Signal averaging, accumulation

Data processing techniques are extremely useful in both pure EPR and electro-chemical-EPR studies. Details of the EPR computer interface are unique to each system and to the goals of each experiment. Since the theory and methodology of these digital operations are similar to those described elsewhere in this book, the discussion will not be reiterated here. There are numerous examples of signal averaging for kinetic measurements and for spectral accumulation using rapid scans. Short-lived species may be studied by these techniques. 

For some mass spectrometers, such as TOF mass spectrometers, for reasons of data transfer speed and data storage capacity, the data acquisition system needs to accumulate data for a period of time in the summing memory, and forward the accumulated data to the data system. Each spectrum is added to the sum of the previous spectra so that a continuous summation process takes place. This type of data acquisition system is called digital signal averaging (DSA) or integrating transient recorder (ITR). Figure 3.8 illustrates the principle of mass spectrum acquisition with this type of system. 

In pulse NMR, there is always noise coherent with the pulse sequence because of the transient nature of the experiment. This is in addition to the usual random noise as well as other systematic noises such as the 50 or 60 Hz "hum from the ac source which are not synchronized with the pulses. These non-synchronous components of noise can be reduced either by appropriate filtering or by multiscan averaging or both. In this section we will deal with noise coherent with the pulse sequence. Even if such noises were very small, they will add upon signal averaging (just like the desired signal) and may, therefore, be troublesome in experiments where very many FID s are accumulated due to inherently poor S/N. 

The long signal accumulation times required by the low repetition rate for C nuclei with long Tis in solids can be circumvented by transferring polarization from abundant H nuclear spins with short Ti s to the nuclei. The repetition rates for signal averaging are now determined by the shorter HJiS (ms-s), because energy is transferred from the proton to the carbon nuclei, a process termed cross-polarization (CP) [38]. 

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