Double stimulated echo sequence

Figure 9.18 compares the diffusion decay profiles of the uncompensated BPP-STE and the compensated BPP-DSTE sequences for a sample of quinine 9.1 in CDCI3 at 313 K. The apparent diffusion coefficient from the STE data of 17.1 x 10 °m s is erroneously high due to the contribution from convection whereas a lower value of 11.1 x 10 m s is provided both by the double stimulated-echo method and for the STE with sample rotation. Whilst providing effective compensation for convection, the double stimulated-echo sequences suffer from the disadvantage of reduced sensitivity as each echo sacrifices half of the available signal, which may make the sequences less attractive in some instances. In contrast, double-echo sequences based on spin-echoes rather than stimulated-echoes do not suffer such sensitivily losses and have been employed in some combined 3D-DOSY sequences [26]. 

Both and Li diffusion have been used in the discrimination of previously characterised aggregates of n-butyUithium in THF solvent, a widely used reagent in synthetic chemistry. Diffusion measurements at 189 K (using the double stimulated-echo sequence to overcome convection) were able to distinguish the H q -CH2 resonances of dimeric and tetrameric aggregates at —1.12 and —1.00 ppm respectively [50]. Whilst Li diffusion measurements were possible for the tetramer, the broader resonance of the faster relaxing... 

Figure 9.13. Signal profiles recorded with the asymmetrical double stimulated-echo (aDSTE) sequence as a test for convection as the ratio Ai A2 is varied, (a) A uniform profile indicates the absence of convection whereas in (b) the asymmetrical profile demonstrates convection to be present.

PRESS, based on a double spin-echo, requires exact 90° and 180° pulses ideally. However PRESS too can be applied in the inhomogeneous surface coil Bi field, provided the out-of-voxel signal from nonideal pulses is removed using phase-cycling schemes and/or large spoiler gradient pulses. The same applies to the stimulated echo sequence STEAM. More complicated versions of PRESS may be implemented... 

Fig. 12. Sequences for volume selective single voxel spectroscopy. Both techniques work with three slice-selective RF-pulses. (a) The Point RESolved Spectroscopy (PRESS) sequence generates a volume selective double spin-echo. The entire time delay between the initial 90° excitation and the echo is sensitive to transverse relaxation, (b) The Stimulated Echo Acquisition Mode (STEAM) sequence generates a stimulated echo. Maximal signal intensity (without relaxation effects) is only half the signal intensity of PRESS under comparable conditions, but slice profiles are often better (only 90° pulses instead of 180° pulses) and the TM interval is not susceptible to transverse relaxation, (c) The recorded echo signal is only generated in a volume corresponding to the intersection of all three slices.

Left two-pulse [(a) primary ESEEM] and three-pulse [(b) stimulated echo ESEEM] sequences t is the (fixed) delay time between pulses one and two and T is a variable delay time. Right frequency domain and time domain (inset) of the two-pulse EESEM spectrum of VO - vanabin, recorded at the m = — 1 /2 line, at 77 K and a pulse width of 20 ns.P l The superhyperfine coupling constant = 4.5 MHz (obtained from the N double-quantum lines at 3.9 and 7.1 MHz) is in accord with amine nitrogen provided by lysines of the vanadium-binding protein. The spin echo due to proton coupling, at 13.7 MHz, was also observed. Reproduced from K. Eukui et al., J. Am. Chem. Soc. 125, 6352-6353. Copyright (2003), with permission from the American Chemical Society. 

In the stimulated echo experiment, also shown in Fig. 6.2.3, the second pulse transfers the system into a mixture of Zeeman and double quantum order (alongandpg). Here, the relevant relaxation times are Ti (longitudinal Zeeman) and T q (double quantum), for which the 45 pulses of the Jeener-Broekaert sequence are replaced by 90v pulses. Again, two echos evolve at T] around the third pulse, and are refocussed by the fourth pulse. The two negative echo amplitudes vary as function of T2, with -[exp(-T2/Tiz) + exp(-T2/Ti3Q)], and both Ti and Tqq can be determined as separate values [14]. 

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