Pulse sequence length

Pulse sequence length This leads us to the third criterion of sequence performance. A method that requires too much time either rims into problems with radiation damping and/or exchange with atoms of interest in the solute... 

A number of parameters have to be chosen when recording 2D NMR spectra (a) the pulse sequence to be used, which depends on the experiment required to be conducted, (b) the pulse lengths and the delays in the pulse sequence, (c) the spectral widths SW, and SW2 to be used for Fj and Fi, (d) the number of data points or time increments that define t, and t-i, (e) the number of transients for each value of t, (f) the relaxation delay between each set of pulses that allows an equilibrium state to be reached, and (g) the number of preparatory dummy transients (DS) per FID required for the establishment of the steady state for each FID. Table 3.1 summarizes some important acquisition parameters for 2D NMR experiments. 

The measurement procedure is known as the pulse sequence, and always starts with a delay prior to switching on the irradiation pulse. The irradiation pulse only lasts a few microseconds, and its length determines its power. The NMR-active nuclei (here protons) absorb energy from the pulse, generating a signal. 

Figure 22 Pulse sequence of the HMBC-RELAY experiment. Filled and open bars represent 90° and 180° pulses, respectively. All other phases are set as x, excepted otherwise stated. A two-phase cycle x, —x is used for the pulse phases (j>, and

Fig. 10 ROCSA pulse sequence based on Cn symmetry. The rectangular blocks in black represent jt/2 pulses. The recoupling period (q) comprises k cycles of Cnln. Each complete cycle of Cnln spans n rotor periods (nzR). The rf phase of each Cq subcycle is set equal to 2nq/n, where q is an index running from 0 to n — 1. Within each Cq subcycle, azR and bzR indicate the position and the duration of the POST composite pulse, respectively. We find that the solution (a, b) = (0.0329,0.467) is a favorable choice for the suppression of the homonuclear dipole-dipole interaction. The bracketed and subscripted values indicate the pulse length and rf phase in radians, respectively. (Figure and caption adapted from [158], Copyright [2003], American Institute of Physics)...

A variety of computer-controlled pulse sequences consisting of two or more pulses of appropriate length, frequency range, power and phase, and separated by variable time intervals, has been developed, giving rise to families of 1-D (one-dimensional) and 2-D (two-dimensional) techniques. These techniques provide additional or more easily interpreted data on coupled nuclei, facilitating the identification of signals from chemically different groups of nuclei and correlations between spectra from different elements in the same compound. 

Fig. 5. Pulse sequence for MR detection of vibration using a radiofrequency field gradient. A binomial 1331 radiofrequency pulse (pulse length D, interpulse delay r) is applied in-phase with the mechanical wave. Thus the vibration period 7V is equal to 4(D + r). The number of cycles can be increased to ensure a better frequency selectivity. The constant RF field gradient generated by a dedicated RF coil allows space encoding without using conventional static field gradients (from Ref. 16 with permission from Elsevier).

As in the case of the HN(CO)CA-TROSY scheme, the HN(CO)CANH-TROSY experiment can be readily expanded to a four-dimensional HN(CO)CANH-TROSY experiment without increasing the overall length of the pulse sequence. This can be accomplished by labelling the 13C (f) chemical shift during additional incremented time delay, implemented into the 13C — 13C INEPT delay. As a result, a well-dispersed 13C (i- 1), 13C (i- 1), 15N(i), Hn(0 correlation map is obtained with minimal resonance overlap albeit with the inherent sensitivity loss by a factor of y/2. 

Accurate measurements of the frequency-resolved transverse spin relaxation T2) of Rb NMR on single crystals of D-RADP-x (x = 0.20, 0.25, 0.30, 0.35) have been performed in a Bq field of 7 Tesla as a function of temperature. The probe head was placed in a He gas-flow cryostat with a temperature stability of 0.1 K. To obtain the spin echo of the Rb - 1/2 -o-+ 1/2 central transition we have used the standard (90 - fi - 180y -ti echo - (2) pulse sequence with an appropriate phase-cycling scheme to ehminate quadrature detection errors and unwanted coherences due to pulse imperfections. To avoid sparking in the He gas, the RF-field Bi had to be reduced to a level where the 7T/2-pulse length T90 equalled 3.5 ps at room temperature. 

