NUCLEAR MAGNETIC RESONANCE 111
lengths for each are slightly different. Some distortion of signal intensity
results from this factor; thus if precise quantitative data is required, the pulse
sequence of Figure 3.14 is necessary. The intensity of a FID is proportional to
the number of nuclei contributing to the signal. When transformed to the so -
called absorption spectrum, integration of the area under the peaks relates to
the number of nuclei resonating at a given frequency.
Each FID signal is accompanied by noise; however, the noise is incoherent
— sometimes positive, sometimes negative — so that it increases more slowly
than the desired nuclear signal. A series of N FIDs has a signal - to - noise ratio
N times better than a single FID, allowing spectroscopists to obtain useful
chemical information from otherwise unreceptive nuclei or from dilute
solution samples having few of the nuclei of interest.
The output of the NMR spectrometer must be transformed from an analog
electrical signal into digital information that can be stored in the computer ’ s
dedicated computer. The minicomputers used in NMR spectroscopy have
memory used for data accumulation, programs for manipulating the data, and
storage devices to store large collections of data for future or additional
manipulation into useful spectral results.
3.4.8 Two - Dimensional (2D) NMR Spectroscopy
Every NMR experiment must have a preparation sequence (inducing the
nuclei to resonate) and detection capability (fi nding out what happened). Two -
dimensional NMR spectroscopy adds two more domains between preparation
and detection. These are an indirect evolution time, t 1 , and a mixing sequence
(see Figure 3.15 ). The two dimensions of two - dimensional NMR spectroscopy
are those of time. In one time domain, FIDs containing frequency and intensity
information about the observed nuclei is collected. The second time dimension
refers to the time that elapses between some perturbation of the system and
the onset of data collection in the time domain. The second time period is
varied, and a series of FID responses are collected for each of the variations.
Figure 3.14 The ideal Fourier transform experiment. (Adapted with permission of
Nelson Thornes Ltd. from Figure 5.12 of reference 21 .)
collect
data
wait wait5 to 10 T 1collect
data90° pulse 90° pulse