Accumulation of FIDs

The remarkable stability and controllability of NMR spectrometers permits not only the precise accumulation of FIDs over several hours, but also the acquisition of long series of spectra differing only in some stepped variable such as an interpulse delay. A peak at any one chemical shift will typically vary in intensity as this series is traversed. All the sinusoidal components of this variation with time can then be extracted, by Fourier transformation of the variations. For example, suppose that the normal ID NMR acquisition sequence (relaxation delay, 90° pulse, collect FID) is replaced by the 2D sequence (relaxation delay, 90° pulse, delay t-90° pulse, collect FID) and that x is increased linearly from a low value to create the second dimension. The polarization transfer process outlined in the previous section will then cause the peaks of one multiplet to be modulated in intensity, at the frequencies of any other multiplet with which it shares a coupling. 

This experiment uses two pulses, 180° and 90°, separated by a delay time x which is varied. For each delay a certain number of FIDs are accumulated the result is a series of spectra in which the individual signals have different intensities. Figure 9 shows the result of an inversion-recovery experiment carried out on 1. 

McLeese et al. [10] studied the accumulation of AP in aquatic fauna including salmon. Fish were mixed with anhydrous sodium sulphate and Soxhlet extracted with ethyl acetate. Lipids were removed by GPC on Biobeads using cyclohexane/dichloromethane (1 1, v/v). The final analysis was performed with GC-FID. The recovery of p-NP was reported as 86%. 

Due to its greatly enhanced sensitivity in comparison to CW NMR, the PFT method has made 13C NMR into a routine method of structure analysis for all molecules having the natural 13C abundance of 1.1%. Additionally, phase-corrected PFT NMR spectra contain all spectral details without the lineskewing and ringing observed in CW spectra. Finally, short-lived molecules can be measured by PFT NMR, and sensitivity enhancement by accumulation of interferograms before Fourier transformation requires much less time than the accumulation of CW NMR spectra, due to the short time required for acquisition of FID signals. 

Figure 10. Fluorine-decoupled, MAS C-13 spectra of PCTFE as a function of temperature. Spectra were obtained with a CP time of 5.0 ms and an experiment repetition time of 2.5 s. Spectra represent a 2K FT in each case the number of FID accumulations varied from IK to 5K (41).

It is obvious from Check it 2.3.3.1 that the accumulation of all four scans where the receiver phase is incremented whilst the excitation pulse phase remains constant results in an observed signal with zero intensity. (This may also be confirmed manually in ID WIN-NMR by using the ProcessIFile Algebra command and clicking on the Add./Sub. button in the button panel to add all four FID or spectra together.) However if the phase... 

A simple but instructive implementation of QD (Pajer and Armitage, 1976) is shown in the next figure. With this setup, we can first accumulate the FID s in one memory block in the... 

Our procedure for a relaxation time run is to specify the number of points to be taken on the recovery curve, the time interval between those points, the number of FID s to be accumulated per point, and then turn the data acquisition over to the computer. Further embellishments include the fact that we calculate the recovery curve from integrals of selected parts of the on-resonance FID s and that we alternate the functions of the two digitizers with each pulse sequence to cope with the differences between the digitizers. After a specified number of FID s are accumulated at a given value of delay between the combs, a new value of delay is transmitted to the timer and the next point is measured. At the end of such a run, there will be a table of the magnetization recovery as a function of time. 

We end this section with another method which is not widely used but deserves mention if for no other reason than its originality. Suppose we accumulate N FID s and then subtract the sum from an accumulation of N other FID s which were taken as a slightly different magnetic field. The coherent noise is cancelled and we only need to worry about what is left which turns out to be quite manageable and yields... 

B) The effect of saturating the 2.338 resonance of the exo isomer at 37°C. Saturation is only transferred from the anti resonance of the exo isomer to the anti resonance of the syn isomer, demonstrating that rotation of the allyl is occurring. The data were collected in a gated decoupling mode i.e., the 2.338 resonance was irradiated for 10 seconds, the irradiation was terminated, and after a 0.1-second delay a 90° pulse was applied, followed by accumulation of the FID. 

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