Fig. 1. Basic pulse sequence and CP diagram for gradient-based spin-locked ID exf>eriments. A 1 (— 1) 2 gradient ratio selects N-type data (solid lines) while 1 (— 1) (—2) selects P-type data (dashed lines). When SL stands for a -filtered DIPSI-2 pulse train, a ge-lD TOeSY is performed. On the other hand, when SL stands for a T-ROESY pulse train, a GROESY experiment is performed. S stands for the gradient length.

Fig. 5. Pulse sequences of NOESY and ROESY with spin-lock purge pulses for water suppression. (A) NOESY pulse sequence. The spin-lock pulses are typically of length 0.5 ms and 2 ms, and r = 1/SW, where SW is the spectral width in the acquisition dimension. Phase cycle (pi = x,—x) 4>2 = 4 x,x,—x,—x) ...

Fig. 1. Pulse sequence for the X/Y H PFG-HSQC experiment as employed for 19F/13C correlation spectroscopy in Ref. 21. 90° and 180° hard pulses are denoted by solid and open bars, respectively groups of two solid and one open bars denote 90° 0 — 180° +9o — 90° pulse sandwiches that serve as composite 180° pulses. 2 are delays of length 1 /(2 Jx,v), and r is a short delay of the same length as the gradient pulse (typically 1 ms). Phase cycles are as in the standard HSQC experiment, and the ratio of gradient pulse strengths is set to G2/G1 = Yy/Yx- Decoupling is employed using WALTZ-16 ( H) and GARP (Y) pulse trains.

Fig. 1. The MR Toolkit MR techniques yield infonnation about chemical and physical processes over length scales of A to cm. Imaging pulse sequences may be integrated with spectroscopy and molecular diffusion measurements providing maps of chemical composition and molecular transport phenomena at spatial resolutions of 30-500 pm.

A schematic representation of how the double-phase encoded DEPT pulse sequence achieves spatial and spectral resolution within the fixed bed of ion-exchange resin is shown in Fig. 45. Typical data acquired in this experiment are shown in Fig. 46. The data were recorded from a vertical section through the center of the bed. The direction of superficial flow was from the bottom to the top of the bed. The spectra shown were recorded at regular intervals along the length of the bed, with a spatial separation of approximately 2.5 mm. With reference to Fig. 46, the... 

Proton NMR measurements were made at 56.4 MHz on a spectrometer that was described previously (J5). Ti was measured with a 180°—f 90° pulse sequence, and lineshapes were determined from free-induction decay signals following a 1-2 fLsec 90° pulse. T p was measured with a 90° x-pulse followed by an attenuated t/-pulse whose length was varied from 10 usec to 40 msec. 

Figure 1. Pulse sequence diagram of a spin-echo experiment with field gradient pulses. The rf-pulses are denoted by 90° and 180° and the field gradient pulses by FGP. TheFGP pulses have a length 5 and are separated by an interval A.

Figure 2. Pulse sequence diagram of a Hahn spin-echo experiment with field gradient pulses. Rf- and field gradient pulses are denoted by 90°, 180° and FGP, respectively. The FGP pulses have a length 5 and are separated by an interval A as in the spin-echo sequence given in Fig. 1. VD is a time delay which may be variable in which case also A is variable. A PFG NMR experiment may also be performed with variable 5 or gradient strength (G) and fixed A. Normally, 6 is chosen between 0 and 10 ms and A between 0 and 400 ms. The time delay t depends on the T1 relaxation time of the pure oil of the emulsion but is normally between 130 and 180 ms.

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