bags and blister packs can be done rapidly using library search routines simply by placing the container
in a specially designed sample compartment. Instruments for the monitoring of specific substances and
based on relatively cheap near infrared emitting diodes, rather than using filters or monochromators to
isolate the desired wavelength, have been developed. The major advantages of NIR spectrometry are its
speed, minimal or no sample preparation and easy operation by unskilled personnel.
9.3—
Nuclear Magnetic Resonance (NMR) Spectrometry
Summary
Principles
Absorption of electromagnetic radiation in the radio-frequency region of the spectrum resulting in
changes in the orientation of spinning nuclei in a magnetic field.
Instrumentation
Powerful and highly homogeneous electromagnet, radio-frequency signal generator and detector circuit,
electronic integrator, glass sample tubes.
Applications
Identification and structural analysis of organic materials and study of kinetic effects, mainly from
proton and carbon-13 spectra. Useful for quantitative analysis but not widely applied.
Disadvantages
Expensive and complex instrumentation. Moderate to poor sensitivity with continuous wave (scanning)
instruments, but greatly enhanced by Fourier transform instruments. Limited range of solvents for
studying proton spectra unless they are deuterated.
Absorption of radiation in the radio-frequency, RF, region of the electromagnetic spectrum can be
observed for those nuclei which are considered to spin about their own axes. The energy changes are
associated with the orientation of the nuclear axis in space relative to an external applied magnetic field
and are of the order of 0.1 J mol–^1 , 10–600 MHz (50 cm–30 m or 3 × 10 –^4 to 2 × 10 –^2 cm–^1 ). This is
considerably smaller than the energy changes associated with vibrational and electronic transitions (pp.
364, 378).
All nuclei are assigned a spin quantum number I which may be zero, half-integral or integral; only those
with a non-zero value can give rise to an NMR spectrum. As the nucleus carries a charge, spinning
about its own axis produces a magnetic moment or dipole μ along the axis. This is analogous to the
magnetic field associated with a current flowing in a loop of wire. The