Calibration of Decoupler Pulse Length

Decoupler pulse length is calibrated periodically because it usually does not change from sample to sample. The calibration is performed indirectly, meaning that the effect of the decoupler pulse on the observed nuclei is calibrated. In principle, any sample enriched with 15N and/or 13C can be used for 15N and 13C decoupler pulse calibration. Samples used for the calibration include 15N, 13C-NAcGly in 90% H20/10% 2H2O, 1 M 15N Urea in DMSO-d6 (Dimethyl Sulfoxide-d6), 0.1% 13C methanol in 99% 2H20/1% H20 (Varian autotest sample), or other samples for indirect calibration. The pulse sequence shown in Figure 4.6 is one of those commonly used to calibrate 15N and 13C decoupler pulses. A delay t is set to 1 /(2Jhx) for calibrating the decoupler pulse of nucleus X.1JHX is measured by the frequency difference between the doublets. It is important to use the correct 1H 90° pulse length and the decoupler offset frequency must be set on the center frequency of the heteronuclear doublets in order to get an accurate calibration at low decoupler power.

To calibrate the decoupler pulse length, the decoupler offset frequency is set to the calibrated value that is in the center of the coupled doublets. When the angle of the heteronuclear X pulse is zero, a doublet is observed. Similar to the 1H pulse calibration, the phase parameters of the first spectrum will be used for the subsequent spectra acquired by arraying the X pulse length. The 90° X pulse will give a null intensity of the doublet. The pulse length at high power is usually calibrated first, and is relatively insensitive to the deviation of decoupler offset frequency. Typical high power 90° pulse lengths for 13C are approximately 15 ^s whereas those for 15N can be as long as 40 ^s. When calibrating the high power X pulses, although the maximum allowed B1 RF field strength ( pulse power) can be used, a pulse power which is 3 dB less than the maximum RF power is commonly chosen because it is in the linear range of the amplifier. However, a properly chosen pulse power is that which gives the above short pulse length.

The 90° pulses for heteronuclear decoupling use much less power and, hence, have a longer pulse length. For instance, the desired 13C 90° pulse is 70 ^s for GARP decoupling, and 300 ^s for SEDUCE decoupling, and 15N 90° decoupling pulses are 200-250 ^s for WALTZ-16 and GARP. Therefore, the decoupling pulses are calibrated with a lower decoupler power. The linearity of the amplifier can be used to estimate the power for the desired pulse length, based on the relationship given by:

in which pw1 and pw2 are the pulse lengths for the hard 90° pulse and the 90° decoupling pulse, respectively. If pulse power is decreased by one dB, the pulse length is increased by dB = 20log10(pw1/pw2)

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