Pulse angle sequences

Irradiation of a sequence of very short resonant RF-pulses with a sum of pulse angles of 0° for on-resonant spins. Spin ensembles with long Tj show nearly full longitudinal magnetization after the pulse sequence. Spin ensembles with short T2 get severe transverse magnetization loss between and during the pulses and therefore is clearly reduced after application of the pulse sequence (e.g., Ref 44). 

To perform a quantitative imaging or spectroscopy experiment, the relaxation time characteristics of all species (and relevant physical states of those species) must be fully characterized so that the pulse angles and delays between pulses are optimized for that particular system. In particular, if successive repetitions of a pulse sequence are applied at time scales ( recycle delays ) of the order of or... 

FIGURE 4.2 (a) Standard l3C pulse sequence with proton decoupling. Rd is relaxation delay, 0 is a variable pulse angle, t2 is acquisition time, (b) FID sinusoidal display of decoupled cholesterol at 150.9 MHz in CDCI3. (c) Expanded small section of FID. 

FIGURE 4.5 Gated proton decoupling pulse sequence. Rd is relaxation delay, 6 is a variable pulse angle, t2 is the acquisition time. 

The DEPT sequence (distortion enhancement by polarization transfer) has developed into the preferred procedure for determining the number of protons directly attached to the individual 13C nucleus. The DEPT experiment can be done in a reasonable time and on small samples in fact it is several times more sensitive than the usual 13C procedure. DEPT is now routine in many laboratories and is widely used in the Student Exercises in this textbook. The novel feature in the DEPT sequence is a variable proton pulse angle 9 (see Figure 4.11) that is set at 90° for one subspectrum, and 135° for the other separate experiment. 

There are two important experimental factors that must be accounted for if we are to be successful in running 15N experiments. The 15N nucleus tends to relax very slowly Tj s of greater than 80 seconds have been measured. Thus, either long pulse delays must be incorporated into our pulse sequence or, alternatively, we could provide another route for spin relaxation. A common procedure is to add a catalytic amount of chromium (III) acetylacetonate, a paramagnetic substance, whose unpaired electrons efficiently stimulate transfer of spin. In cases where Tt s are not known (and not intended to be measured), pulse delays and pulse angles must be considered carefully because the signal from one (or more) 15N resonance can accrue too slowly or be missed altogether. 

Figure 2.27. The overall efficiency of a 2-pulse MQ pulse sequence for different excitation (3Q, 5Q, 7Q, 9Q) for different spins with the quadrupole frequency scaled by the spin factor to allow direct comparison of the different spins with the optimum pulse angle as given in Table 2.9, after Amoureux and Fernandez (1998).

Fig. 13. (a) The J-WISE pulse sequence.18 (b) The constant time J-WISE pulse sequence.18 The 0m pulse angle is the angle between the applied magnetic field of the NMR experiment and the direction of the effective field in the homonuclear decoupling sequence applied to the 1H. 6m> — 90°—6m. 

A common modification of the basic COSY sequence is one in which the 90° mixing pulse is replaced with one of shorter tip angle, P, usually of 45 or 60 degrees (Fig. 5.52). These experiments are typically acquired and presented as absolute-value experiments since, strictly speaking, the use of a pulse angle less than 90° does not produce purely amplitude-modulated data. 

The parameter optimizer routine can be used to find the optimum delay for a particular ID sequence or to determine the pulse length for a specific power level and pulse angle combination. A series of spectra are calculated that differ by a constant increment in the parameter being optimized, the optimum delay or pulse length is then determined by examining the signal intensities in the simulated spectra. 

The calculated spectra may be represented either as a ID spectrum, a series of ID spectra or as a pseudo 2D spectrum. As such the delay or pulse angle in any ID pulse sequence can be optimized without the pulse sequence having to be modified in anyway. For 2D experiments the pulse program must be converted to its ID analogue. The optimization process is started using the menu bar command Go 1 Optimize parameter. Fig. 4.19 shows the Parameter optimizer dialog box where a number of different options may be selected. 

In the basic ATP sequence the combination of 90° excitation pulse and high repetition rates leads to intensity loss due to the incomplete relaxation. Therefore before discussing specific extensions to the APT sequence it is of interest to consider replacing the 90° excitation pulse with a pulse adapted to the Ernst angle condition in a similar manner as for one-pulse experiments. However simply replacing the 90° pulse by an Ernst pulse angle fails because the spin echo is based on a complete transfer of the z-magnetization into x- or y-coherence. Consequently it is necessary to append a second... 

In the DEPT experiment, results similar to those described here for the APT experiment are obtained. A variety of pulse angles and delay times are incorporated into the pulse sequence. The result of the DEPT experiment is that methyl, methylene, methine, and quaternary carbons can be distinguished from one another. 

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