Broadband inversion technique

This equation, demonstrated in [62], is indeed independent of the m/z ratio. Thus broadband excitation will bring all the ions onto the same radius, but at frequencies depending on their m/z ratio, provided that the voltage is the same at each frequency. This can be best performed by applying a waveform calculated by the inverse Fourier transform, namely SWIFT [63], As usual for a technique based on Fourier transform, the resolution depends on the observation time, which is linked with the disappearance of the detected signal (relaxation time). Here the disappearance of the signal mainly results from the ions being slowed by... 

Figure 20. Schematics of an C NMR experiment (A) without (B) and with different H decoupling techniques C) broadband decoupling D) gated decoupling and E) inverse gated (igated) decoupling...

For proton NMR the main consideration is ensuring complete relaxation between successive pulses for all the different types of hydrogen atoms present. This requires the interpulse delay to be at least 5 times the longest Tj. In addition, for nuclei such as C. where broadband decoupling is usually required, the inverse gated technique (Section 18.3.4.2) should be used to prevent the occurrence of NOE effects. A further consideration in the case of spectra from nuclei such as C and F, which may have very wide spectral widths, is whether the RF pulse has sufficient power to irradiate all the nuclei equally effectively. The digital resolution and data processing requirements for a particular application also require careful selection. 

An alternative method to the J-B sequence is the selective inversion (SI) technique [2.21]. Both the J-B and SI methods give the same efficiency in measuring Tig, but the SI method is inferior in determing T z because of a decrease of a factor of 2 in the signal-to-noise ratio. It is noted that because of Eq. (2.77), a separate experiment is required for each distinct quadrupole doublet. Alternatively, broadband excitation may be used to irradiate several doublets identically using a composite J-B sequence [2.22, 2.23]. Recently, this broadband J-B excitation sequence was used to create quadrupolar orders with same efficiency on all the labeled sites in a liquid crystal [2.24]. This is particularly useful if the NMR spectrum represents a powder sample. The modification of the J-B sequence (Table 2.3) to produce a broadband excitation sequence is shown in Table 2.4. This has been used to minimize the number of separate experiments required to give Tiz and Tig for various deuterons in several deuterated nematogens [2.25]. 

---