Purge pulse

Refocusing period of the INEPT+ experiment, the arrow denotes the additional pulse (purge pulse) in comparison to the refocused INEPT sequence. 

The two main methods to solve the flooding problem are (1) opening the pulse purge valve to allow a pressure wave of hydrogen to push the water out of the flow field and (2) raising the operating temperature to decrease the RH, turning the liquid water into stream that is then expelled with the reactant gas as exhaust. 

The pulse sequence for the ID ROESY experiment using purged half-Gaussian pulses is shown in Fig. 7.7. The purging is required to remove the dispersive components, since these are not completely eliminated by the weak spin-lock field employed in the ID ROESY experiment. 

Figure 7.7 A ID ROESY pulse sequence with purged half-Gaussian excitation. (Reprinted from Mag. Reson. Chem. 29, H. Kessler et al., 527, copyright (1991), with permission from John Wiley and Sons Limited, Baffins Lane, Chichester, Sussex P019 lUD, England.)...

Instrument. Before the H2 chemisorption, each sample was heated in pure H2 at 300°C for 90 min, subsequently it was heated in He at 290°C for 60 min in order to desorb hydrogen from the sample. The chemisorption measurement was performed at 0°C by several H2 pulses with an Ar purge in between, in order to desorb physisorbed hydrogen. A 1/1 H/Pt ratio was used to estimate the Pt dispersion. Values in the range 50-70% have been obtained on the fresh samples Pt—Ba/ y-Al203 (1/20/100 w/w) catalyst. 

FIGURE 14.18 Flow diagram of split flow capillary LC system. 1. Solvent reservoirs. 2. Model 5000 syringe pump (Varian, Walnut Creek, California). 3. Static mixer. 4. Injection port. 5. Column. 6. Detector. 7. Pressure transducer. 8. Pulse dampener. 9. Purge valve. 10. U-flow controlling device. 11. Waste. 

The last 90° pulse on 13C acts as a purge pulse for the undesired dispersive magnetization.47,48 The function of the pulse is to convert any magnetization remaining antiphase with respect to the 13C spin into unobservable multiple-quantum coherence. This will provide cross peaks with pure lineshapes and with higher resolution, and consequently establishes reliable determination of coupling constants.47,48... 

Fig. 17.4 Common filter elements a X-half filter based on X pulse phase cycling [16, 17], b X-half filter with purge gradient [18], c X-half filter as in a, but with refocusing period for the hetero-nuclear antiphase magnetization [16, 17]. Sequences d [22], e [23] and f [18] show double filters based on single filter elements the delays r and r can be set to slightly different values to cover a broader range of ]J coupling constants (see text for a more detailed description).

The simplest double tuned filter can be constructed by a concatenation of two X-half filters and removal of redundant 180° pulse pairs (Fig. 17.4d) [22]. Alternatively, it can also be realized by keeping the 180° pulse pairs and adding short spin-lock periods (to dephase the 1H-13C magnetization which is orthogonal to the spin-lock axis, Fig. 17.4e) [23], or it is based on the gradient-purging scheme of Fig. 17.4b, resulting in the double filter shown in Fig. 17.4f [18]. 

Fig. 17.7 2D NOESY experiment with 13C, 15N fil- The sequence uses double filters for 13C, single ter in both H dimensions, selecting intramolecu- filters for 15N, and additional 13C, 15N purge lar NOEs within the unlabeled components [22]. pulses during the NOE mixing time ( ).

In addition to these specialized adsorption designs there are a number of variations on regeneration and cooling processes. The pulse regeneration technique has been practiced for several decades. In this variation we introduce a pulse of hot purge gas over a somewhat limited duration and then follow that with a cooling... 

Fig. 6. Spectra of cinnamic aldehyde 4 dissolved in CDClj and obtained with the heteronu-clear multiple selective COSY experiments, (a) Spectra acquired with pulse sequence IVa dedicated to detect and measure Jen connectivities after selective perturbation of carbons C7, C3 and Cl. Spurious TOCSY peaks caused by short purge pulses (not shown in fig. I) are marked with an asterisk, (b) Spectra acquired with pulse sequence IVb dedicated to detect and measure " Jch connectivities after selective perturbation of carbons C-7, C-1 and C-4. Normal H spectra at the bottom.

Looking at the series of subspectra obtained with both sequences, the perfect suppression of central signals is noticeable. This must be attributed on the one hand to the highly effective combination of proton broadband decoupling, the purge pulses and phase cycling and on the other hand to... 

---