Calibrations decoupler pulses

Fig. 11. A Pulse sequence to invert direct (one-bond) responses based on HSQC-TOCSY (Domke 1991). B Pulse sequence to invert direct responses based on HMQC-TOCSY (Martin et al. 1992). When the adjustable pulse, P, is set to 180°, the direct responses are inverted as in the original work of Domke (1991). In contrast, when p = 90°, direct responses will be canceled in a manner analogous to the procedure used to calibrate decoupler pulses (Thomas et al. 1981 Bax 1983b). In experiments when the direct response is to be canceled, there is no need of broadband heteronuclear decoupling during acquisition, allowing higher levels of digital resolution in F2 than would otherwise be possible...

In Check it 5.2.1.4 the calibration of the decoupler pulse is demonstrated using a CH fragment. Experimentally CHCI3 (S = I = IH) would be a suitable test sample. 

Load the file ch5214.cfg (File I Experiment setup I Load from file...). Check the pulse lengths of pi 2.5u (90° pulse) and p3 0.5u (4.5° pulse) (Go Check Experiment Parameters). In the Options I NMR-Sim settings... dialog box select the Modify RF field option. To simulate the decoupler pulse calibration, open the parameter optimizer dialog box (Go I Optimize parameter). Select the Show results as 1D series, N 8 and p3 for optimization. Click on the OK button. In the next dialog box enter the start value p3 0.5u and increment size inpO 2.0u. Click on the OK button and enter then the path and name for the calculated and saved files. Run the series of simulations. In 1D WIN-NMR the last simulated FID will be automatically loaded into the spectrum window. Process the FID (zero filling Sl(r+i) 16k, apodization EM, LB 1.0 [Hz]) amd... 

WALTZ-16 sequence (Chapter 10). A 0.3 Hz line broadening is used and the noise recorded over the same region, with the peak height determined for the tallest aromatic resonance. Tuning of the proton coil and appropriate calibration of the proton decoupling pulses are required in this case for optimum results. Test samples for other common nuclei are summarised in Table 3.8. Should you have frequent interests in other nuclei, a suitable standard should be decided upon and used for future measurements. 

If delayed extraction increases the mass resolution without degradation of sensitivity compared with continuous extraction, it also has limitations. Indeed, delayed extraction complicates the mass calibration procedure. It can only be optimized for part of the mass range at a time and is less effective at high mass. Delayed extraction partially decouples ion production from the flight time analysis, thus improving the pulsed beam definition. However, calibration, resolution and mass accuracy are still affected by conditions in the source. For instance, in the usual axial MALDI-TOF experiments, optimum focusing conditions depend on laser pulse width and fluence, the type of sample matrix, the sample preparation method, and even the location of the laser spot on the sample. 

Probe tuning is necessary for a number of reasons. Other than the fundamental requirement for maximising sensitivity, it ensures pulse-widths can be kept short which in turn reduces off-resonance effects and minimises the power required for broadband decoupling. A properly tuned probe is also required if previously calibrated pulse-widths are to be reproducible, an essential feature for the successful execution of multipulse experiments. 

The approach is to begin with 0 very small and to phase the spectrum so that the doublet lines are in antiphase. As 0 increases the doublet intensity will decrease and become zero when 0 is 90", whilst beyond this the doublet reappears but with inverted phase (Fig. 3.53). If it is necessary to perform the calibration with an AaX group, the delay A should be 1/4J and it is the outer lines of the triplet that behave as described above whilst the centre line remains unaffected. When calibrating lower powers for the purpose of broadband decoupling, it is usually more convenient to set the duration of the 0 pulse according to the decoupler bandwidth required (Section 9.2) and to vary the output attenuation to achieve the null condition. 

Figure 9.21. Soft pulse calibration sequences for (a) amplitude and (b) hard/soft phases differences for selective pnlses applied on the indirect (decoupler) channel. SL is a spin-lock applied to decouple A and X during the soft pulse.

The pulse calibration on the decoupler channel cannot be performed using the direct method since the pulse which is to be calibrated is transmitted on a different rf channel and resonance frequency to the observed nucleus. To calibrate the rf pulse on the decoupler channel it is necessary to determine the pulse length from the effect the pulse has on the nucleus observed on the transmitter/receiver channel. This indirect calibration is achieved by using a coupled IS spin system and transferring the detectable antiphase... 

The radiofrequency field strengths must be carefully calibrated, preferably using the 2D experiment described by Nielsen et al. [15]. Strong radiofrequency fields are most convenient because they increase the efficiency of the multiple-pulse homonuclear decoupling, while high stability of the power and phases of the radiofrequency pulses is crucial. 

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