Observation Channel

The operating concept of PFT NMR can be recognized by following the arrows in Fig. 2.34, going from pulse programmer via probehead and computer to plotter. 

After Fourier transformation, the effects on the spectrum of data manipulations, such as phase adjustments, can be controlled on a display before giving final calculating commands. Communication with the computer is generally via keyboard and graphic display. Light pen control via oscilloscope is also possible. 

The PFT NMR spectrum can then be recorded in analog or digital form by a plotter controlled by the computer. If the computer core memory allows, additional subroutines may be stored which automatically compute signal intensities and positions (in ppm values relative to some standard or in Hz) and outputs them to the printer. 

Programs and data are stored on disks or diskettes. Using a bank of chemical shift data and coupling constants of several thousand compounds, stored on magnetic tapes or disks, and the appropriate software, partial structural assignments by the computer are possible [48], 

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In this way the child spectrum is transformed into a spectrum as if measured on the parent instrument. In a more refined implementation one establishes the highest correlating wavelength channel through quadratic interpolation and, subsequently, the corresponding intensity at this non-observed channel through linear interpolation. In this way a complete spectrum measured on the child instrument can be transformed into an estimate of the spectrum as if it were measured on the parent instrument. The calibration model developed for the parent instrument may be applied without further ado to this spectram. The drawback of this approach is that it is essentially univariate. It cannot deal with complex differences between dissimilar instruments. 

Figure 3 shows a block diagram of the spectrometer. The modules depicted inside the area surrounded by the bold lines have been built into the FPGA chip. By default, the spectrometer can equip with up to three equivalent and independent RF transmitters, while there is a single receiver. The observation channel is selected by manually plugging the cable from one of the three reference signals of the transmitters to the receiver (Figure IB). 

The basic components of the solid state spectrometer are the same as the solution-phase instrument data system, pulse programmer, observe and decoupler transmitters, magnetic system, and probes. In addition, high-power amplifiers are required for the two transmitters and a pneumatic spinning unit to achieve the necessary spin rates for MAS. Normally, the observe transmitter for 13C work requires broadband amplification of approximately 400 W of power for a 5.87-T, 250-MHz instrument. The amplifier should have triggering capabilities so that only the radiofrequency (rf) pulse is amplified. This will minimize noise contributions to the measured spectrum. So that the Hartmann-Hahn condition may be achieved, the decoupler amplifier must produce an rf signal at one-fourth the power level of the observe channel for carbon work. 

The T, Inversion-Recovery experiment is not restricted to C nuclei, but may also be applied to other nuclei, e.g, protons. In this case, the pulse sequence for the observe channel is the same, but no broadband decoupling is used. 

These types of experiments call for efficient doubly tuned coils, ideally with a separate deuterium lock channel. For more complex molecules, such as proteins, considerably more intricate NMR pulse sequences, such as (HNCO) [32,33], require the probe to operate at three or four distinct frequencies. High efficiency is demanded from the proton observe channel. Ideally, the additional circuitry allowing multiple tuning should not interfere with the proton efficiency when compared to a singly tuned proton coil. In practice, some reduction is tolerated. The two most important design criteria for such... 

B, oscillating magnetic field vector of the observing channel... 

The normal convention places the I spin on the x-axis and the indirectly detected S nucleus on the y-axis. The I dimension is referred to as F2, the S dimension as FI. This does not correspond to the channel labelling on Bruker spectrometers, fl (observe channel), f2 (1 decoupler), G (2 decoupler), etc. 

The weak-coupling approximation (7.132,7.140) can be verified within the context of the coupled-channels-optical method. Equns. (7.123) may be solved with a particular channel, defined by the target state i), included in either P space or Q space. If it is in P space the channel i is fully coupled. The approximation is verified if the two solutions agree. In practice the lowest dipole-excited channels should be included in P space with the experimentally-observed channels, but the approximation is closely verified for higher channels in Q space. However, computation of (7.123) is not difficult and it is common to include all discrete channels in P space that are necessary for convergence. 

The measuring cell which serves as a basis of the CFMIO method [3] can also be used in a stopped-flow mode by positioning of one or more observation channels perpendicular to the flow tube. This combined stopped-flow, continuous-flow method [5] was used to determine the effect of surface-active substances (sodium dodecyl sulfate or dodecyltrimethylammonium chloride) on electron-transfer reactions between metal complexes. 

Lastly, the action spectra in Ca-HF close to the energy threshold to chemiluminescence display very narrow lines, indicating the partial closure of the corresponding reactive excited-state channel. This indicates that the observed channel luminescence is independent of the ground-state channel that could also contribute to the broadening of the lines in the spectrum and confirms the model of the 4s electron hop separated from the 3d hop. We shall now examine the effect of this hop on the efficiency of production of excited states. 

Figure 3.51. I ilse 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 360° condition can be used for the calibration (but be sure to know which of these you are observing ).

Precise control of the relative phase between pulses is crucial to the success of many multi-pulse NMR experiments and some correction to the phase of a soft pulse may be required to maintain these relationships when both hard and soft pulses are to be applied to the same nucleus. When soft pulses are used on the observe channel the phase difference (which may arise because of the potentially different rf paths used for high and low power pulses) may be determined by direct inspection of two separate ID pulse-acquire spectra recorded with high- and low-power pulses but under otherwise identical... 

The instrument is built on the heterodyne principle with 10 MHz as the intermediate frequency. Both the lock frequency and the spectrometer frequency are derived from the synthesizer, which ensures that any source frequency drift would appear equally in both the lock and the observe channels thus resulting in maintaining the resonance condition. 

The frequency of the RF applied to the sample for the observe channel, also the frequency of the rotating frame for the observe nuclide. 

Fig. 14. Pulse sequence of the 5-ir-pulse experiment used in the pseudo-twodimensional spinning-sideband-suppression experiment. All pulses in the observe channel are ir pulses. The constant delay T must be a multiple of the rotor period T, but not 3t,. (Reproduced from Gan " with permission.)...

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