Page 103 - Tandem Techniques

Page 84

Consider a sample containing protons, situated in a strong magnetic field and irradiated at the transition
frequency. If the sample is then scanned by a second, low intensity magnetic field, when transition
actually occurs energy will be absorbed and can be detected.

The manner in which the experimental measurements are made will be discussed later, but consider the
results of an experiment using alcohol as the sample. Assume that the device is examined on a low-
resolution spectrometer (a concept which will also be discussed in due course). The sample is irradiated
with electromagnetic waves of the calculated frequency. The magnetic field intensity is then scanned
over a narrow range close to that where the absorption of energy is expected to take place. The energy
of absorption, which is sensed electronically, is plotted against field strength to provide the NMR
spectrum. The spectrum for ethyl alcohol would look something like that depicted in Figure 2.25.

Figure 2.25
A Low-Resolution NMR Spectrum of Ethyl Alcohol

There are several points of interest arising from the simple spectrum. First, the three peaks have areas in
the proportions of 1:2:3 indicating they are from the proton on the oxygen, the methylene protons and
the methyl protons. Thus the spectrum discloses the number of protons associated with each peak from
the relative peak areas. Second, the proton peaks appear at