Recovery delay

The elongation of a stretched fiber is best described as a combination of instantaneous extension and a time-dependent extension or creep. This viscoelastic behavior is common to many textile fibers, including acetate. Conversely, recovery of viscoelastic fibers is typically described as a combination of immediate elastic recovery, delayed recovery, and permanent set or secondary creep. The permanent set is the residual extension that is not recoverable. These three components of recovery for acetate are given in Table 1 (4). The elastic recovery of acetate fibers alone and in blends has also been reported (5). In textile processing strains of more than 10% are avoided in order to produce a fabric of acceptable dimensional or shape stabiUty. 

Fiber Immediate elastic recovery, % Delayed recovery, % Permanent set, %... 

A) Recorded with 256 increments, 4 transients, and a recovery delay of 1 s. (B) Recorded with 256 increments, 8 transients, and a recovery delay of 0.2 s. The flip angle a was set to 90°. The typical F2 rows (at the 13C chemical shift of C-18, indicated by the horizontal arrows and depicted on the top of the spectra) show the signal enhancements. 

Fig. 4. The HNCO-TROSY experiment for recording solely interresidual 1HN, 15N, 13C correlations in 13C/15N/2H labelled proteins. All 90° (180°) pulses for the 13C and 13C spins are applied with a strength of 2/ /l5 (p/ /3), where 2 is the frequency difference between the centres of the 13C and 13Ca regions. All 13Ca pulses are applied off-resonance with phase modulation by Q. A = 1/(4/hn) Tn = l/(4/NC ) S = gradient + field recovery delay 0 < k < TN/z2,max- Phase cycling i = y 4>2 = x, — x + States-TPPI 03 = x 0rec = x, — x.

Fig. 14. SeqHNCA-TROSY experiment for establishing sequential 1HN(i), 15N(i), 13Ca(i— 1) correlations in 13C/15N/2H enriched proteins. Durations of transfer delays A = 1/(4/Hn) 2Ta = 20-27 ms, depending on rotational correlation time of protein 2Tc = 5-7 ms S = gradient + field recovery delay 0 < k < Ta/t2,max- Phase cycling i = y (j>2 = y, — y + States-TPPI 0 = x 0ret. = x, — x. Semi-selective decoupling of 13C spins is attained using a SEDUCE-1 decoupling sequence.95...

Fig. 19. Pulse scheme of the MP-HNCA-TROSY experiment. Delay durations A = 1/(4/hn) 2T a = 27 ms 2Ta= 18-27 ms 2TN = 1/(2JNC-) <5 = gradient + field recovery delay 0 < k < Ta/t2,inax- Phase cycling scheme for the in-phase spectrum is 0i = y 02 = x, — x + States-TPPI 03 = x 0rec = x, — x 0 = y. For the antiphase spectrum, f is incremented by 90°. The intraresidual and sequential connectivities are distinguished from each other by recording the antiphase and in-phase data sets in an interleaved manner and subsequently adding and subtracting two data sets to yield two subspectra.

A typical spectrum was acquired with Bloch decays excited by 4 ysec pulses separated by 10 second recovery delays, and the data should give a reasonably quantitative estimate of the Si content. Previous work on other types of zeolites has demonstrated the importance of checking for complete relaxation if the spectra are to be used for quantitative studies. 

During normal acquisition time of approximately 1 s, these nuclei cannot relax between pulses the spin system becomes saturated and the signal-to-noise ratio decreases. A common solution for slow relaxation is a recovery delay after signal acquisition. The pulse delay should be of the order of four to five times the longest Tj in the sample, allowing all nuclei to return to the Boltzmann equilibrium state before being flipped by the next pulse. 

Degassed samples may require inconveniently long recovery delays in two-dimensional experiments. 

Figure 4.1. The essential elements of the single-pulse NMR experiment the relaxation (recovery) delay, the pulse excitation angle and the data acquisition time.

Scenario (a) transplants acquisition parameters from a typical ID proton spectrum into the second dimension leading to unacceptable time requirements, whereas (b) and (c) use parameters more appropriate to 2D acquisitions. All calculations use phase cycles for f quad-detection and axial peak suppression only and, for (b) and (c), a recovery delay of Is between scans. A single zero-filling in f] was also employed for (b) and (c). 

Figure 6.12. The BIRD variant of the HMQC experiment. The conventional HMQC sequence is employed, but is preceded by the BIRD inversion element and an inversion recovery delay, t. This procedure ultimately leads to saturation of the unwanted parent resonances.

Figure 6.13. Elimination of parent H- C proton resonances through the BIRD-inversion recovery sequence. At the start of data collection no longitudinal proton magnetisation exists but this reappears during the acquisition period and subsequent recovery delay (RD) through spin relaxation. The BIRD element selectively inverts only those protons attached to carbon-12, which then continue to relax during the inversion recovery delay, r. With an appropriate choice of x, the magnetisation has no longitudinal component when the HMQC sequence starts, so does not contribute to the detected FID.

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