shift. This precessing process is captured as a free induction decay (FID) by
monitoring the variation in an induced current in receiver coils positioned
perpendicular to theX-,Y- axis. The FID is typically monitored for duration of
milliseconds to seconds. This basic NMR process is called a one pulse, one-
dimensional (1D) experiment.
A high power, short duration (hard) rf pulse will simultaneously perturb all
the^1 H,^13 C,or^15 Nnuclei in a molecule creating a FID that includes the NMR
resonances of all these nuclei. The observed FID is a complex combination of
sine and cosine wave oscillations at multiple frequencies in the time domain.
The FID is transformed to a conventional frequency-domain NMR spectrum
by applying a FT as depicted in Fig. 12.2.
12.3 NMR Hardware
The major components of an NMR spectrometer include a with a magnet,
probe, rf electronics, and a desktop computer as depicted in Fig. 12.3 and
further described in Table 12.2. Significant advancements in NMR hardware
over the years have yielded dramatic increases in experimental sensitivity and
resolution. One such advancement has been the recent introduction of the
shielded 900 MHz magnet. A 900 MHz magnet results in an 84% improvement
in sensitivity and a 150% improvement in resolution relative to a standard
600 MHz magnet (Kupce, 2001). The shielded aspect of the magnet reduces the
FIGURE 12.2 Scheme of one pulse, one-dimensional NMR experiment.372 INTRODUCTION TO NMR AND ITS APPLICATION