Intensity of Absorption Bands
The intensity of a spectral transition is related to the magnitude of the change in dipole moment and the
relative populations of the energy levels between which the transition occurs (p. 274). In addition,
spectroscopic selection rules may 'forbid' certain transitions, although, in some cases, 'forbidden'
transitions may give rise to weak bands because of perturbation effects which relax the rules. The
largest changes in dipole moment are associated with electronic transitions, and visible or ultraviolet
spectrometry is therefore generally the most sensitive of the three techniques for quantitative analysis.
Nuclear magnetic resonance spectrometry is the least sensitive in this respect because populations of
nuclear spin energy levels are very similar.
Dissipation of Absorbed Energy
Most absorbed energy is converted into translational energy which appears as heat, the process
occurring within a fraction of a second and enabling spectra to be recorded rapidly or re-recorded as
necessary. However, after the absorption of radiation in the ultraviolet region of the spectrum, energy
may be re-emitted, usually at a longer wavelength, in the form of fluorescence. This occurs in a number
of complex organic molecules such as quinine, anthracene and fluorescein and in some inorganic
compounds where the lifetime of the excited state is prolonged by resonance stabilization. The
measurement of fluorescence radiation, which provides a sensitive means of quantitative analysis, is
known as fluorimetry (p. 373).
Instrumentation
The basic instrumentation used for spectrometric measurements has already been described in Chapter 7
(p. 277). The natures of sources, monochromators, detectors, and sample cells required for molecular
absorption techniques are summarized in Table 9.1. The principal difference between instrumentation
for atomic emission and molecular absorption spectrometry is in the need for a separate source of
radiation for the latter. In the infrared, visible and ultraviolet regions, 'white' sources are used, i.e. the
energy or frequency range of the source covers most or all of the relevant portion of the spectrum. In
contrast, nuclear magnetic resonance spectrometers employ a narrow waveband radio-frequency
transmitter, a tuned detector and no monochromator.
Recording spectrophotometers in the infrared, visible and ultraviolet regions necessarily employ the
double-beam principle of operation (Figures 9.1(a) and (b) and p. 277), but for quantitative
measurements at fixed wavelength single-beam instruments are often preferred. Diode array UV/visible
spectrophotometers provide a means of acquiring a full-range spectrum in under 1 second. The optics
are reversed in the sense that the monochromator disperses the radiation onto an array of photodiodes
after it has passed through the sample (Figure 9.2(a)). The array, which replaces the usual
photomultiplier detector, consists of up to about 300 photodiodes