Spin echo reproducibility

Figure 4. Spin-echo technique for removing initial transients from signal. Reproduced with permission from Ref. 1. Copyright 1982, The American Physical Society.

Fig. 12. (a) 64 MHz H STEAM MRS spectrum of a brain tumor in vivo, and 500 MHz H HRMAS NMR spectrum of a biopsied sample of the same tumour using (b) presaturation, and (c) a spin echo sequence. Metabolites are identified directly on the spectra. For HRMAS, the samples were rotated at 4 kHz. Reproduced with permission from Ref. 128. Copyright 1999 Elsevier. 

The reproducibility of spin-echo measurements is a sensitive function of the temperature stability of the sample during measurement. Changes in temperature affect both the unattenuated echo height (mainly via T2) and the degree of attenuation (via D). 

The use of NMR spectroscopy as an analytical technique is well established ( 1 8). In order to quantitate our spin-echo height to the number of protons present, we performed an independent calibration using standard solutions of naphthalene in carbon tetrachloride. Concentrations for the standards were chosen to correspond to the anticipated supercritical C02 solubilities, and all calibration measurements were performed using a sample cell of the same dimensions as the solubility sample cell previously described. The response of our spectrometer to the standard solutions was linear over the concentration range. The reproducibility for independent measurements of the calibration curve was 3 . Throughout the experiment, all spectrometer conditions (pulse lengths, phases, receiver amplifier gain, etc.) were closely monitored, and frequent checks on the calibration of the spectrometer were performed. In this way we were able to obtain the molar solubility of solid naphthalene in supercritical carbon dioxide to an estimated experimental accuracy of 6%. 

FIGURE 25 SEM micrographs (A), XRD spectrum (B) and spin-echo mapping NMR spectroscopy of active catalysts derived from VPA (row 1), VPO (row 2), and VPD (row 3) precursors. Reproduced with permission from Ref. (39). Copyright 1996 Elsevier. 

As an example of how to determine the electronic ground state of a low-spin iron(lll) compound, we present work on the ferric low-spin heme complex [TPPFe(NH2PzH)2]Cl, which has been shown to have a (dxy) (dxz, dj, ) electronic ground state. The field-dependent Mossbauer spectra of [TPP Fe(NH2PzH)2]Cl displayed in Fignre 12 are well reproduced by simulations, which yield 5 =0.25mms , AEq = ( )2.50imns, an asymmetry parameter rj = —3, and an anisotropic A tensor of" / nMn = (-47.6, 6.7, 18.3)T. The g values necessary for the 5" = 1 /2 spin Hamiltonian (g z = 2.39, gyy = 2.28, and gxx = L87) have been taken from a combined EPR and electron spin echo envelope modulation spectroscopy ESEEM analysis. 

Fig. 28. Schematic representation of the NMR self-diffusion measurements (a-d) and their applications in the spin-echo technique (e-g). Broken lines in (c) and (d) indicate the behaviour with molecular migration, (a) RF pulses, (b) gradient pulses, (c) transverse magnetization M of different regions, (d) total transverse magnetization M equal to vector sum of (c), (e) RF pulses, (f) gradient pulses and (g) magnetization (Reproduced by permission of John Wiley Sons, Inc., Chichester).

Figure 6-41 The double pulse field gradient spin echo (DPFGSE) NOE experiment with llp-hydroxyproges-terone (a) the unirradiated spectrum (b)-(g) spectra with irradiation at selected frequencies. (Reproduced with permission from K. Stott,...

Fig. 6.26, Signal decay of the water peak (a, b) and one representative peak of 15c (c, d) as a function of the gradient strength (C) in the CDCI3 solution of [ 1 5c)6 H2O)8] collected with a BPLED (a, c) and spin echo diffusion (b, d) sequence along with the normalized signal decay (In I/Iq) as a function of the b-values taken with the sequences shown in Figs. 6.1 and 6.4 for (e) the water peak and (f) one of the peaks of 15c [62]. (Reproduced with permission from ref. [62]. Copyright 2005 American Chemical Society.)...

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. 

Figure 11 Solid-state chlorine NMR spectra of i-lysine hydrochloride. Experimental spectra are shown in (A) Cl at 11.75 T and (C) Cl at 11.75 T. The Cl NMR spectrum of a stationary sample obtained at 11.75 T is shown in (E). A hyperbolic secant spin-echo sequence was used to acquire the spectra. Simulated spectra are shown above each experimental spectrum. The effects of CSA are evident in the spectrum of the stationary sample (E) shown in trace (G) is the simulated spectrum obtained with an isotropic CS tensor. The relative orientation of the EFG and CS tensor PASs are shown in the inset. From Ref. 179. Reproduced by permission of the American Chemical Society.

Fig. 8. Plot of spin-echo intensity (on a logarithmic scale) against the gradient wave vector q for water in a randomly packed bed of polystyrene spheres with average diameter 15.8 pm. The diffusion time A was 20 ms (squares), 40 ms (triangles), 70 ms (circles) and 110 ms (diamonds). A coherence peak is observed at a position corresponding to the average interpore spacing. (Reproduced with permission from ref. 147, 1992, American Institute of Physics.)...

In the basic two-pulse or primary echo experiment, two pulses separated by a time T are applied. The second pulse is twice as long as the first. At time t after the last pulse a transient response appears from the sample, the so called spin echo. By monitoring the echo amplitude as a function of the time t, a spin echo envelope can be recorded. The hyperfine couplings are obtained either by trial-and-error simulations to reproduce the modulations superimposed on the decaying echo amplitude (the original procedure) or by a Fourier transform to obtain nuclear frequencies in modern instruments as in Fig. 2.20. The frequencies are the same as obtained in ENDOR. Contrary to ENDOR, combination peaks at the sum and difference frequencies may also occur. 

Figure 1. Diagram showing how the two-pulse echo envelope may be obtained by repeating the electron spin echo cycle with gradually increasing values of t. The "modulation" of the envelope is due to electron-nuclear coupling and is caused by interference between allowed and semi-forbidden microwave transitions (see e.g. Figure 2). The periods observed in the echo envelope correspond to the nuclear shfs, or ENDOR frequencies. (Reproduced with permission from Biochemistry 15, 3863 (1976).)...

Figure 3. Transmitter pulse sequence and spin echo signals observed in a stimulated echo experiment. In order to obtain the envelope function, t is set to a fixed value and T is slowly increased. Superhyperfine frequencies appear as terms of the form cos(u (T+t)). The unwanted echoes C, B, A are two-pulse echoes generated by combinations of pulse III with pulse I, with pulse II, or with the first two-pulse echo E. Overlap of echoes B, A, with the stimulated echo, SE, occurs when T = t and T = 2t and may cause glitches to appear in the echo envelope function. (Reproduced with permission from the Journal of Biological Chemistry.)...

Figure 14 (A) Planning images for magnetic resonance spectroscopy (MRS) of the brain top and middle spin-echo image TE = 96 ms, TR = 3 s. Bottom gradient echo image TE = 1.9 ms, TR = 116 ms. (B) Chemical shift metabolic image (CSI) of the brain overlaid on the anatomic image. Spectra are displayed for the selected regions. Chemical shift range is from 4.3 to 0.5 ppm. (Spectra were obtained by Dr JA Hopkins, GE Medical Systems and reproduced by permission of GE Medical Systems, Milwaukee, Wl.)...

Figure 3 Schematization of pulsed-gradient spin-echo (PGSE) data editing for a hypothetical mixture composed by a large (red), slow diffusing, and a small (blue), fast diffusing, species. (Reproduced from Ref. 11. CRC Press, 2011.)...

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