Calibrations pulse widths

It is important to avoid saturation of the signal during pulse width calibration. The Bloch equations predict that a delay of 5 7 will be required for complete restoration to the equilibrium state. It is therefore advisable to determine the values an approximate determination may be made quickly by using the inversion-recovery sequence (see next paragraph). The protons of the sample on which the pulse widths are being determined should have relaxation times of less than a second, to avoid unnecessary delays in pulse width calibration. If the sample has protons with longer relaxation times, then it may be advisable to add a small quantity of a relaxation reagent, such as Cr(acac) or Gd(FOD)s, to induce the nuclei to relax more quickly. 

The whole sequence of successive pulses is repeated n times, with the computer executing the pulses and adjusting automatically the values of the variable delays between the 180° and 90° pulses as well as the fixed relaxation delays between successive pulses. The intensities of the resulting signals are then plotted as a function of the pulse width. A series of stacked plots are obtained (Fig. 1.40), and the point at which the signals of any particular proton pass from negative amplitude to positive is determined. This zero transition time To will vary for different protons in a molecule. 

40 Stacked plots of H-NMR spectra for ethylbenzene. This experiment can be used to measure the spin-lattice relaxation time, T.  

There are many other methods known for accurate calibration of pulse widths, but such discussion is beyond the scope of this text (see Thomas et al., 1981 Lawn and Jones, 1982 Bax, 1983 Wesener and Gunther, 1985 Nielsen et ai, 1986). 

In practice it is usually unnecessary to determine exact pulse widths for each sample we can use approximate values determined for each probe-head, except in certain 2D experiments in which the accuracy of pulse widths employed is critical for a successful outcome. Proper tuning of the probehead is advisable, since pulse widths will normally not vary beyond 10% with well-tuned probeheads. 

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Another approach to a source of vapors to calibration of instruments, and similar to that described above, was that of Davies et al. [67] who used a computer-controlled pulsed vapor generator with TNT, RDX, and PETN. The explosive solid was coated on quartz beads, which were then packed into a stainless steel tube. The tube was coiled and placed into a temperature-controlled chamber. Ultrapure air was passed through the coil at temperature and vapors of explosives were vented from the coil at rates or concentrations governed by coil temperature, airflow rate, and pulse width. Calibrations could reach the picogram to nanogram range when an IMS analyzer was used as the calibrating instrument. 

Figure 2-14 A typical 90° pulse width calibration plot.

Modem multipulse NMR experiments are critically dependent on the application of rf pulses of known duration (the pulse width) that correspond to precise magnetisation tip angles, most frequently 90° and 180°. Pulse width calibrations are normally defined for the 90° pulse (PW90), from which all other... 

Figure 3.59. Pulse width calibration for the observe channel. A sequence of experiments is recorded with a progressively incremented excitation pulse. The maximum signal is produced by a 90° pulse and the first null with a 180° pulse. Either the 180° or the 360° condition can be used for the calibration (but be sure to know which of these you are observing).

Why is accurate calibration of pulse widths and delays essential for the success of a 2D NMR experiment ... 

Tuning the probe assures that the resonant frequency of the probe coil is the same as the RF frequency you will be using and matching the probe matches the probe coil as a load to the impedance (internal electrical resistance) of the amplifiers. This gives maximum efficiency of transfer of RF power from the amplifiers to your sample nuclei and maximum sensitivity in detecting the FID. Each sample modifies the resonant frequency and matching of the probe, so these have to be reoptimized with each new sample. Tuning the probe is not necessary for routine XH spectra, but for advanced experiments it is important if you wish to use standard values for pulse widths without the need to calibrate for each sample. 

Likewise if we calibrate the pulse width to give the maximum peak height in the spectrum (90° pulse), we can calculate the pulse amplitude in hertz ... 

To double the pulse power, simply increase the power by 3 dB, as log(2) = 0.3. Because pulse power is the square of pulse amplitude B, to double the amplitude we need to multiply pulse power by a factor of 4, which corresponds to increasing power by 6 dB, as log (4) = 0.6. This leads to a simple rule of thumb Every time you increase the pulse power by 6 dB, you will cut the 90° pulse (fp) in half (because B is doubled). Likewise, each 6 dB decrease in pulse power will double the 90° pulse width. This is a good rule of thumb, but as the actual power settings are not precise, you will normally have to calibrate the 90° pulse at the new power setting to be sure. To make matters worse, Bruker uses the dB scale to describe power attenuation rather than power itself, so that the higher the dB value the lower the power. This is the opposite of Varian s system. Be careful whenever you are setting power levels If you get it wrong, you can burn up the probe, the amplifiers, and your sample ... 

You can then use the decibel scale to estimate power settings. For example, suppose you calibrated the 90° pulse on a Bruker 500 to be 17.6 pis for XH at a power setting of 3 dB, and you want to know the power setting that will give a 30 pis 90° pulse (yB l2n = 1/(4 x 30 pis) = 8333 Hz). Just plug in the ratio of pulse widths ... 

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. 

Despite numerous applications, conventional CRAMPS still remains one of the most demanding solid state NMR experiments as it requires the use of specially prepared spherical samples to minimise radiofrequency inhomogeneity effects and the careful calibration and setting of pulse widths and phases. Further modifications of the experiment that do not require the complicated and extended set-up procedures have been suggested recently. These are known as rotor-synchronised CRAMPS, which combines a new multiple pulse sequence [21], and its modification which uses a standard WHH-4 sequence at ultrafast MAS frequencies (e.g. 35 kHz) [22]. 

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