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fluorescent light to monitor, or scan the fluorescent light and provide an emission fluorescent spectrum.
Light from the grating passes to another photocell which monitors its intensity. The instrument is rather
complex and, as a result, rather expensive. However, from the point of view of measuring fluorescence
spectra it is extremely versatile. Much less sample is required to produce useful fluorescence spectra
compared to UV spectra and, in general, fluorescence spectra contain more fine detail and are thus more
useful for solute identification where sample size is limited. It is seen that as both the wavelength of the
excitation light and that of the fluorescent light are exclusively selected, consequently the spectra are
not liable to same errors as those of from the diode array discussed previously.

Infrared Spectroscopy

IR spectroscopy involves the absorption of light having a wavelength longer than the visible spectrum,
i.e. between 2 and 15 micron. In the practice of IR spectroscopy, spectra are usually displayed using
wavenumbers as the independent variable, as opposed to wavelength. Both forms of presentation are as
shown in Figure 1.7. The wavenumber is the number of waves per centimeter taken as (1/l) and
expressed in reciprocal centimeters (cm -1). Optically, the IR spectroscopic system is very similar to
that used to measure UV absorption except that the materials of construction must be different. The
optical components must be transparent to IR light of the pertinent wavelength as opposed to being
transparent to UV light. As already stated, UV absorption occurs at frequencies where the radiation
energy is equal to that of specific electronic transitions in the molecules of the substance. In contrast, IR
adsorption occurs at frequencies where the radiation energy is equal to that of changes in vibrational
and rotational energy of the molecule. A molecule can be regarded as an assembly of balls (atomic
nuclei) and springs (chemical bonds). Such a system can vibrate in a very complex manner and for most
compounds, a molecule containing (n) atoms will have (3n-6) modes of vibration. Consequently a
characteristic fundamental frequency and an adsorption band will be associated with each vibration
mode. Both UV and IR absorption provide spectra that are characteristic