Free induction decay digitization

A two-dimensional data set is composed of a matrix of [LX(R + /)] data points. Here L is the number of increments in t, and R and i are the number of real and imaginary data points digitized into memory locations thus R = i. The two-dimensional data set is collected by storing a signal-averaged, free induction decay for each value of t,. Typically, the free induction decays are sequentially stored on a disk drive as individual data blocks. For the first block of data, the duration of t is the inverse of the desired spectral width that is equivdent to the dwell time of the one-dimension, free induction decay digitization. The evolution time is then incremented by the dwell time L times to create L data blocks. L should be chosen to be a power of 2 and is usually 128, 256, or 512. For the homonuclear, chemicsd-shift correlation experiment, when L r, = t2 and L = /, the two-dimensional data set will be symmetrical after double Fourier transformation. 

Free Induction Decay (FID) Interference pattern of decaying cosine waves collected by Fourier Transform spectrometers, stored digitally prior to Fourier Transformation. 

Fig. 10.1. The top panel shows the free induction decay (FID) acquired for a sample of strychnine (1) at an observation frequency of 500 MHz. The spectrum was digitized with 16 K points and an acquisition time of 2 s. Fourier transforming the data from the time domain to the frequency domain yields the spectrum of strychnine presented as intensity versus frequency shown in the bottom panel.

Digital Processing of the Free-induction, Decay Signal. 50... 

Fig. 5.4.3. a Free induction decay pattern FID(t). b NMR signal obtained after Fourier transformation of the FID, and the relation between acquisition parameters and digitization... 

Figure 5.10 Radio frequency variation of My(t) transverse magnetisation observed, acquired and stored digitally with time is known as a Free Induction Decay (FID). Stored FID either singly or averaged, are processed by fourier series transformation (FT) from time domain signal information, SnoCti), into frequency domain (spectral) information, /nmr( i)- Only chemically equivalent nuclei without spin-spin coupling and with an equivalent Lamor frequency, V, are being observed here hence only a single signal will result of frequency Vi.

Free induction decay, FID The analog signal induced in the receiver coil ofan NMR instrument caused bythexy component of the net magnetization. Sometimes the FID is also assumed to be the digital array of numbers corresponding to the FID S amplitude as a function of time. 

Acquisition time, at (Varian), or AQ (Broker). Syn. detection period. The amount of time that the free induction decay (FID) is digitized to generate an array of numbers denoted "at" on a Varian instrument, or"AQ"on a Bruker instrument. 

The free induction decay of 512 points was padded with another 512 zeroes to increase digital resolution. Data were provided by Drs. T. R. Tritton and I. M. Armitage of Yale University. [Taken from ref. 9.]... 

The task of an NMR spectrometer system in performing a two-dimensional experiment may be divided into three parts. The first is the acquisition of data, which requires the generation of accurately timed sequences of pulses followed by the digitization of the resultant free induction decays. The second task is the processing of this matrix of time-domain data to yield a two-dimensional spectrum, and the third and final stage is the presentation of the data in a form suitable for analysis. No commercial spectrometer is ideally suited to this type of experiment, but almost all spectrometers have the basic hardware required. The implementation of 2D NMR techniques on a given instrument usually reduces to the development of the necessary computer software for carrying out the steps outlined above these will now be discussed in a little more detail. 

Once generated, the free induction decays are detected, filtered, digitized and stored just as in a normal FT NMR experiment. 

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