5.4.11 [Cal 11 ] Modulated gradient NMR for probing spectral densities of diffusive translational motion. The pulse sequence (left) consists of a CPMG echo train with interdispersed gradient pulses G(t) which produces the time-dependent wave vector k(t). The spectrum K(co) of k(t) probes the spectral density of diffusive motion at a single frequency (right).

The PGSE NMR method relies on the use of two sharp gradient pulses separated by a well-defined time interval and is therefore naturally suited to time-domain analysis of motion. However, it is important to realize that this particular form of two-pulse gradient modulation is not unique. In particular, a number of other time-modulation schemes are possible in which the molecular motion is detected in a different manner. However, as we shall see, whenever modulated gradients are used to encode the spin magnetization for motion rather than position, it is appropriate to refocus any phase shift due to absolute spin position by means of a spin echo. Consequently, we refer to this more general type of experiment as modu-... 

Topgaard, D., Malmborg, C. and Soderman, O. Restricted self-diffusion of water in a highly concentrated W/O emulsion studied using modulated gradient spin-echo NMR, /. Magn. Reson., 156, 195, 2002. 

CEMS = conversion electron Mossbauer spectroscopy DFT = density functional theory EFG = electric field gradient EPR = electron paramagnetic resonance ESEEM = electron spin echo envelope modulation spectroscopy GTO = Gaussian-type orbitals hTH = human tyrosine hydroxylase MIMOS = miniaturized mossbauer spectrometer NFS = nuclear forward scattering NMR = nuclear magnetic resonance RFQ = rapid freeze quench SAM = S -adenosyl-L-methionine SCC = self-consistent charge STOs = slater-type orbitals TMP = tetramesitylporphyrin XAS = X-ray absorption spectroscopy. 

A very similar sequence is the ll sequence, which actually belongs to a family of binomial pulse sequences. Of these sequences, the 1331 (often referred to as 1—3) is the most popular due to its wider water suppression window (see Figure l).22 Due to the frequency-dependent sine modulation of the resonance amplitudes (the so-called excitation profile ), the resonances on either side of the water signal (carrier frequency) have opposite signs. In addition, the binomial sequences suffer from baseline distortions due to a strong linear phase gradient (see Figure 1). This baseline distortion can be particularly troublesome in multidimensional experiments and therefore the binomial sequences have not proved popular for multidimensional NMR studies. 

Signal separation in NMR spectra is not restricted to NMR parameters. Other physical properties can be used to create indirect spectral dimensions. He et al. reported 3D EP COSY experiment which utilises electrophoretic migration rates to separate resonances from individual components in mixed solutions. Additional modulation of COSY resonances is introduced in the experiment by a stepwise increase of the electric field. Due to the presence of gradient pulses in the sequence the amplitude of the resonances has a cosine dependence on the field strength with the period depending on the electrophoretic mobility. In the processed spectra, COSY spectra of individual components are observed in separate planes. 

Fig. 2.2.5 Phase and frequency encoding of the NMR signal (RX receiver) following an rf excitation pulse (TX transmitter). The space information in the y-direction is encoded in the signal phase by a gradient pulse Gy of length ti. The phase-modulated signal is aquired at discrete time intervals Afi in the presence of a gradient Gx which encodes the space information in a -direction in the frequency of the signal.

Multi-quantum transitions can only be observed indirectly by a modulation of the detected signal with the phase of the multi-quantum coherence. This modulation is achieved in an experiment by variation of an evolution time prior to detection. Repetitive detection of the signal for different evolution times provides the information about the evolution of the multi-quantum coherence. The indirect detection of spectroscopic information based on phase or amplitude modulation of the detected signal is the principle of multi-dimensional NMR spectroscopy [Eml]. Thus multi-quantum NMR is a special form of 2D NMR. Also, NMR imaging can be viewed as a special form of multi-dimensional NMR spectroscopy, where the frequency axes have been coded by the use of magnetic field gradients to provide spatial information. 

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