Pulse calibration

Pulse calibration. The correlation of RF pulse duration (at a given transmitter power) to net magnetization tip angle. 

To tip the net magnetization of a spin ensemble by W, we apply a timed pulse of RF at the correct frequency and amplitude to our sample. To ensure our tip angle is 90°, we first determine the amplitude and duration of the pulse of the RF pulse to use in a process called pulse calibration. Pulse calibration may be carried out only periodically or with each sample. 

For conducting 2-D NMR experiments such as the HSQC, HMBC, and NOESY experiments, we may elect to tune and calibrate the 90° pulse for each sample. 

Calibration of pulse widths is often done with a standard sample containing a copious amount of solute (or one that is enriched). Calibration of N pulse widths also benefits from the use of an iso-topically enriched standard sample. Once we put in our real world sample (with a concentration of perhaps less than 5 mM), often the best we can do is tune the probe and hope the calibration arrived at while using a different sample will be sufficiently accurate. We must assume that pulse calibrations determined using a standard are valid for our sample as well. 

Rotating frame. An alternate Cartesian coordinate system (x, y, z ) sharing its z-axis with that of the 

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We use common sense to find the correct power level We know we want lower power, and for Bruker that means a larger number. So we add this to 3 dB to get a power setting of 58.4 dB. As this power level corresponds to the maximum power of the Gaussian shaped pulse, we can set this power level for our shaped pulse and get a 180° rotation. This would be the starting point for the pulse calibration. 

Inasmuch as the irradiation dose can change from pulse to pulse, it is desirable and often necessary to monitor the dose delivered by each pulse. This can be done by means of various devices a toroidal coil placed around the electron beam [116], a secondary emission chamber placed just before the exit window of the accelerator [13], a charge collector placed in the proximity of the irradiation cell [117], or, electron beam energy permitting, behind the irradiation cell [118], With these devices, a parameter is obtained representing the relative dose of each pulse calibration against chemical dosimeters provides the knowledge of the absolute irradiation dose associated with each individual pulse. 

Fig. 13. Pulse sequences for indirect pulse calibration (a) 90° pulses, (b) 180° pulses, (c) modified sequence for indirect calibration of 90° pulses on spin-1 nuclei."...

Errors in DEPT editing may arise from a number of sources. The most likely is incorrect setting of the proton pulses, especially the 0 pulse used for editing, which may often be traced to poor tuning of the proton channel. Even small errors in 0 can lead to the appearance of small unexpected peaks in the DEPT-90 if 0 is too small it approaches DEPT-45 whilst too big and it approaches DEPT-135. Usually, because of their low intensity, these spurious signals are easily recognised and should cause no problems. Even with correct pulse calibrations, errors can arise when the setting for the delay period is very far from that demanded by Uch [35] and in particular CH3 resonances... 

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.

It is crucial that the pulses we use in NMR experiments have the correct flip angles. For example, to obtain the maximum intensity in the pulse-acquire experiment we must use a 90° pulse, and if we wish to invert magnetization we must use a 180° pulse. Pulse calibration is therefore an important preliminary to any experiment. 

Fig. 3.18 Illustration of how pulse calibration is achieved. The signal intensity varies as (sin p) as shown by the curve at the bottom of the picture. Along the top are the spectra which would be expected for various different flip angles (indicated by the dashed lines). The signal is a maximum for a flip angle of 90° and goes through a null at 180° after that, the signal goes negative.

A careful pulse calibration experiment determines that the 180° pulse is 24.8 lis. How much attenuation, in dB, would have to be introduced into the transmitter in order to give a field strength, (a) /lit), of 2 kHz ... 

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... 

Stack plot of the ID spectrum series of the indirect pulse calibration of p3 and pulse sequence scheme (upper part). The arrow denotes the transition of p3 < 90° to p3 > 90°... 

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... 

The pulse calibration procedure for a selective pulse is more difficult than for a hard pulse. In calibrating a particular flip angle the pulse length is kept constant while the pulse power is varied. 

Since the intensity of the electron adduct band decreases rapidly below 4000 A. the first two spectra approximate to those of the H atom and OH radical adducts, while the third represents the difference between them. The qualitative conclusions drawn from visual examination of the gross spectra are confirmed the H atom adduct spectrum reaches a peak at —3900 A. compared with —3600-3800 A. for the much broader, less intense OH radical adduct spectrum, while the negative values of ge(e0H-ee) above 3900 A. suggest that the electron adduct spectrum extends a little below 4000 A., although its intensity is much less than that of the H atom adduct. From the measured dose in the pulse, calibrated against thiocyanate, and assuming the primary yields to be g(OH) = 2.9, g(H) = 0.6, g(e aq) = 2.3, we calculate the following approximate values for the extinction coefficients of the transients ... 

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