Preparation pulse

The proton was estimated by subtracting the signal intensity with 0.5 ms contact after 9.5 ms delay from the signal intensity with 10 ms contact time, as above, but after the insertion of a variable delay (20/rs to 2 ms) after the proton preparation pulse. 

A clear demonstration of the effects of the magnetic coupling are shown in Fig. 16 where the liquid proton signal intensity is plotted as a function of the frequency offset of a preparation pulse (84,87). The essence of the... 

Fig. 16. Cross-relaxation or Z-spectra derived from the solvent proton spectra of cross-linked bovine serum albumin gel supported in a ternary solvent system consisting of 8.8% H2O, 8.7% acetone, and 8.8% methanol and 63% D20.The offset axis represents the frequency offset of the off-resonance preparation pulse which partly saturates the immobilized spin. In this case the 3 s preparation had an amplitude of 880 Hz at a Larmor frequency of 500 MHz (87).

In FFC relaxometry, the most conspicuous pulser-controlled device (apart from the RF excitation channel) is the magnet system. In other words, we generate Bq field pulses of considerable amplitude, often switching the magnet field between zero and a maximum value of over 1 T, and we rigorously synchronize such Bq pulses with the RF signal-excitation and/or preparation pulses. This, moreover, does not exclude the possibility to control other devices as well. 

Fig. 1—Schematic representation of the prepared pulsed release system a, layer containing the immediately available dose of drug (first dose) b, barrier of swellable polymeric material c, layer containing the second dose of drug d impermeable film container.

Further, as discussed in Section 3.1, the inability to control the product ratio by shaping the pulse can be overcome by photodissociating not just one EX) bound . state but a superposition of several bound states )) (as was done, e.g., with bichro-, matic control). Such a superposition state can be created separately by an initial preparation pulse, as in the case of pump-dump control scenario Sections 3.5 and. 4.1). Alternatively, the superposition state can be created by the photolysis pulse itself (by, e.g., a stimulated Raman process), provided that the bandwidth of the -pulse is comparable to the energy spacings between the Ef) levels. r, ... 

Morgan KR, Newman RH (1987) Estimation of the tannin content of eucalypts and other hardwoods by carbon-13 nuclear magnetic resonance Appita 40 450-454 Newman RH (1987) Effects of finite preparation pulse power on carbon-13 cross-polarization NMR spectra of heterogeneous samples J Magn Reson 72 337-340 Newman RH (1989) Carbon 13 NMR studies of lignin in solid samples - a review Chemistry Division Report, DSIR, New Zealand... 

Ulanski P, Kadlubowski S, Rosiak JM (2002) Synthesis of poly(acrylic acid) nanogels by preparative pulse radiolysis. Radiat Phys Chem 63(3-6) 533-537... 

Use of very short preparation pulses, however, is not ideal since the size of the spin packet that potentially gives an ENDOR signal is small (i.e., a longer pulse affects more spins). There is a trade off between selectivity and signal intensity of the hyperfine selected nucleus. As shown by Fan etal, who give numerous quantitative examples on metalloproteins, the ideal situation actually obtains for... 

The computational procedure follows closely the steps of an actual m.p. experiment see Fig. 1. The spin system, which is initially in thermal equilibrium, is hit by a preparation pulse Pp. Thereafter, one component of the transverse nuclear magnetization created by Pp, say My, is measured and the measurement is repeated at intervals of the cycle time The resulting time series My(qtJ,q = 0,...,(2 " - 1), if Fourier transformed. For simulations we accordingly first specify the initial condition of the spin system, that is, the initial value of the spin density matrix g(t) in the rotating frame. Our standard choice Pp, = P implies p(0) fy == the sum running over k = We then follow the evolution... 

Errors of the flip angles of individual pulses affect the m.p. linewidths. This is demonstrated in Fig. 8c, where the +x pulses are stipulated to be too long by 1%. Actually we find that an error of the flip angle of the +x pulses broadens the lines somewhat stronger than does an error of the same size of the +y pulses. Most likely this is a consequence of the fact that the preparation pulse we have chosen creates y magnetization and that we observe y magnetization. 

Heteronuclear multiple-quantum coherence (HMQC) allows one to edit all of the different isotopomers of a Pt cluster using the sequence described in ref. [20] with the preparation pulse only being cycled between +x and —x. More sophisticated phase cycling procedures lead to the selective editing of the different isotopomers. An example of Pt3(CO)3(PPh2 Pr)3 is presented in Figure 2. 

Fig. 8.4.2 Proton multi-quantum spectra (bottom) of an adamantane phantom (top) without (a) and with (b) application of a static gradient of 48 mT/m. Evolution time and phase of the preparation pulse sequence were incremented in 32 steps of 0.1 p,s and 2 r/32, respectively. Only even-order multi-quantum coherences were detected. The signals from orders 8 through 14 are also displayed on an expanded scale. The multi-quantum spectra demonstrate the increase in the spatial resolution of the two cylinders with increasing coherence order p. Adapted from [Garl] with permission from the American Physical Society.

All these sequences include a (90°) preparation pulse which flips the magnetization of the spin system into the xy plane. The evolution period is then the period of free precession of the spin system in the xy plane, in the Hahn spin echo experiment (Fig. 6). The almost infinite variations in the mixing strategies serve the purpose of bringing out those features of the correlated motions in the system of coupled spins which one wishes to observe. 

As for ID data, fi quadrature detection requires two data sets to be collected which differ in phase by 90 , thus providing the necessary sine and cosine amplitude-modulated data. Since the fi dimension is generated artificially, there is strictly no reference rf to define signal phases so it is the phase of the pulses that bracket ti that dictate the phase of the detected signal. Thus, for each tj increment two data sets are collected, one with a 90 preparation pulse (ti sine modulation), the other with 90j, (ti cosine modulation), and both stored separately (Fig. 5.17). These two sets are then equivalent to the... 

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